This is it

Squashed commit of all the exploration history. Development starts here. Signed-off-by: Kim Altintop <kim@eagain.io>
author: Kim Altintop <kim@eagain.io> 2023-01-09 13:18:33 +0100
committer: Kim Altintop <kim@eagain.io> 2023-01-09 13:18:33 +0100
commit: d2f423521ec76406944ad83098ec33afe20c692b (patch)
tree: afd86bcb088eebdd61ba4e52fa666ff0f41c42a2
90 files changed, 18006 insertions, 0 deletions
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+In addition, other licenses may also apply. In particular, individual source
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+checksum = "49874b5167b65d7193b8aba1567f5c7d93d001cafc34600cee003eda787e483f"
+
+[[package]]
+name = "versions"
+version = "4.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ee97e1d97bd593fb513912a07691b742361b3dd64ad56f2c694ea2dbfe0665d3"
+dependencies = [
+ "itertools",
+ "nom",
+ "serde",
+]
+
+[[package]]
+name = "wasi"
+version = "0.9.0+wasi-snapshot-preview1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cccddf32554fecc6acb585f82a32a72e28b48f8c4c1883ddfeeeaa96f7d8e519"
+
+[[package]]
+name = "wasi"
+version = "0.11.0+wasi-snapshot-preview1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9c8d87e72b64a3b4db28d11ce29237c246188f4f51057d65a7eab63b7987e423"
+
+[[package]]
+name = "winapi"
+version = "0.3.9"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5c839a674fcd7a98952e593242ea400abe93992746761e38641405d28b00f419"
+dependencies = [
+ "winapi-i686-pc-windows-gnu",
+ "winapi-x86_64-pc-windows-gnu",
+]
+
+[[package]]
+name = "winapi-i686-pc-windows-gnu"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ac3b87c63620426dd9b991e5ce0329eff545bccbbb34f3be09ff6fb6ab51b7b6"
+
+[[package]]
+name = "winapi-util"
+version = "0.1.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "70ec6ce85bb158151cae5e5c87f95a8e97d2c0c4b001223f33a334e3ce5de178"
+dependencies = [
+ "winapi",
+]
+
+[[package]]
+name = "winapi-x86_64-pc-windows-gnu"
+version = "0.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f"
+
+[[package]]
+name = "windows-sys"
+version = "0.36.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ea04155a16a59f9eab786fe12a4a450e75cdb175f9e0d80da1e17db09f55b8d2"
+dependencies = [
+ "windows_aarch64_msvc 0.36.1",
+ "windows_i686_gnu 0.36.1",
+ "windows_i686_msvc 0.36.1",
+ "windows_x86_64_gnu 0.36.1",
+ "windows_x86_64_msvc 0.36.1",
+]
+
+[[package]]
+name = "windows-sys"
+version = "0.42.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5a3e1820f08b8513f676f7ab6c1f99ff312fb97b553d30ff4dd86f9f15728aa7"
+dependencies = [
+ "windows_aarch64_gnullvm",
+ "windows_aarch64_msvc 0.42.0",
+ "windows_i686_gnu 0.42.0",
+ "windows_i686_msvc 0.42.0",
+ "windows_x86_64_gnu 0.42.0",
+ "windows_x86_64_gnullvm",
+ "windows_x86_64_msvc 0.42.0",
+]
+
+[[package]]
+name = "windows_aarch64_gnullvm"
+version = "0.42.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "41d2aa71f6f0cbe00ae5167d90ef3cfe66527d6f613ca78ac8024c3ccab9a19e"
+
+[[package]]
+name = "windows_aarch64_msvc"
+version = "0.36.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9bb8c3fd39ade2d67e9874ac4f3db21f0d710bee00fe7cab16949ec184eeaa47"
+
+[[package]]
+name = "windows_aarch64_msvc"
+version = "0.42.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dd0f252f5a35cac83d6311b2e795981f5ee6e67eb1f9a7f64eb4500fbc4dcdb4"
+
+[[package]]
+name = "windows_i686_gnu"
+version = "0.36.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "180e6ccf01daf4c426b846dfc66db1fc518f074baa793aa7d9b9aaeffad6a3b6"
+
+[[package]]
+name = "windows_i686_gnu"
+version = "0.42.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fbeae19f6716841636c28d695375df17562ca208b2b7d0dc47635a50ae6c5de7"
+
+[[package]]
+name = "windows_i686_msvc"
+version = "0.36.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e2e7917148b2812d1eeafaeb22a97e4813dfa60a3f8f78ebe204bcc88f12f024"
+
+[[package]]
+name = "windows_i686_msvc"
+version = "0.42.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "84c12f65daa39dd2babe6e442988fc329d6243fdce47d7d2d155b8d874862246"
+
+[[package]]
+name = "windows_x86_64_gnu"
+version = "0.36.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4dcd171b8776c41b97521e5da127a2d86ad280114807d0b2ab1e462bc764d9e1"
+
+[[package]]
+name = "windows_x86_64_gnu"
+version = "0.42.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bf7b1b21b5362cbc318f686150e5bcea75ecedc74dd157d874d754a2ca44b0ed"
+
+[[package]]
+name = "windows_x86_64_gnullvm"
+version = "0.42.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "09d525d2ba30eeb3297665bd434a54297e4170c7f1a44cad4ef58095b4cd2028"
+
+[[package]]
+name = "windows_x86_64_msvc"
+version = "0.36.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c811ca4a8c853ef420abd8592ba53ddbbac90410fab6903b3e79972a631f7680"
+
+[[package]]
+name = "windows_x86_64_msvc"
+version = "0.42.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f40009d85759725a34da6d89a94e63d7bdc50a862acf0dbc7c8e488f1edcb6f5"
+
+[[package]]
+name = "zeroize"
+version = "1.5.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c394b5bd0c6f669e7275d9c20aa90ae064cb22e75a1cad54e1b34088034b149f"
+dependencies = [
+ "zeroize_derive",
+]
+
+[[package]]
+name = "zeroize_derive"
+version = "1.3.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "44bf07cb3e50ea2003396695d58bf46bc9887a1f362260446fad6bc4e79bd36c"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+ "synstructure",
+]
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..b5ced98
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,79 @@
+[package]
+name = "it"
+version = "0.1.0"
+authors = ["Kim Altintop <kim@eagain.io>"]
+license = "GPL-2.0-only WITH openvpn-openssl-exception"
+
+edition = "2021"
+rust-version = "1.60"
+
+[features]
+default = ["vendored-libgit2"]
+vendored-libgit2 = ["git2/vendored-libgit2"]
+sha1dc = ["sha1collisiondetection"]
+
+[dependencies]
+anyhow.features = ["backtrace"]
+anyhow.version = "1"
+base64.version = "0.13"
+clap.features = ["derive", "env", "string", "wrap_help"]
+clap.version = "4.0"
+clap_complete.version = "4.0"
+clap_mangen.version = "0.2"
+console.default-features = false
+console.version = "0.15"
+directories.version = "4.0"
+either.version = "1.8"
+erased-serde.version = "0.3"
+git2.default-features = false
+git2.version = "0.15"
+globset.version = "0.4.9"
+hex.features = ["serde"]
+hex.version = "0.4"
+log.features = ["std"]
+log.version = "0.4"
+multipart.default-features = false
+multipart.features = ["client"]
+multipart.version = "0.18"
+num_cpus.version = "1.13"
+once_cell.version = "1.13"
+rand_core.features = ["getrandom"]
+rand_core.version = "0.6"
+serde.features = ["derive", "std", "rc"]
+serde.version = "1"
+serde_json.version = "1.0"
+sha2.version = "0.10"
+shlex.version = "1.1"
+signature.version = "1.6"
+ssh-encoding.version = "0.1"
+ssh-key.features = ["alloc", "ecdsa", "ed25519", "encryption", "p256", "rsa"]
+ssh-key.version = "0.5"
+tempfile.version = "3.3"
+thiserror.version = "1.0"
+threadpool.version = "1.8"
+time.features = ["serde-well-known"]
+time.version = "0.3.11"
+tiny_http.features = ["ssl-openssl"]
+tiny_http.version = "0.11"
+unicode-normalization.version = "0.1.21"
+ureq.default-features = false
+ureq.features = ["gzip", "json", "native-tls"]
+ureq.version = "2.5"
+url.features = ["serde"]
+url.version = "2.2"
+versions.features = ["serde"]
+versions.version = "4.1"
+zeroize.version = "1.5.7"
+
+#
+# Optionals
+#
+sha1collisiondetection.default-features = false
+sha1collisiondetection.optional = true
+sha1collisiondetection.version = "0.2"
+
+#
+# Platform specifics
+#
+[target.'cfg(windows)'.dependencies]
+uds_windows = "1.0"
diff --git a/DCO b/DCO
new file mode 100644
index 0000000..d53d3ad
--- /dev/null
+++ b/DCO
@@ -0,0 +1,40 @@
+                  Developer Certificate of Origin
+
+By making your contribution, you are making the declaration set out in the
+Linux Foundation’s Developer Certificate of Origin version 1.1 as set out
+below, in which the “open source licence indicated in the file” is GPLv3.
+
+Developer Certificate of Origin
+Version 1.1
+
+Copyright (C) 2004, 2006 The Linux Foundation and its contributors.
+
+Everyone is permitted to copy and distribute verbatim copies of this
+license document, but changing it is not allowed.
+
+
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.
diff --git a/Documentation/.gitignore b/Documentation/.gitignore
new file mode 100644
index 0000000..2d19fc7
--- /dev/null
+++ b/Documentation/.gitignore
@@ -0,0 +1 @@
+*.html
diff --git a/Documentation/Makefile b/Documentation/Makefile
new file mode 100644
index 0000000..0ad63a9
--- /dev/null
+++ b/Documentation/Makefile
@@ -0,0 +1,4 @@
+default: spec.html
+
+%.html: %.adoc
+	asciidoctor -v $<
diff --git a/Documentation/getting-started.adoc b/Documentation/getting-started.adoc
new file mode 100644
index 0000000..a5ca4bc
--- /dev/null
+++ b/Documentation/getting-started.adoc
@@ -0,0 +1,310 @@
+// Copyright © 2023 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: CC-BY-SA-4.0
+
+= Getting started with _it_
+:stylesheet: monospace.css
+:source-highlighter: pygments
+
+This document walks you through the very basics of _it_. Many of the
+interactions below may be automated away eventually, but our goal here is to
+provide an overview of what is going on under the hood.
+
+
+== Prerequisites
+
+We are going to assume you have the _it_ executable installed using
+
+    cargo install --git https://git.eagain.io/it
+
+Chances are that you already have an SSH key handy. If not, or if you want to
+use a key specifically for this exercise, generate one using
+
+    ssh-keygen -t ed25519
+
+It is also a good idea to add this key to your `ssh-agent`, so you don't have to
+type the password every time it is used for signing. Typing `ssh-add` usually
+does the trick.
+
+Next, we'll need to teach git to use our SSH key for signing. If you followed
+above recommendation and are using an agent for signing, the following commands
+will set it up as a default:
+
+    git config --global gpg.format ssh
+    git config --global user.signingKey "key::$(cat /path/to/your_key.pub)"
+
+If you prefer to not mess with your existing git configuration, you can also
+arrange for the key to be recognised by _it_ itself by running the following
+command instead:
+
+    git config --global it.signingKey "key::$(cat /path/to/your_key.pub)"
+
+Lastly, we'll create an _it_ xref:spec.adoc#_identities[identity] using this
+key:
+
+    it id init
+
+The command's output will look similar to this:
+
+[source,json]
+----
+{
+  "committed": {
+    "repo": "~/.local/share/it/ids",
+    "ref": "refs/heads/it/ids/671e27d4cce92f747106c7da90bcc2be7072909afa304d008eb8ecbfdebfbfe2",
+    "commit": "e08c34df95cd28aa212a4d110ecfb8acec2a102c"
+  },
+  "data": {
+    "signed": {
+      "_type": "eagain.io/it/identity",
+      "spec_version": "0.1.0",
+      "prev": null,
+      "keys": [
+        "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIDtt6XEdNVInhiKkX+ccN++Bk8kccdP6SeBPg0Aq8XFo"
+      ],
+      "threshold": 1,
+      "mirrors": [],
+      "expires": null,
+      "custom": {}
+    },
+    "signatures": {
+      "ddc27a697903b8fe3ae3439818af81eaac20ba65e51a4170e3c81eb25abd1767": "5a460b26099ddd42912b7a52ee0c478619425ddfe4a562fd2ffd427d84cde6ab32effd8971308cfcdb64b08ac920e7a2c2a69d11b0ca7fe293e39306cd4d7c01"
+    }
+  }
+}
+----
+
+The `data` object is exactly what is stored in the repository `repo` at branch
+`ref`, which we invite you to convince yourself of using normal git commands.
+
+Identities can describe multiple keys, and carry additional custom metadata, but
+we'll skip over this for now.
+
+
+== Local drop
+
+_it_ is organised around patches. You know, like in the olden days, but
+xref:spec.adoc#_patches[not quite]. Patches are recorded onto a log dubbed
+xref:spec.adoc#_drops["`drop`"].
+
+If you have a git repository to toy around with, you can initialise a drop
+adjacent to the "normal" branches in it. You can record patches (which you may
+have received from elsewhere) onto that local drop, and push it to wherever you
+like.
+
+To initialise a drop in this way, just run:
+
+    it drop init --description "my project"
+
+This will drop you into `$EDITOR` to give you an opportunity to customise the
+drop's metadata, which will look similar to this:
+
+[source,json]
+----
+{
+  "description": "my project",
+  "roles": {
+    "drop": {
+      "ids": [
+        "671e27d4cce92f747106c7da90bcc2be7072909afa304d008eb8ecbfdebfbfe2"
+      ],
+      "threshold": 1
+    },
+    "snapshot": {
+      "ids": [
+        "671e27d4cce92f747106c7da90bcc2be7072909afa304d008eb8ecbfdebfbfe2"
+      ],
+      "threshold": 1
+    },
+    "mirrors": {
+      "ids": [
+        "671e27d4cce92f747106c7da90bcc2be7072909afa304d008eb8ecbfdebfbfe2"
+      ],
+      "threshold": 1
+    },
+    "branches": {
+      "refs/heads/main": {
+        "ids": [
+          "671e27d4cce92f747106c7da90bcc2be7072909afa304d008eb8ecbfdebfbfe2"
+        ],
+        "threshold": 1,
+        "description": "the default branch"
+      }
+    }
+  },
+  "custom": {}
+}
+----
+
+You may want to check if _it_ has guessed your mainline branch correctly (in the
+`branches` section), but otherwise just save and exit to finish the
+initialisation step. Run
+
+    git log -p refs/it/patches
+
+to see the effect.
+
+
+We want source code patches to be against the `refs/heads/main` branch, so we
+need to teach the drop about what the current state is:
+
+    it merge-point record
+
+Again, you may want to run `git log` as above to see what changed. You'll notice
+a line starting with "Re:" in the latest commit message: this is the
+xref:spec.adoc#_topics[topic] of a patch, and a xref:spec.adoc#mergepoints[merge
+point] is just a patch with a well-known topic. Run
+
+    it topic ls
+
+to see that this topic now exists, and
+
+    it topic show c44c20434bfdaa0384b67d48d6c3bb36d755b87576027671f606c404b09d9774
+
+to display the metadata recorded in it.
+
+Whenever you update `refs/heads/main`, run `merge-point record` again to convey
+the new head to the drop. ``show``ing the topic as above will give you a log of
+every such update.
+
+
+Finally, let's create a patch: make some changes on a feature branch, like you
+normally would, and then run
+
+    it patch record
+
+This will drop you into `$EDITOR`, asking you to describe what the patch is
+about. After you save and exit, a new record will be committed onto the drop,
+and a new topic will have been created:
+
+    $ it topic ls
+    {
+      "topic": "2d2d3c97df62b18d3d1476342fe9d6df0989592f6d55d151350422795da714d8",
+      "subject": "Just testin"
+    }
+    {
+      "topic": "c44c20434bfdaa0384b67d48d6c3bb36d755b87576027671f606c404b09d9774",
+      "subject": "Merges"
+    }
+
+You can post more patches to an existing topic, and reply to a specific entry
+within the topic. Because a patch in _it_ is really a combination of commentary
+and source code changes, and source code changes are actually optional, we have
+a handy shortcut to just, well, comment:
+
+    it topic comment record 2d2d3c97df62b18d3d1476342fe9d6df0989592f6d55d151350422795da714d8
+
+Type your comment into `$EDITOR`, save and exit. The result may look like this:
+
+    $ it topic show 2d2d3c97df62b18d3d1476342fe9d6df0989592f6d55d151350422795da714d8
+    {
+      "header": {
+        "id": "11337eb409fbd16a034d0323dfa8d879b5a0f36c",
+        "author": {
+          "name": "Kim Altintop",
+          "email": "kim@eagain.io"
+        },
+        "time": "2023-01-09T09:39:15+01:00",
+        "patch": {
+          "id": "8da0f98009aae98e7ca9df926125aa386a4f6a644c2036e9ec86a0810a7b8a62",
+          "tips": []
+        },
+        "in-reply-to": "0c9b7c0b437a3a072f3a1eead17703d22a0bf8f1"
+      },
+      "message": {
+        "_type": "eagain.io/it/notes/basic",
+        "message": "Ship it"
+      }
+    }
+    {
+      "header": {
+        "id": "0c9b7c0b437a3a072f3a1eead17703d22a0bf8f1",
+        "author": {
+          "name": "Kim Altintop",
+          "email": "kim@eagain.io"
+        },
+        "time": "2023-01-09T09:23:51+01:00",
+        "patch": {
+          "id": "502b3c4dcf709c9b16df2b58aece9a8966405347a2bf6ccbb305711120984951",
+          "tips": [
+            "refs/it/bundles/502b3c4dcf709c9b16df2b58aece9a8966405347a2bf6ccbb305711120984951/heads/main"
+          ]
+        }
+      },
+      "message": {
+        "_type": "eagain.io/it/notes/basic",
+        "message": "Just testin"
+      }
+    }
+
+
+Notice the `patch.tips` array? If the patch contains references which are
+conventionally recognised as source code changes (i.e. `refs/heads/...`,
+`refs/tags/...`), their physical location inside the drop's repository will be
+shown here. _it_ is currently lacking a nice UI for this, but you can just do
+
+    git diff refs/it/bundles/502b3c4dcf709c9b16df2b58aece9a8966405347a2bf6ccbb305711120984951/heads/main
+
+to see the diff against your currently checked-out branch. If you're satisfied,
+go ahead and merge this ref into your local `main` branch. Don't forget to thank
+yourself for the contribution by commenting on the topic!
+
+
+To wrap it up, you may be wondering how _it_ stored everything in your
+repository, and perhaps clean it up. Run
+
+    git for-each-ref refs/it
+
+to poke around the references _it_ uses to maintain its state. Note, however,
+that this structure is not part of any public API, and may change without
+further notice!
+
+The actual xref:spec.adoc#_bundles[patch bundles] can be found in
+`.git/it/bundles`. Note that a patch bundle is self-contained -- you can send
+them over email, store them in IPFS, or whatever is convenient to move them from
+one place to another.
+
+
+== Remote drop
+
+We said that you could receive patches over whatever channel, and apply them to
+your local drop. A more tangible way is to serve the drop over HTTP, allowing
+anyone to submit patches to it. While it's possible to do this from your working
+repository, it is preferable to create a dedicated repo for the drop:
+
+    it drop init --git-dir /the/drop.git --description "my public drop"
+    it merge-point record --git-dir /the/drop.git --source-dir .
+    cd /the/drop.git
+    RUST_LOG=debug it serve
+
+In a second terminal, cd into your working repo and add the drop as a regular
+git remote:
+
+    git remote add dropit /the/drop.git
+    git remote update dropit
+
+You can now submit to it by replacing `record` with `submit` for the respective
+commands, and specifying `--drop dropit/patches` to use the remote drop as the
+reference.
+
+Currently, an extra command `it drop bundles sync` is needed to receive the
+patch bundles after updating the remote. This is not particularly smart yet,
+especially given that we do support inspecting individual topics (as
+opposed to the entire drop history) by `it topic unbundle`. We'll get there.
+
+
+== Loose ends
+
+If you've used email to send around patches, or even the excellent
+https://git.kernel.org/pub/scm/utils/b4/b4.git[b4] tool, this may all seem
+vaguely familiar to you: instead of `mbox` archives we have binary git bundles,
+what gives?
+
+That's fair, we haven't really detailed how _it_ permits much richer
+interactions and datatypes, for lack of a UI. For brevity, we also haven't shown
+that patch bundles can be stored on IPFS, the "commit bit" can be extended to
+co-maintainers, or how more complex topologies can be created by drop
+aggregation (and without resorting to HTTP POST).
+
+We invite you to play around with the available commands, read the
+xref:spec.adoc[spec], and perhaps consider to contribute where you see _it_ is
+currently lacking :)
diff --git a/Documentation/monospace.css b/Documentation/monospace.css
new file mode 100644
index 0000000..056ed3e
--- /dev/null
+++ b/Documentation/monospace.css
@@ -0,0 +1,72 @@
+/* SPDX-License-Identifier: MIT
+ *
+ * Based on https://github.com/darshandsoni/asciidoctor-skins/blob/c98a8ab9b27571e5b63d75912a3c753cc72ed8e4/css/monospace.css
+ *
+ */
+
+@import url(https://cdn.jsdelivr.net/gh/asciidoctor/asciidoctor@2.0/data/stylesheets/asciidoctor-default.css);
+
+:root {
+--maincolor:#FFFFFF;
+--primarycolor:#000000;
+--secondarycolor:#000000;
+--tertiarycolor: #000000;
+--sidebarbackground:#CCC;
+--linkcolor:#000000;
+--linkcoloralternate:#f44336;
+--white:#FFFFFF;
+--black:#000000;
+--font: SFMono-Regular,Menlo,Monaco,Consolas,"Liberation Mono","Courier New",monospace;
+}
+
+html,body {
+    font-size: 80%;
+}
+body {
+    font-family: var(--font);
+}
+h1 {
+    color: var(--primarycolor) !important;
+    font-family: var(--font);
+}
+h2,h3,h4,h5,h6 {
+    color: var(--secondarycolor) !important;
+    font-family: var(--font);
+}
+pre,code {
+    font-family: var(--font);
+    white-space: pre;
+}
+
+#header,#content,#footnotes,#footer {
+    max-width: 72em;
+}
+#footer {
+    display: none;
+}
+#footnotes .footnote {
+    font-size: 1em;
+}
+.title {
+    color: var(--tertiarycolor) !important;
+    font-family: var(--font) !important;
+    font-style: normal !important;
+    font-weight: normal !important;
+}
+.content>pre {
+    font-size: 1rem;
+}
+#toctitle {
+    font-family: var(--font);
+    font-size: 2em;
+    color: var(--primarycolor);
+}
+.sectlevel1 {font-family: var(--font)!important; font-size: 1.0625rem;}
+.sectlevel2 {font-family: var(--font)!important;}
+.sectlevel3 {font-family: var(--font)!important;}
+.sectlevel4 {font-family: var(--font)!important;}
+
+.lst,dl {
+    padding-left: 5%;
+    padding-right: 10%;
+}
diff --git a/Documentation/spec.adoc b/Documentation/spec.adoc
new file mode 100644
index 0000000..0816a11
--- /dev/null
+++ b/Documentation/spec.adoc
@@ -0,0 +1,1373 @@
+// Copyright © 2022-2023 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: CC-BY-SA-4.0
+
+= it: zero-g git
+Kim Altintop <kim@eagain.io>
+:revdate: 2022
+:revnumber: 0.1.0
+:revremark: draft
+:attribute-missing: warn
+:listing-caption: Figure
+:reproducible:
+:sectanchors:
+:sectnums:
+:stylesheet: monospace.css
+:toc: preamble
+:toclevels: 3
+:xrefstyle: short
+
+_it_ aims to augment git with primitives to build integrated, cryptographically
+verifiable collaboration workflows around source code. It maintains the
+distributed property of git, not requiring a central server. _it_ is transport
+agnostic, and permits data dissemination in client-server, federated, as well as
+peer-to-peer network topologies.
+
+
+// Footnotes. Ironically at the head, due to asciidoc's evaluation order
+:fn-content-hash: pass:q,a[footnote:hash[ \
+Hashing with both the SHA-1 and SHA-256 algorithms allows internally-linked \
+data to roam between git repositories with different object formats. We hope \
+that when and if git introduces support for a new hash algorithm post SHA-256, \
+it will also have interoperability implemented. Otherwise, the burden will \
+fall on _it_ implementations. \
+]]
+:fn-peel: footnote:peel[ \
+"Peeling" is git jargon for dereferencing the natural target of a git object \
+until an object of the desired type is found. \
+]
+:fn-commit-signing: pass:q,a[footnote:commit-signing[ \
+_it_ does not prescribe whether commits or tags pertaining source code \
+histories must be cryptographically signed. Due to the non-commutativity of \
+git commits (their identity changes when reordered), it is highly dependent on \
+the development model whether author signatures are preserved in published \
+histories. Thus, we leave it to users to decide if signatures should be \
+applied at the git level, or other forms of attestation (e.g. via topic \
+entries) are employed. \
+]]
+:fn-resolve-id: footnote:resolveid[ \
+Normally, identities must be resolvable within the same tree as the drop \
+metadata. However, resolution may be substituted if e.g. the client believes \
+to have more up-to-date identity data elsewhere. \
+]
+//
+
+
+== Introduction
+
+=== Motivation
+
+The checks and balances of Free and Open Source Software (FOSS) is the ability
+for anyone to contribute to or diverge from ("`fork`") a line of development
+freely and cheaply. As FOSS is defined by the community developing it, this
+extends to all artefacts of communication and collaboration, not just the source
+code itself. In other words, an open development model is a transparent process.
+
+It is easy to see that this model necessitates _data sovereignty_: control over
+the data implies controlling participation.
+
+Traditionally, this property has been approximated by using internet email for
+collaboration. While its simplicity as a medium has its merits, email is clearly
+declining in popularity for our purpose. We attribute this to mainly two
+weaknesses: intended primarily as a free-form medium, email is lacking the
+_programmability_ of the web, impeding innovation in both tooling and services.
+Secondly, the protocol is inherently prone to abuse by permitting unsolicited
+messages, and the response measures implemented over the years have amplified
+monopolization: today, it takes significant effort and expertise to maintain a
+mail exchanger independent of large providers (let alone one which hosts a
+mailing list fanning out messages to a potentially large number of subscribers).
+
+It is not obvious, however, how an alternative could look like on a protocol
+level. Among the tradeoffs to consider is the tension between openness,
+addressability and availability -- and it highly depends on the situation which
+one has higher priority. It thus seems unlikely that it can be resolved once and
+for all. Instead, we recognise it as desirable to provide the user with choices
+of transport methods. Or, put differently, that "`the network is optional`", as
+Kleppmann et al. have called for in their essay on <<local-first,"Local-first
+software">>.
+
+Git is prototypical of the _local-first_ idea, providing data sovereignty -- for
+as long as we do not consider bidirectional collaboration: git commits do not
+commute, and so concurrent modifications do not converge, but must be explicitly
+linearised. This is not satisfying if we want to eliminate both intermediaries
+and online rendezvous. It is tempting to design a source code management and
+collaboration system from the ground up with commutativity in mind, yet git is
+so ubiquitous that we feel that we cannot forgo to present a solution which
+preserves the ability to use its existing toolchain and ecosystem. It turns out
+that, while it would be difficult to retrofit git into a proper, idealised
+_local-first_ application, it is perfectly suitable for _hosting_ such an
+application which models the collaboration process itself.
+
+
+=== Overview
+
+_it_ is essentially a collection of datatypes.
+
+We start by establishing identities (<<Identities>>), which for our purposes
+only need to certify ownership of public keys. By using an extensible,
+human-readable metadata format, we leave it to the user to bind the identity to
+external identifiers or extend it with "`profile`" information in order to
+convey a _persona_. As the metadata can be conveniently managed using git, it
+can be published easily.
+
+_it_ inherits the paradigm of most distributed version control systems, where
+changes are exchanged as small increments ("`patches`", <<Patches>>), but
+generalises the concept to include both source code changes and associated data
+such as commentary. An _it_ patch is thus similar to an email message, but
+mandates the associated data to be structured (as opposed to free-form).
+Ordering with respect to related patches is determined via git's commit graph,
+optionally allowing for sophisticated shared state objects to be constructed if
+a <<CRDT>>-based payload is used.
+
+Patches are recorded onto a log structure ("`drop`", <<Drops>>), for which we
+define a representation as a git commit history. The patch contents are,
+however, not stored directly in this structure, but redistributed verbatim. This
+is done so as to reduce data dissemination to mostly (static) file transmission,
+which opens up more choices for alternative transport protocols and minimises
+resource consumption imposed by dynamic repacking.
+
+The drop is responsible for ensuring that the dependencies (or: prerequisites)
+of a patch are satisfied before recording it, enforcing that the partial
+ordering of related patches can be recovered. Apart from that, a drop does not
+provide any ordering guarantees, which means that independent drops may converge
+even though their (commit) hashes differ.
+
+Finally, a drop is secured by a trust delegation scheme which authorises
+operations modifying its state. It also serves as a PKI, allowing verification
+of all signed objects it refers to.
+
+Networking is exemplified by a simple HTTP API (<<HTTP API>>), hinting at
+alternative protocols where appropriate. We envisage patch submission to give
+rise to gateway services, which may be elaborated on in future revisions of this
+document.
+
+
+// TODO: Related work?
+
+== Conventions and Terminology
+
+The key words "`MUST`", "`MUST NOT`", "`REQUIRED`", "`SHALL`", "`SHALL NOT`",
+"`SHOULD`", "`SHOULD NOT`", "`RECOMMENDED`", "`NOT RECOMMENDED`", "`MAY`", and
+"`OPTIONAL`" in this document are to be interpreted as described in <<RFC2119>>
+and <<RFC8174>> when, and only when, they appear in all capitals, as shown here.
+
+Familiarity with git concepts and datastructures is assumed, and terminology
+used without further explanation. Refer to the <<gitglossary>> instead.
+
+== Formats
+
+=== Signed Values
+
+Signed data items in _it_ are encoded as a subset of JSON which disallows
+floating point numbers, and requires string values and object keys to be UTF-8
+encoded. Signatures are obtained over the SHA-512 hash of the
+<<Canonical-JSON,canonical form>> of the JSON object (hashing is used to
+minimise the payload size, which may be sent to an agent process for signing).
+
+JSON values SHOULD be stored in pretty-printed form, with object keys sorted
+lexicographically.
+
+Empty optional fields SHOULD NOT be omitted from the output, but be set to
+`null` if the value is a custom type represented by a JSON string, or the
+neutral element of the JSON type otherwise.
+
+Unless otherwise noted, JSON arrays SHALL be treated as sets.
+
+Where JSON data is signed inline, it is wrapped in an object:
+
+[source,subs="+macros"]
+----
+{
+    "signed": <<OBJECT>>,
+    "signatures": {
+        <<KEYID>>: <<SIGNATURE>>,
+        ...
+    }
+}
+----
+
+[[OBJECT]]OBJECT::
+    A JSON object. Its canonical form is obtained as per <<Canonical-JSON>>.
+
+[[KEYID]]KEYID::
+    The identifier of the key signing the OBJECT, which is the SHA-256 hash of
+    the canonical form of the key, in hexadecimal.
+
+[[SIGNATURE]]SIGNATURE::
+    The hex-encoded signature of the SHA-512 hash of the canonical form of
+    OBJECT.
+
+=== Common Types
+
+[[BLOB_HASH]]BLOB_HASH::
+    Hash of the payload `p`, as if created by <<git-hash-object>>. That is, for
+    a hash algorithm `H`:
++
+[source]
+----
+H('blob ' || LEN(p) || NUL || p)
+----
+
+
+[[CONTENT_HASH]]CONTENT_HASH::
+    Dictionary of both the SHA-1 and SHA-256 <<BLOB_HASH>> of the referenced
+    object{fn-content-hash}:
++
+[source,subs="+macros"]
+----
+{
+    "sha1": <<BLOB_HASH>>,
+    "sha2": <<BLOB_HASH>>
+}
+----
+
+
+[[DATETIME]]DATETIME::
+    Date-time string in <<RFC3339>> format, e.g. "`2022-08-23T14:48:00Z`".
+
+[[OBJECT_ID]]OBJECT_ID::
+    Hexadecimal git object id.
+
+[[SPEC_VERSION]]SPEC_VERSION::
+    Version of this specification in "`dotted triple`" format, currently
+    {revnumber}. The semantics loosely follows the <<semver,"Semantic
+    Versioning">> convention, but gives no significance to leading zeroes.
+
+[[URL]]URL::
+    A URL as per the <<WHATWG-URL, WHATWG specification>>.
+
+[[VARCHAR]]VARCHAR(N)::
+    A UTF-8 encoded string of at most length `N` (in bytes).
+
+
+== Identities
+
+Like most decentralised systems, _it_ relies on public key cryptography to
+ensure authenticity of data. In order to manage and distribute public keys, _it_
+defines a <<id-metadata,simple, JSON-based format>> which can conveniently be
+stored in git.
+
+The subject of an _it_ identity is not inherently a human, it could just as well
+be a machine user such as a CI- or merge bot, or a group of users extending
+ultimate trust to each other. Consequently, it should not be assumed that
+ownership of the keys constituting the identity lies with a single actor in the
+system. It is, however, illegal to reuse keys for multiple identities within the
+same context.
+
+The context of an identity is generally a <<Drops,drop>>. Thus, a subject may
+create as many identities as they see fit (provided keys are not reused).
+Conversely, the `*custom*` attribute of an <<id-json,id.json>> document permits
+to associate an _it_ identity with external methods certifying the subject's
+_persona_, such as custodial identity providers or <<DID>> controllers (for
+example by embedding a DID document in the `*custom*` section).
+
+In general, _it_ does not specify how trust is initially established in an
+identity.
+
+Identities in _it_ are self-certifying, in that introduction or revocation of
+keys are signed by a threshold of the specified keys themselves. A threshold
+greater than one reduces the probability of identity compromise, even if a
+subset of its keys is compromised. For usability reasons, owners of personal
+identities may want to set the `threshold` to `2` and carry a certification key
+on a portable device.
+
+For practical reasons, it is RECOMMENDED for implementations to use the widely
+deployed <<OpenSSH>> suite for signing purposes, including for git commits.
+Verification of SSH-signed git commits (available since git version 2.34) MUST
+be supported. Via the <<ssh-agent>> protocol, alternative tooling is not
+precluded. All key algorithms and signature schemes supported by OpenSSH MUST be
+supported by _it_ implementations. To make it easy for users to visually match
+output from OpenSSH with <<id-json,id.json>> documents, <<KEY,keys>> are encoded
+in the format used by OpenSSH.
+
+Additional key algorithms, signature schemes or public key encodings may be
+introduced in the future.
+
+[#id-metadata]
+=== Metadata
+
+Identity information is stored in a JSON file, conventionally named `id.json`.
+The file's contents can be amended using a threshold signature scheme, and
+revisions are hash-linked to their predecessors.
+
+The `*signed*` portion of the `id.json` file is defined as follows:
+
+[source#id-json,subs="+macros"]
+----
+{
+    "_type": "eagain.io/it/identity",
+    "spec_version": <<SPEC_VERSION>>,
+    "prev": <<CONTENT_HASH>> | null,
+    "keys": [
+        <<KEY>>,
+        ...
+    ],
+    "threshold": <<THRESHOLD>>,
+    "mirrors": [
+        <<URL>>,
+        ...
+    ],
+    "expires": <<DATETIME>> | null,
+    "custom": <<CUSTOM>>
+}
+----
+
+[[KEY]]KEY::
+    Public key in SSH encoding, specified in <<RFC4253>>, <<RFC5656>> and
+    <<RFC8709>>. The comment or label part after the base64-encoded key SHOULD
+    be omitted in the document.
++
+Example:
++
+----
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIDtt6XEdNVInhiKkX+ccN++Bk8kccdP6SeBPg0Aq8XFo
+----
+
+[[THRESHOLD]]THRESHOLD::
+    An integer number of keys whose signatures are required in order to consider
+    the identity metadata to be properly signed. Must be between 1 and the
+    number of `*keys*` in the metadata file.
+
+[#id-verification]
+=== Verification
+
+Verification of an identity history proceeds as follows:
+
+[.lst]
+    . Load the latest known <<id-json,id.json>> metadata
+
+    . If the `*expires*` attribute is not `null`, check that the specified
+      <<DATETIME>> does not lie in the past. Otherwise, abort and report an
+      error.
+
+    . Verify that at least `*threshold*` of `*keys*` have provided valid
+      signatures
+
+    . If `*prev*` is not `null`, load the corresponding previous revision of the
+      metadata
+
+    . Verify that at least `*threshold*` of `*keys*` of the _previous_ revision
+      have provided valid signatures over the _current_ revision
+
+    . Repeat steps 4. and 5. until `*prev*` is `null`
+
+    . [[IDENTITY_ID]]Compute the SHA-256 hash over the canonical form of the
+      initial revision. This is the *_identity id_*.
+
+    . If a particular identity id was expected, check that it matches the
+      computed one
+
+
+== Patches
+
+A source code patch is traditionally a differential between source code files.
+In practice, however, such diffs are seldomly exchanged without additional
+context, usually prose describing and motivating the change.
+
+During the process of accepting a patch into the mainline history of a project,
+collaborators may leave comments on the original submission, reference points
+may be annotated ("`tagged`"), and revised versions of the patch may be
+submitted. The degree to which this process is formalised varies between
+projects, as does the preference for capturing it in formal datastructures such
+as ticketing systems. A common property of all these different contributions to
+a code base is that they can be seen as state transitions, where the git commit
+chain helpfully provides a way to establish a partial ordering.
+
+_it_ seeks to unify all kinds of contributions into a single exchange format, a
+<<Bundles,bundle>>, which is already native to git. The semantics of a bundle,
+apart from causal ordering, is defined by its contents, which makes the format
+amenable for future extensions.
+
+In that sense, _it_ aspirationally uses the term "`patch`" in the generalised
+way described by theoretical work such as <<Darcs>>, <<CaPT>>, and <<HoPT>>.
+When describing the more mundane processing procedures, the term "`patch
+bundle`" is also used, alluding to the container format.
+
+=== Bundles
+
+A patch bundle is a <<gitformat-bundle,git bundle>> of either version supported
+by git (v2 or v3). If v3 is used, only the `object-format` capability is
+recognised, specifying an object `filter` is illegal.
+
+For compatibility with git, prerequisite object ids MUST refer to commit
+objects, even though the format specification permits any object type.
+
+The pack data of the bundle MUST NOT contain objects unreachable from the tips
+specified in the bundle header.
+
+[NOTE]
+====
+Enforcing this rule on the receiving end of a patch bundle may not be practical
+in some circumstances. Unreachable objects will automatically be purged if and
+when <<snapshots,snapshots>> are taken (which imply repacking), but it is worth
+noting that there might be security implications of redistributing patch bundles
+which have not been verified to adhere to this rule, as it is possible to
+"`hide`" arbitrary objects in the bundle.
+====
+
+The bundle references may contain zero or more branches, tags or notes. A
+<<Topics,topic>> ref MUST be present. If <<Identities,identities>> need to be
+added or updated, zero or more `ids` refs may be present whose target either
+resolves directly to an updated <<id-json,id.json>>, or is peelable{fn-peel} to
+a tree containing the updated document in a blob named `id.json` at the root.
+
+Where more than one occurrence is permissible, the receiver MAY limit the total
+number of occurrences (see also <<drop-validation>>).
+
+More formally, the permissible references are (in ABNF notation):
+
+[source#bundle-refs,abnf,subs="+macros"]
+----
+refname  = topic / *identity / *branch / *tag / *note
+
+topic    = "refs/it/topics/" <<TOPIC_ID>>
+identity = "refs/it/ids/" <<IDENTITY_ID>>
+branch   = "refs/heads/" name
+tag      = "refs/tags/" name
+note     = "refs/notes/" name
+----
+
+[[TOPIC_ID]]TOPIC_ID::
+    SHA-256 hash, in hexadecimal. The preimage is opaque to _it_, but should be
+    chosen by the initiator of a <<Topics,topic>> such that the probability of
+    collisions with independently initiated topics is very low (for example the
+    contents of the initial message combined with a random nonce).
+
+
+The pack data section of a bundle MAY be encrypted using either <<age>> or GPG.
+
+=== Topics
+
+A topic is conceptually similar to a mailing list thread or structured data such
+as a "`Pull Request`", in that it groups together related information. The
+<<TOPIC_ID,stable identifier>> of a topic is a SHA-256 hash, the preimage of
+which is opaque to _it_.
+
+A patch bundle MUST contain a topic commit history (`refs/it/topics/...`)
+containing objects which represent _interactions_ such as free-form comments,
+code review annotations, attestations ("`signoffs`") or results from CI services.
+The set of all histories referring to the same topic identifier forms a directed
+acyclic graph (DAG), usually a tree, yielding a partial order of topic entries.
+
+If topic entries form a <<CRDT>>, sophisticated "`mutable`" state objects can be
+constructed, resembling concepts commonly managed in a centralised fashion such
+as "`Issues`", "`Task trackers`" or automated merge queues. However, not all
+workflows require this level of sophistication (namely the ability to change
+state collaboratively), and traversing a DAG of semi-structured, easily
+parseable data in topological order is sufficient. Examples of this include
+mailing-list style conversations or archives of external communication systems.
+
+Hence, _it_ mandates that topic histories can have one of two types: message
+based or CRDT based.
+
+[.lst]
+    - [[message-topic]]*Message based* topics consist of a single JSON object
+      per commit, found in a file named `*m*` at the root of the commit's tree.
+      A message based topic is represented by its commit graph.
+
+    - [[crdt-topic]]*CRDT based* topics consist of a single
+      <<automerge-change,change object>> per commit, found in a file named `*c*`
+      at the root of the commit's tree. CRDT based topics are represented by a
+      single object, to which changes are applied in the topological order of
+      the commit graph.
+
+[NOTE]
+====
+The <<Automerge>> CRDT is chosen for its generality. Future versions of this
+document may allow for other CRDTs to be used.
+
+The exact encoding of Automerge changes for use with _it_ is still under
+consideration. Since binary operation payloads are likely to be undesirable for
+the intended use, it may be preferable to define a textual encoding (such as
+JSON), which would make the stored data easier to inspect without specialised
+tooling.
+====
+
+Changing the type of a topic is illegal, and should result in the offending
+patch to be rejected, or be omitted during topic traversal.
+
+In both paradigms, authenticity of authorship is inferred from the cryptographic
+signature of the individual commits. Dependencies, respectively reply-to
+relationships, are expressed as commit parents.
+
+Note that no type or schema information is imposed. It is up to the client to
+interpret the data payload, and potentially omit unknown entries from the
+output.
+
+[#patch-equivalence]
+=== Equivalence
+
+
+Depending on context, two patch bundles are considered equivalent if:
+
+[.lst]
+- The set of bundle reference targets is equal
++
+This means that the bundles logically carry the same information, which is
+preserved even if repacked (e.g. when <<snapshots,snapshots>> are used). This
+equivalence is captured in the <<BUNDLE_HEADS>> value, which is the value a
+patch submitter signs and which determines whether a patch has been received
+before by a <<Drops,drop>>.
+
+- The union of the reference targets and prerequisite objects is equal
++
+When applied to an existing object database, the packfiles require the same
+objects to be present, and result in the same (reachable) state afterwards, and
+so are for practical purposes "`the same`". +
++
+However, packfile generation is not formally guaranteed to be deterministic
+across git implementations or -versions, namely the order of entries in the
+file. For long-term storage, patch bundles are thus referred to by their
+<<BUNDLE_HASH>>.
+
+- Or, the exact file contents are equal
++
+When downloading bundles from untrusted sources, or from content-addressable
+storage, the checksum of the exact contents should be verified. This information
+is preserved as the <<BUNDLE_CHECKSUM>>.
+
+
+== Drops
+
+A _drop_ (as in _-box_ or _deadletter-_) is a hash-linked log which timestamps
+the reception of <<Patches,patches>>. In git terms, it is a history of
+(single-parent) commits, where integrity is ensured through git itself. To add
+authenticity, drops carry additional <<drop-metadata,metadata>> which is secured
+using a scheme based on <<TUF,The Update Framework Specification (TUF)>>.
+
+A drop also carries all <<Identities,identities>> needed to verify cryptographic
+signatures on metadata, <<Patches,patches>>, and optionally git
+commits{fn-commit-signing}, thus forming a PKI. Identities are themselves
+updated through patches.
+
+Importantly, the drop history does _not_ carry the patch payload itself. Patch
+bundles are kept and redistributed as received, and so can make heavy use of
+content distribution networks. At the same time, the drop history itself remains
+fairly small even if not delta-encoded. Together, this allows to operate even
+public drops on relatively constrained hardware.
+
+A drop is a strictly _local-first_ concept -- the drop history may never leave a
+single machine. In order to be able to accept patch proposals, however, a drop
+may make itself _externally addressable_, for example by exposing an HTTP API
+(see <<HTTP API>>).
+
+It is important to note that drop histories, even if they logically describe the
+same project, are not in principle required to converge. In git terms, this
+means that two drop histories may refer to the same set of patch bundles, but
+differ in the ordering of the commits (or other parameters which change the
+commit identity). Conversely, the respective sets of patch bundles may also be
+distinct, to the extent permitted by the connectivity requirement (see
+<<record-patch>>).
+
+An exception to this rule are *mirrors*, whose network addresses are published
+as part of the drop <<mirrors-json,metadata>>: the addresses listed therein are
+interchangeable, i.e. obtaining the drop history from any of them MUST result in
+the exact same state.
+
+[#aggregation]
+Instead of or in addition to exposing a public means of patch submission, drops
+may *aggregate* patches from other drops. That is, they may follow other drops
+just like a normal git remote, and apply patch records to their own history. By
+specifying <<alternates-json,alternates>> in the metadata, a drop promises to
+aggregate submissions from those locations. Aggregation is, however, not limited
+to published alternates: for example, a contributor may maintain their own
+private drop recording only the patches created by that contributor. Another
+drop for the same project may be made aware of a mirror URL for that private
+drop, and update itself from there periodically.
+
+[#drop-metadata]
+=== Metadata
+
+The authenticity of drops is ensured by a trust delegation scheme derived from
+<<TUF>>. There, a role-based threshold signature scheme is used to prove
+authenticity of updates to certain parts of an abstract "`repository`",
+including the metadata containing the trust delegations itself.
+
+For our purposes, some of the properties of a "`repository`" are upheld by git
+itself, while other roles are specific to _it_. There are four roles defined for
+_it_ drops:
+
+[.lst]
+    . Root role
+    . Snapshot role
+    . Mirrors role
+    . Branch roles
+
+Like in TUF, the mirrors role is optional. Also like TUF, we note that it is
+possible to instantiate a drop with a single <<Identities,identity>> (and even
+with a single key) -- which is not considered to be secure, but may be
+convenient in some scenarios.
+
+Root role::
+
+The root role delegates trust to specific <<Identities,identities>> trusted for
+all other roles, by means of being eligible to sign updates to the
+<<drop-json,drop.json>> metadata file.
++
+Delegating to identities instead of directly to keys permits to rotate the
+respective keys independently, thus weakening the requirement for air-gapped
+storage of all root keys.
+
+Snapshot role::
+
+The snapshot role permits signing commits onto the drop history.
++
+This applies mainly to new <<record-json,records>>, but note that it may also
+include updates to the metadata files, yet does not render those updates valid
+as their signatures are verified independently.
++
+The snapshot role is typically granted to machine users on public drop servers.
++
+Snapshot signatures are regular git commit signatures. Pending a practical
+method to obtain multiple signatures on a git commit, `*threshold*` values other
+than `1` are not currently supported.
+
+Mirrors role::
+
+The mirrors role permits signing the <<mirrors-json,mirrors.json>> and
+<<alternates-json,alternates.json>> metadata files.
++
+This role is optional, as not all drop operators may find it practical or useful
+to publish signed mirrors/alternates lists.
+
+[[branch-roles]]Branch roles::
+
+Branch roles are keyed by concrete reference names, which the listed
+<<Identities,identities>> are trusted to update (see <<Mergepoints>>).
+
+
+The metadata files establishing the scheme are described in the following
+sections.
+
+[#drop-json]
+==== `drop.json`
+
+The `drop.json` metadata file is signed by the root role and indicates which
+<<Identities,identities>> are authorised for all roles, including the root role
+itself.
+
+The `*signed*` portion the `drop.json` metadata file is defined as follows:
+
+[source,subs="+macros"]
+----
+{
+    "_type": "eagain.io/it/drop",
+    "spec_version": <<SPEC_VERSION>>,
+    "description": <<DESCRIPTION>>,
+    "prev": <<CONTENT_HASH>> | null,
+    "roles": {
+        "root": <<ROLE>>,
+        "snapshot": <<ROLE>>,
+        "mirrors": <<ROLE>>,
+        "branches": {
+            <<REFNAME>>: <<ANNOTATED_ROLE>>,
+            ...
+        }
+    },
+    "custom": <<CUSTOM>>
+}
+----
+
+[[ANNOTATED_ROLE]]ANNOTATED_ROLE::
+    Like a <<ROLE>>, but with an additional field `*description*` of type
+    <<DESCRIPTION>>.
++
+[source#annotated-role,subs="+macros"]
+----
+{
+    "ids": [
+        <<IDENTITY_ID>>,
+        ...
+    ],
+    "threshold": <<THRESHOLD>>,
+    "description": <<DESCRIPTION>>
+}
+----
+
+[[CUSTOM]]CUSTOM::
+    An arbitrary JSON object carrying user-defined data. To avoid conflicts, it
+    is RECOMMENDED to key custom objects by a URL-like identifier. For example:
++
+[source#example-custom]
+----
+{
+    "custom": {
+        "eagain.io/it/emojicoin": {
+            "insert-here": "lol1u2vgx76adff"
+        }
+    }
+}
+----
+
+[[DESCRIPTION]]DESCRIPTION::
+    A UTF-8 string with a maximum length of 128 bytes, i.e. a
+    <<VARCHAR,VARCHAR(128)>>.
+
+[[REFNAME]]REFNAME::
+    A full git refname (i.e. starting with "`refs/`"), well-formed as per
+    <<git-check-ref-format>>.
+
+[[ROLE]]ROLE::
+    Dictionary of a set of <<IDENTITY_ID,identity ids>> assigned to that role,
+    paired with a <<THRESHOLD,threshold>>. I.e.:
++
+[source#role,subs="+macros"]
+----
+{
+    "ids": [
+        <<IDENTITY_ID>>,
+        ...
+    ],
+    "threshold": <<THRESHOLD>>
+}
+----
++
+Example:
++
+[source#example-role,json]
+----
+{
+    "ids": [
+        "671e27d4cce92f747106c7da90bcc2be7072909afa304d008eb8ecbfdebfbfe2"
+    ],
+    "threshold": 1
+}
+----
+
+[#mirrors-json]
+==== `mirrors.json`
+
+The `mirrors.json` file is signed by the mirrors role. It describes known
+network addresses of read-only copies of the drop, believed to be kept in-sync
+with the drop within a reasonable time window by its operators.
+
+The `*signed*` portion of the `mirrors.json` file is defined as follows:
+
+[source,subs="+macros"]
+----
+{
+    "_type": "eagain.io/it/mirrors",
+    "spec_version": <<SPEC_VERSION>>,
+    "mirrors": [
+        <<MIRROR>>,
+        ...
+    ],
+    "expires": <<DATETIME>> | null
+}
+----
+
+[[MIRROR]]MIRROR::
+    A dictionary describing a mirror.
++
+[source#mirror,subs="+macros"]
+----
+{
+    "url": <<URL>>,
+    "kind": <<MIRROR_KIND>>,
+    "custom": <<CUSTOM>>
+}
+----
+
+[[MIRROR_KIND]]MIRROR_KIND::
+    Hint at what retrieval method is offered by the mirror. Unknown values MUST
+    be accepted during parsing and signature verification. Defined values are:
+
+[.lst]
+    - *bundled*: the mirror is expected to serve patch bundles at the well-known
+      <<http-fetch-bundle,HTTP>> endpoint relative to `*url*`, if `*url*`
+      denotes a HTTP URL
+    - *packed*: the mirror is a plain git server, but the client may reify
+      bundles by requesting the appropriate objects over the regular git network
+      protocol
+    - *sparse*: the mirror does not host bundle data at all, only the drop
+      history. This can be useful in constrained environments such as
+      peer-to-peer storage if (and only if) the <<record-json,record.json>>
+      entries specify stable bundle URIs.
+
+
+[#alternates-json]
+==== `alternates.json`
+
+The `alternates.json` file is signed by the mirrors role. It describes known
+network addresses of writeable (e.g. via <<HTTP API,HTTP>>) drops where
+<<Patches,patches>> pertaining the same project may be submitted. The method of
+submission is described by the alternate's URL. A drop publishing an
+`alternates.json` file implicitly promises to <<aggregation,aggregate>> patches
+from the alternates listed, although it is free to do so only selectively.
+
+The `*signed*` portion of the `alternates.json` file is defined as follows:
+
+[source,subs="+macros"]
+----
+{
+    "_type": "eagain.io/it/alternates",
+    "spec_version": <<SPEC_VERSION>>,
+    "alternates": [
+        <<URL>>,
+        ...
+    ],
+    "custom": <<CUSTOM>>,
+    "expires": <<DATETIME>> | null
+}
+----
+
+[#drop-verification]
+=== Verification
+
+To verify a drop, the <<drop-json,drop.json>> metadata file must be verified
+first:
+
+[.lst]
+    . From the latest known commit of the drop history, load the
+      <<drop-json,drop.json>> file
+
+    . For each <<IDENTITY_ID,identity id>> in the `*root*` role of the file,
+      resolve the corresponding <<Identities,identity>> and
+      <<id-verification,verify>> it{fn-resolve-id}
+
+    . Verify that no key is being referenced by more than one identity
+
+    . Verify that the <<drop-json,drop.json>> file is signed by a threshold of
+      identities as specified in the `*threshold*` attribute of the `*root*`
+      role. Signatures by multiple keys from the same identity are allowed, but
+      don't count toward the threshold.
+
+    . If `*prev*` is not `null`, load the corresponding previous revision of the
+      metadata
+
+    . Verify that the threshold specified in the _previous_ revision is met on
+      the _current_ revision, loading and verifying additional identities as
+      needed
+
+    . Repeat steps 5. and 6. until `*prev*` is `null`
+
+Having obtained a verified <<drop-json,drop.json>> metadata file, it can now be
+verified that the head commit of the drop history is signed by a key belonging
+to an identity which is assigned the `*snapshot*` role.
+
+If a <<mirrors-json,mirrors.json>> and/or <<alternates-json,alternates.json>> is
+present in the head commit's tree, it should be verified as follows:
+
+[.lst]
+    . Load the metadata file
+
+    . If the `*expires*` attribute is not `null`, check that the specified
+      <<DATETIME>> does not lie in the past
+
+    . For each <<IDENTITY_ID,identity id>> in the `*mirrors*` role of the
+      <<drop-json,drop.json>> file, resolve the corresponding
+      <<Identities,identity>> and <<id-verification,verify>> it{fn-resolve-id}
+
+    . Verify that the metadata file is signed by a threshold of identities as
+      specified in the `*threshold*` attribute of the `*mirrors*` role.
+      Signatures by multiple keys from the same identity are allowed, but don't
+      count toward the threshold.
+
+Verification of mirror- and alternates-lists MAY be deferred until they are
+actually used. Failure to verify <<mirrors-json,mirrors.json>> or
+<<alternates-json,alternates.json>> does not render the drop metadata invalid.
+
+[#history-repr]
+=== History representation
+
+A drop history is stored as a git commit history. Initially, it contains only
+the metadata, organised in a tree with the following layout:
+
+.Drop metadata tree
+[source#drop-tree,subs="+macros"]
+----
+.
+|-- <<drop-json,drop.json>>
+|-- <<mirrors-json,mirrors.json>>
+|-- <<alternates-json,alternates.json>>
+`-- ids
+    |-- <<IDENTITY_ID,identity-id>>
+    |   `-- <<id-json,id.json>>
+    `-- ...
+----
+
+[NOTE]
+====
+In this document, tree entries are ordered for legibility, which is not
+necessarily how they are ordered by git.
+====
+
+In <<drop-tree>>, the `mirrors.json` and `alternates.json` files are
+optional. The `ids` hierarchy contains at least all <<Identities,identities>>
+needed to verify the metadata files, where the `id.json` file represents the
+most recent revision of the identity. It is up to the implementation how to make
+previous revisions available, although most are expected to opt for a "`folded`"
+representation where previous revisions are stored as files in a subdirectory.
+
+A commit which updates metadata files may carry a free-form commit message. Data
+created by a previous patch commit SHOULD be removed from the tree.
+
+To <<record-patch,record a patch>>, the <<record-json,record.json>> is written
+to the tree adjacent to the other metadata files. If the patch contains identity
+updates, the `ids` subtree is updated accordingly.
+
+The patch <<Topics,topic>> is written as a <<git-interpret-trailers,trailer>>
+keyed "`Re:`", as shown in <<example-topic-commit>>. This allows to collect
+patches for a particular topic from the drop history without having to access
+objects deeper than the commit.
+
+
+.Simplified topic commit
+[#example-topic-commit]
+----
+commit ccd1fd5736bed6fb6342e34c9d8cbc2b9db7f326
+Author: Kim Altintop <kim@eagain.io>
+Date:   Mon Dec 12 10:47:32 2022 +0100
+
+    Re: 1fdc53e27b01b440839ff1b6c14ef81c3d63d0f2b39aae8fb4abd0b565ea0b10
+----
+
+Lastly, the <<BUNDLE_HEADS>> (cf. <<patch-equivalence>>) are written to a file
+`heads` adjacent to the `record.json` file in the tree. Provided appropriate
+atomicity measures, this provides a reasonably efficient way to determine if a
+patch has been received before by simply probing the object database for
+existence of the corresponding <<BLOB_HASH>>.
+
+==== Location-independent storage
+
+Since the drop history only stores metadata, it should be suitable for
+location-independent storage inheriting some of git's data model, e.g. <<IPFS>>,
+<<Hypercore>>, or <<SSB>>. Those systems come with their own limitations,
+perhaps the most severe one in our context being the lack of a reliable and
+efficient way to propagate contributions from _unknown_ identities back to the
+root drop. Thus, exact mappings are deferred to a future revision of this
+document.
+
+We note, however, that distributing <<gitformat-bundle,git bundle>> snapshots of
+the drop history itself over protocols which support some form of name
+resolution (such as <<IPNS>>) may present an attractive bandwidth-sharing
+mechanism.
+
+
+[#record-patch]
+=== Recording patches
+
+Once a patch has passed <<drop-validation,validation>>, its reception is
+recorded in the drop history as a file containing metadata about the patch. The
+file's schema may be extended over time, where the currently defined properties
+are:
+
+.`record.json`
+[source#record-json,subs="+macros"]
+----
+{
+    "bundle": {
+        "len": <<BUNDLE_SIZE>>,
+        "hash": <<BUNDLE_HASH>>,
+        "checksum": <<BUNDLE_CHECKSUM>>,
+        "prerequisites": [
+            <<OBJECT_ID>>,
+            ...
+        ],
+        "references": {
+            <<REFNAME>>: <<OBJECT_ID>>,
+            ...
+        },
+        "encryption": "age" | "gpg",
+        "uris": [
+            <<URL>>,
+            ...
+        ]
+    },
+    "signature": {
+        "signer": <<CONTENT_HASH>>,
+        "signature": <<SIGNATURE>>,
+    }
+}
+----
+
+[[BUNDLE_SIZE]]BUNDLE_SIZE::
+    Size in bytes of the bundle file as received.
+
+[[BUNDLE_HASH]]BUNDLE_HASH::
+    SHA-256 hash over the sorted set of object ids (in bytes) referenced by the
+    bundles, i.e. both the prerequisites and reference heads.
+
+[[BUNDLE_CHECKSUM]]BUNDLE_CHECKSUM::
+    SHA-256 hash over the bundle file as received.
+
+[[BUNDLE_SIGNATURE]]BUNDLE_SIGNATURE::
+    Signature over the <<BUNDLE_HEADS>>, in hexadecimal.
+
+[[BUNDLE_HEADS]]BUNDLE_HEADS::
+    SHA-256 hash over the sorted set of object ids (in bytes) or the reference
+    heads of the bundle (i.e. without the prerequisites).
+
+The `*signature*` field captures the signature made by the submitter of the
+patch. Multiple signatures may be supported in a future revision of this
+document.
+
+The `*uris*` field enumerates alternate network addresses from which the bundle
+file may be downloaded. Since the recorded information is immutable, this is
+mainly intended for content-based addresses, such as IPFS CIDs.
+
+Additionally, the drop will want to record the hashed reference heads in an
+efficiently retrievable form, such that it can be quickly determined if a patch
+has been received before (see <<patch-equivalence>>, <<history-repr>>).
+Similarly for the patch <<Topics,topic>>.
+
+
+[#drop-validation]
+==== Validation
+
+Accepting a patch for inclusion in the drop history is subject to validation
+rules, some of which depend on preferences or policies. A public drop server
+will want to apply stricter rules before accepting a patch than a user who is
+applying a patch to a local (unpublished) drop history.
+
+The *mandatory* validations are:
+
+[.lst]
+    . The bundle file MUST be available locally before creating a log entry
+    . The bundle MUST be connected, i.e. its prerequisite objects must be
+      present in bundles received prior to the one under consideration
+    . The bundle MUST NOT have been received before (cf. <<patch-equivalence>>)
+    . The bundle MUST conform to the conventions specified in <<Patches>>
+    . The bundle MUST be signed and the signer's (i.e. submitter's)
+      <<Identities,identity>> resolvable, either from the drop state or the
+      bundle contents (or both)
+    . If the bundle contains identity updates, they MUST pass
+      <<id-verification,verification>> and MUST NOT diverge from their
+      previously recorded history (if any)
+
+[NOTE]
+====
+Validation 5. entails that a patch submission message must carry the
+<<CONTENT_HASH>> of the submitter's identity head revision.
+====
+
+Additional RECOMMENDED validations include:
+
+[.lst]
+    - restricting the size in bytes of the patch bundle
+    - restricting the number of references a bundle can convey
+    - restricting the number of commits, or total number of objects a bundle can
+      contain
+    - rejecting patches whose <<Topics,topic>> is not properly signed by the
+      submitter, does not cleanly apply to a merged history of previously
+      received patches on the same topic, or contains otherwise invalid data
+
+Beyond that, a drop may also decide to reject a patch if it is encrypted, or if
+its contents do not pass content analysis proper (e.g. Bayesian filtering).
+
+
+[#snapshots]
+=== Snapshots
+
+Over time, a drop will accumulate many small patch bundles. Repacking them into
+larger bundles is likely to reclaim storage space by means of offering more
+opportunities for delta compression. It can also be beneficial for data
+synchronisation (especially non-incremental) to avoid too many network
+roundtrips.
+
+In principle, a drop could employ a dynamic repacking scheme, and either serve
+larger than requested data when individual bundles are requested, or offer a way
+to dynamically discover snapshotted alternatives via the bundle-uri negotiation
+mechanism (see <<http-fetch-bundle>>). This would, however, preclude drops which
+delegate bundle storage entirely (such as packed or sparse
+<<MIRROR_KIND,mirrors>>) from benefiting from this optimisation. Therefore, we
+define a convention for publishing snapshots as patches on the drop itself.
+
+A snapshot is a <<Patches,patch>> posted to the well-known topic
+`SHA256("snapshots")`, i.e.:
+
+[source]
+----
+2b36a6e663158ffd942c174de74dbe163bfdb1b18f6d0ffc647e00647abca9bb
+----
+
+A snapshot bundle may either capture the entire history of the drop, or depend
+on an earlier snapshot. The bundle references capture all references of the
+patch bundles received prior to the snapshot, up until the previous snapshot if
+the snapshot is incremental. In order to be unique within the snapshot bundle,
+the patch bundle references are rewritten as follows:
+
+[.lst]
+    . Strip the `refs/` prefix
+    . Prepend `refs/it/bundles/<<BUNDLE_HEADS>>/`
+
+For example:
+
+[source#example-snapshot-refs]
+----
+refs/it/bundles/107e80b2287bc763d7a64bee9bc4401e12778c55925265255d4f2a38296262b8/heads/main 77ce512aa813988bdca54fa2ba5754f3a46c71f3
+refs/it/bundles/107e80b2287bc763d7a64bee9bc4401e12778c55925265255d4f2a38296262b8/it/topics/c44c20434bfdaa0384b67d48d6c3bb36d755b87576027671f606c404b09d9774 65cdd5234e310efc1cb0afbc7de0a2786e6dd582
+----
+
+The payload of the <<Topics,topic>> entry associated with a snapshot is not
+defined normatively. It is RECOMMENDED to use a <<message-topic,message based
+topic>>, where a payload schema could be:
+
+[source#snapshot-topic-payload,subs="+macros"]
+----
+{
+    "_type": "eagain.io/it/notes/checkpoint",
+    "kind": "snapshot",
+    "refs": {
+        <<REFNAME>>: <<OBJECT_ID>>,
+        ...
+    }
+}
+----
+
+Taking a snapshot implies privileged access to the drop repository, and can only
+be submitted by the snapshot role.
+
+After publishing a snapshot, a drop MAY prune patch bundles recorded prior to
+the snapshot, possibly after a grace period (for example, by only pruning
+bundles older than the N-1st snapshot). When synchronising with a drop, clients
+which encounter a snapshot record should thus prefer fetching only snapshots
+from this point on in the drop history.
+
+
+[#mergepoints]
+=== Mergepoints
+
+It is often useful for a drop to convey cryptographically verifiable reference
+points for contributors to base source code changes on, i.e. long-running
+branches.
+
+While the process of agreeing on what changes are to be finalised into such
+branches can vary widely between projects, and could even involve the evaluation
+of <<crdt-topic,CRDT state>>, the final statement can be reduced to restricting
+the set of allowed signers of a patch bundle (which updates a certain set of
+branches). This is what the <<branch-roles,branch roles>> in the
+<<drop-json,drop.json>> metadata file are for: they make certain
+<<Identities,identities>> eligible for submitting _mergepoints_ affecting named
+long-running branches.
+
+A mergepoint is a <<Patches,patch>> posted to the well-known topic
+`SHA256("merges")`, i.e.:
+
+[source]
+----
+c44c20434bfdaa0384b67d48d6c3bb36d755b87576027671f606c404b09d9774
+----
+
+A mergepoint bundle may contain one or more references matching exactly the
+names specified in the drop's <<branch-roles,branch roles>>, and MUST only be
+accepted if the submitter(s) identities are allowed as per the role definition.
+
+As with <<snapshots,snapshots>>, the topic payload is not defined normatively.
+It is RECOMMENDED to use <<message-topic,message based topic>>, where a payload
+schema could be:
+
+[source#mergepoint-topic-payload,subs="+macros"]
+----
+{
+    "_type": "eagain.io/it/notes/checkpoint",
+    "kind": "merge",
+    "refs: {
+        <<REFNAME>>: <<OBJECT_ID>>,
+        ...
+    }
+}
+----
+
+Upon encountering a mergepoint properly signed by the applicable branch roles, a
+client may update the targets of a local representation of the mergepoint
+references _iff_ the local targets are in the ancestry path of the mergepoint
+targets.
+
+
+=== HTTP API
+
+<<Drops,Drops>> MAY expose an HTTP API for accepting and serving patch bundles.
+Drops listed as alternates in the drop <<alternates-json,metadata>> MUST conform
+to this API (endpoint paths are interpreted as relative to the alternate URL).
+The defined endpoints of the API are as follows:
+
+[#http-fetch-bundle]
+==== Fetching patch bundles
+
+---
+
+[source,subs="+macros"]
+----
+GET /bundles/<<BUNDLE_HASH,bundle-hash>>[.bundle|.uris]
+----
+
+---
+
+Without a file extension suffix, this endpoint conforms to the git
+<<bundle-uri>> specification: the server may either respond by sending the
+bundle file identified by <<BUNDLE_HASH,bundle-hash>>, or a bundle list.
+
+When responding with a bundle list:
+
+[.lst]
+    - `mode` MUST be `any`
+    - `<id>` segments MUST be treated as opaque by the client
+    - entries specifying a `filter` MUST be ignored by the client
+
+In addition to regular `uri` values (relative, `http://`, `https://`), `ipfs://`
+URLs are accepted. If encountered, a client MAY rewrite them to
+<<IPFS-GATEWAY,gateway URLs>> to fetch the bundle from.
+
+By specifying the `.bundle` suffix, a client instructs the server to either
+respond with the bundle file, or a 404 status, but never with a bundle list.
+Correspondingly, by specifying `.uris`, the server MUST respond with a bundle
+list, or a 404 status, but never with a bundle file.
+
+.Example bundle list
+[source]
+----
+[bundle]
+    version = 1
+    mode = any
+    heuristic = creationToken
+
+[bundle "8aea1a1c20b09ed9ad4737adc6319203d65a0026ac86873f84f7961bd42f132c"]
+    uri = /bundles/6c4d3d4e4db8e37c698f891e94780c63e1b94f87c67925cd30163915c7d7923e.bundle
+
+[bundle "816dc1231cb1b82a91144ebb9e325c3655f4b4da30f806a84fa86fdb06ca9c04"]
+    uri = https://it.example.com/bundles/6c4d3d4e4db8e37c698f891e94780c63e1b94f87c67925cd30163915c7d7923e.bundle
+    creationToken = 1670838467
+
+[bundle "f4ecc80c9339ecdbc2a8f4c0d19e990f8ee9631e6b7f3e044b86c35fe69505d3"]
+    uri = ipfs://QmVTw4vVFWkPBN6ZT7To4BHoNBfaBNjVJ17wK15tci6bn1
+    creationToken = 1670839391
+----
+
+
+[#http-submit-patch]
+==== Submitting patches
+
+---
+
+[source,subs="+macros"]
+----
+POST /patches
+<<HEADER_SIGNATURE>>
+----
+
+---
+[[HEADER_SIGNATURE]]HEADER_SIGNATURE::
+    A <<BUNDLE_SIGNATURE>> and corresponding identity <<CONTENT_HASH>>, encoded
+    suitable for use as a HTTP header value:
++
+[source,subs="+macros"]
+----
+X-it-signature: s1={<<BLOB_HASH>>}; s2={<<BLOB_HASH>>}; sd={<<BUNDLE_SIGNATURE>>}
+----
+
+The body of this request is a bundle file. The bundle signature is transmitted
+as a HTTP header, allowing for the bundle file to be streamed directly from
+disk.
+
+Once the drop server has received the request body, it attempts to
+<<record-patch,record the patch>>, and responds with the corresponding
+<<record-json,record.json>> document, or an error.
+
+Optionally, the server MAY accept a request of the form:
+
+---
+
+[source#request-pull,subs="+macros"]
+----
+POST /patches/request-pull
+Content-Type: application/x-www-form-urlencoded
+<<HEADER_SIGNATURE>>
+
+url=<<URL>>
+----
+
+---
+
+If accepted, the server attempts to fetch the bundle file from the URL given in
+the form field before continuing as if the bundle was submitted directly in the
+request body. Otherwise, the server responds with an error code in the 4xx range
+to indicate that this method of submission is not supported.
+
+
+== Future work
+
+We found that git bundles are a simple yet effective container format. They are,
+however, not extensible: git, being the reference implementation, rejects
+bundles whose header does not exactly conform to the specified format. While
+compatibility with upstream git was a design goal for the current iteration of
+_it_, we may want to evolve the format independently in the future, e.g. by
+embedding cryptographic signatures directly in the file.
+
+We have deliberately not mandated strict schema checking for topic payloads
+respectively CRDT objects, although we acknowledge that interoperability will
+eventually demand for some method to be devised. Since the design space is quite
+large -- ranging from static schema definitions to runtime evaluation of a
+dynamic interpreter -- this would have been well beyond the scope of the
+current specification.
+
+---
+
+[discrete]
+== Acknowledgements
+
+The author would like to thank Alex Good for a perpetual supply of ideas worth
+considering.
+
+---
+
+[discrete]
+== Copyright notice
+
+Copyright © 2022-2023 Kim Altintop. This work is made available under the
+<<CC-BY-SA-4,Creative Commons Attribution 4.0 International License>>. To the
+extent portions of it are incorporated into source code, such portions in the
+source code are licensed under either the <<Apache-2,Apache License 2.0>> or the
+<<MIT,MIT license>> at your option.
+
+
+[bibliography]
+== References
+
+// IETF
+* [[[RFC2119]]]: https://datatracker.ietf.org/doc/html/rfc2119
+* [[[RFC3339]]]: https://datatracker.ietf.org/doc/html/rfc3339#section-5.6
+* [[[RFC4253]]]: https://datatracker.ietf.org/doc/html/rfc4253
+* [[[RFC5656]]]: https://datatracker.ietf.org/doc/html/rfc5656
+* [[[RFC8174]]]: https://datatracker.ietf.org/doc/html/rfc8174
+* [[[RFC8709]]]: https://datatracker.ietf.org/doc/html/rfc8709
+* [[[ssh-agent]]]: https://datatracker.ietf.org/doc/html/draft-miller-ssh-agent
+
+// Other specs
+* [[[automerge-change]]]: https://alexjg.github.io/automerge-storage-docs/#change-reference
+* [[[Canonical-JSON]]]: http://wiki.laptop.org/go/Canonical_JSON
+* [[[DID]]]: https://www.w3.org/TR/did-core
+* [[[semver]]]: https://semver.org
+* [[[TUF]]]: https://theupdateframework.github.io/specification/latest
+* [[[WHATWG-URL]]]: https://url.spec.whatwg.org
+
+// Licenses
+* [[[Apache-2]]]: https://www.apache.org/licenses/LICENSE-2.0
+* [[[CC-BY-SA-4]]]: https://creativecommons.org/licenses/by/4.0
+* [[[MIT]]]: https://spdx.org/licenses/MIT.html
+
+// git
+* [[[bundle-uri]]]: https://git-scm.com/docs/bundle-uri
+* [[[git-check-ref-format]]]: https://git-scm.com/docs/git-check-ref-format
+* [[[git-format-patch]]]: https://git-scm.com/docs/git-format-patch
+* [[[git-hash-object]]]: https://git-scm.com/docs/git-hash-object
+* [[[git-interpret-trailers]]]: https://git-scm.com/docs/git-interpret-trailers
+* [[[git]]]: https://git-scm.com
+* [[[gitformat-bundle]]]: https://git-scm.com/docs/gitformat-bundle
+* [[[gitglossary]]]: https://git-scm.com/docs/gitglossary
+
+// Patch Theory
+* [[[Darcs]]]: https://en.wikibooks.org/wiki/Understanding_Darcs/Patch_theory
+* [[[CaPT]]]: https://arxiv.org/abs/1311.3903
+* [[[HoPT]]]: https://www.cambridge.org/core/journals/journal-of-functional-programming/article/homotopical-patch-theory/42AD8BB8A91688BCAC16FD4D6A2C3FE7
+
+// Misc
+* [[[age]]]: https://age-encryption.org/v1
+* [[[Automerge]]]: https://automerge.org
+* [[[CRDT]]]: https://en.wikipedia.org/wiki/Conflict-free_replicated_data_type
+* [[[Hypercore]]]: https://hypercore-protocol.org
+* [[[IPFS-GATEWAY]]]: https://docs.ipfs.tech/concepts/ipfs-gateway
+* [[[IPFS]]]: https://ipfs.tech
+* [[[IPNS]]]: https://docs.ipfs.tech/concepts/ipns
+* [[[local-first]]]: https://www.inkandswitch.com/local-first/
+* [[[OpenSSH]]]: https://www.openssh.com
+* [[[SSB]]]: https://scuttlebutt.nz
diff --git a/LICENSES/Apache-2.0 b/LICENSES/Apache-2.0
new file mode 100644
index 0000000..d645695
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+++ b/LICENSES/Apache-2.0
@@ -0,0 +1,202 @@
+
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new file mode 100644
index 0000000..7d4f96c
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+          extent necessary to allow You to exercise the Licensed
+          Rights, but not otherwise.
+
+       2. Patent and trademark rights are not licensed under this
+          Public License.
+
+       3. To the extent possible, the Licensor waives any right to
+          collect royalties from You for the exercise of the Licensed
+          Rights, whether directly or through a collecting society
+          under any voluntary or waivable statutory or compulsory
+          licensing scheme. In all other cases the Licensor expressly
+          reserves any right to collect such royalties.
+
+
+Section 3 -- License Conditions.
+
+Your exercise of the Licensed Rights is expressly made subject to the
+following conditions.
+
+  a. Attribution.
+
+       1. If You Share the Licensed Material (including in modified
+          form), You must:
+
+            a. retain the following if it is supplied by the Licensor
+               with the Licensed Material:
+
+                 i. identification of the creator(s) of the Licensed
+                    Material and any others designated to receive
+                    attribution, in any reasonable manner requested by
+                    the Licensor (including by pseudonym if
+                    designated);
+
+                ii. a copyright notice;
+
+               iii. a notice that refers to this Public License;
+
+                iv. a notice that refers to the disclaimer of
+                    warranties;
+
+                 v. a URI or hyperlink to the Licensed Material to the
+                    extent reasonably practicable;
+
+            b. indicate if You modified the Licensed Material and
+               retain an indication of any previous modifications; and
+
+            c. indicate the Licensed Material is licensed under this
+               Public License, and include the text of, or the URI or
+               hyperlink to, this Public License.
+
+       2. You may satisfy the conditions in Section 3(a)(1) in any
+          reasonable manner based on the medium, means, and context in
+          which You Share the Licensed Material. For example, it may be
+          reasonable to satisfy the conditions by providing a URI or
+          hyperlink to a resource that includes the required
+          information.
+
+       3. If requested by the Licensor, You must remove any of the
+          information required by Section 3(a)(1)(A) to the extent
+          reasonably practicable.
+
+  b. ShareAlike.
+
+     In addition to the conditions in Section 3(a), if You Share
+     Adapted Material You produce, the following conditions also apply.
+
+       1. The Adapter's License You apply must be a Creative Commons
+          license with the same License Elements, this version or
+          later, or a BY-SA Compatible License.
+
+       2. You must include the text of, or the URI or hyperlink to, the
+          Adapter's License You apply. You may satisfy this condition
+          in any reasonable manner based on the medium, means, and
+          context in which You Share Adapted Material.
+
+       3. You may not offer or impose any additional or different terms
+          or conditions on, or apply any Effective Technological
+          Measures to, Adapted Material that restrict exercise of the
+          rights granted under the Adapter's License You apply.
+
+
+Section 4 -- Sui Generis Database Rights.
+
+Where the Licensed Rights include Sui Generis Database Rights that
+apply to Your use of the Licensed Material:
+
+  a. for the avoidance of doubt, Section 2(a)(1) grants You the right
+     to extract, reuse, reproduce, and Share all or a substantial
+     portion of the contents of the database;
+
+  b. if You include all or a substantial portion of the database
+     contents in a database in which You have Sui Generis Database
+     Rights, then the database in which You have Sui Generis Database
+     Rights (but not its individual contents) is Adapted Material,
+     including for purposes of Section 3(b); and
+
+  c. You must comply with the conditions in Section 3(a) if You Share
+     all or a substantial portion of the contents of the database.
+
+For the avoidance of doubt, this Section 4 supplements and does not
+replace Your obligations under this Public License where the Licensed
+Rights include other Copyright and Similar Rights.
+
+
+Section 5 -- Disclaimer of Warranties and Limitation of Liability.
+
+  a. UNLESS OTHERWISE SEPARATELY UNDERTAKEN BY THE LICENSOR, TO THE
+     EXTENT POSSIBLE, THE LICENSOR OFFERS THE LICENSED MATERIAL AS-IS
+     AND AS-AVAILABLE, AND MAKES NO REPRESENTATIONS OR WARRANTIES OF
+     ANY KIND CONCERNING THE LICENSED MATERIAL, WHETHER EXPRESS,
+     IMPLIED, STATUTORY, OR OTHER. THIS INCLUDES, WITHOUT LIMITATION,
+     WARRANTIES OF TITLE, MERCHANTABILITY, FITNESS FOR A PARTICULAR
+     PURPOSE, NON-INFRINGEMENT, ABSENCE OF LATENT OR OTHER DEFECTS,
+     ACCURACY, OR THE PRESENCE OR ABSENCE OF ERRORS, WHETHER OR NOT
+     KNOWN OR DISCOVERABLE. WHERE DISCLAIMERS OF WARRANTIES ARE NOT
+     ALLOWED IN FULL OR IN PART, THIS DISCLAIMER MAY NOT APPLY TO YOU.
+
+  b. TO THE EXTENT POSSIBLE, IN NO EVENT WILL THE LICENSOR BE LIABLE
+     TO YOU ON ANY LEGAL THEORY (INCLUDING, WITHOUT LIMITATION,
+     NEGLIGENCE) OR OTHERWISE FOR ANY DIRECT, SPECIAL, INDIRECT,
+     INCIDENTAL, CONSEQUENTIAL, PUNITIVE, EXEMPLARY, OR OTHER LOSSES,
+     COSTS, EXPENSES, OR DAMAGES ARISING OUT OF THIS PUBLIC LICENSE OR
+     USE OF THE LICENSED MATERIAL, EVEN IF THE LICENSOR HAS BEEN
+     ADVISED OF THE POSSIBILITY OF SUCH LOSSES, COSTS, EXPENSES, OR
+     DAMAGES. WHERE A LIMITATION OF LIABILITY IS NOT ALLOWED IN FULL OR
+     IN PART, THIS LIMITATION MAY NOT APPLY TO YOU.
+
+  c. The disclaimer of warranties and limitation of liability provided
+     above shall be interpreted in a manner that, to the extent
+     possible, most closely approximates an absolute disclaimer and
+     waiver of all liability.
+
+
+Section 6 -- Term and Termination.
+
+  a. This Public License applies for the term of the Copyright and
+     Similar Rights licensed here. However, if You fail to comply with
+     this Public License, then Your rights under this Public License
+     terminate automatically.
+
+  b. Where Your right to use the Licensed Material has terminated under
+     Section 6(a), it reinstates:
+
+       1. automatically as of the date the violation is cured, provided
+          it is cured within 30 days of Your discovery of the
+          violation; or
+
+       2. upon express reinstatement by the Licensor.
+
+     For the avoidance of doubt, this Section 6(b) does not affect any
+     right the Licensor may have to seek remedies for Your violations
+     of this Public License.
+
+  c. For the avoidance of doubt, the Licensor may also offer the
+     Licensed Material under separate terms or conditions or stop
+     distributing the Licensed Material at any time; however, doing so
+     will not terminate this Public License.
+
+  d. Sections 1, 5, 6, 7, and 8 survive termination of this Public
+     License.
+
+
+Section 7 -- Other Terms and Conditions.
+
+  a. The Licensor shall not be bound by any additional or different
+     terms or conditions communicated by You unless expressly agreed.
+
+  b. Any arrangements, understandings, or agreements regarding the
+     Licensed Material not stated herein are separate from and
+     independent of the terms and conditions of this Public License.
+
+
+Section 8 -- Interpretation.
+
+  a. For the avoidance of doubt, this Public License does not, and
+     shall not be interpreted to, reduce, limit, restrict, or impose
+     conditions on any use of the Licensed Material that could lawfully
+     be made without permission under this Public License.
+
+  b. To the extent possible, if any provision of this Public License is
+     deemed unenforceable, it shall be automatically reformed to the
+     minimum extent necessary to make it enforceable. If the provision
+     cannot be reformed, it shall be severed from this Public License
+     without affecting the enforceability of the remaining terms and
+     conditions.
+
+  c. No term or condition of this Public License will be waived and no
+     failure to comply consented to unless expressly agreed to by the
+     Licensor.
+
+  d. Nothing in this Public License constitutes or may be interpreted
+     as a limitation upon, or waiver of, any privileges and immunities
+     that apply to the Licensor or You, including from the legal
+     processes of any jurisdiction or authority.
+
+
+=======================================================================
+
+Creative Commons is not a party to its public
+licenses. Notwithstanding, Creative Commons may elect to apply one of
+its public licenses to material it publishes and in those instances
+will be considered the “Licensor.” The text of the Creative Commons
+public licenses is dedicated to the public domain under the CC0 Public
+Domain Dedication. Except for the limited purpose of indicating that
+material is shared under a Creative Commons public license or as
+otherwise permitted by the Creative Commons policies published at
+creativecommons.org/policies, Creative Commons does not authorize the
+use of the trademark "Creative Commons" or any other trademark or logo
+of Creative Commons without its prior written consent including,
+without limitation, in connection with any unauthorized modifications
+to any of its public licenses or any other arrangements,
+understandings, or agreements concerning use of licensed material. For
+the avoidance of doubt, this paragraph does not form part of the
+public licenses.
+
+Creative Commons may be contacted at creativecommons.org.
diff --git a/LICENSES/GPL-2.0 b/LICENSES/GPL-2.0
new file mode 100644
index 0000000..d159169
--- /dev/null
+++ b/LICENSES/GPL-2.0
@@ -0,0 +1,339 @@
+                    GNU GENERAL PUBLIC LICENSE
+                       Version 2, June 1991
+
+ Copyright (C) 1989, 1991 Free Software Foundation, Inc.,
+ 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+                            Preamble
+
+  The licenses for most software are designed to take away your
+freedom to share and change it.  By contrast, the GNU General Public
+License is intended to guarantee your freedom to share and change free
+software--to make sure the software is free for all its users.  This
+General Public License applies to most of the Free Software
+Foundation's software and to any other program whose authors commit to
+using it.  (Some other Free Software Foundation software is covered by
+the GNU Lesser General Public License instead.)  You can apply it to
+your programs, too.
+
+  When we speak of free software, we are referring to freedom, not
+price.  Our General Public Licenses are designed to make sure that you
+have the freedom to distribute copies of free software (and charge for
+this service if you wish), that you receive source code or can get it
+if you want it, that you can change the software or use pieces of it
+in new free programs; and that you know you can do these things.
+
+  To protect your rights, we need to make restrictions that forbid
+anyone to deny you these rights or to ask you to surrender the rights.
+These restrictions translate to certain responsibilities for you if you
+distribute copies of the software, or if you modify it.
+
+  For example, if you distribute copies of such a program, whether
+gratis or for a fee, you must give the recipients all the rights that
+you have.  You must make sure that they, too, receive or can get the
+source code.  And you must show them these terms so they know their
+rights.
+
+  We protect your rights with two steps: (1) copyright the software, and
+(2) offer you this license which gives you legal permission to copy,
+distribute and/or modify the software.
+
+  Also, for each author's protection and ours, we want to make certain
+that everyone understands that there is no warranty for this free
+software.  If the software is modified by someone else and passed on, we
+want its recipients to know that what they have is not the original, so
+that any problems introduced by others will not reflect on the original
+authors' reputations.
+
+  Finally, any free program is threatened constantly by software
+patents.  We wish to avoid the danger that redistributors of a free
+program will individually obtain patent licenses, in effect making the
+program proprietary.  To prevent this, we have made it clear that any
+patent must be licensed for everyone's free use or not licensed at all.
+
+  The precise terms and conditions for copying, distribution and
+modification follow.
+
+                    GNU GENERAL PUBLIC LICENSE
+   TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
+
+  0. This License applies to any program or other work which contains
+a notice placed by the copyright holder saying it may be distributed
+under the terms of this General Public License.  The "Program", below,
+refers to any such program or work, and a "work based on the Program"
+means either the Program or any derivative work under copyright law:
+that is to say, a work containing the Program or a portion of it,
+either verbatim or with modifications and/or translated into another
+language.  (Hereinafter, translation is included without limitation in
+the term "modification".)  Each licensee is addressed as "you".
+
+Activities other than copying, distribution and modification are not
+covered by this License; they are outside its scope.  The act of
+running the Program is not restricted, and the output from the Program
+is covered only if its contents constitute a work based on the
+Program (independent of having been made by running the Program).
+Whether that is true depends on what the Program does.
+
+  1. You may copy and distribute verbatim copies of the Program's
+source code as you receive it, in any medium, provided that you
+conspicuously and appropriately publish on each copy an appropriate
+copyright notice and disclaimer of warranty; keep intact all the
+notices that refer to this License and to the absence of any warranty;
+and give any other recipients of the Program a copy of this License
+along with the Program.
+
+You may charge a fee for the physical act of transferring a copy, and
+you may at your option offer warranty protection in exchange for a fee.
+
+  2. You may modify your copy or copies of the Program or any portion
+of it, thus forming a work based on the Program, and copy and
+distribute such modifications or work under the terms of Section 1
+above, provided that you also meet all of these conditions:
+
+    a) You must cause the modified files to carry prominent notices
+    stating that you changed the files and the date of any change.
+
+    b) You must cause any work that you distribute or publish, that in
+    whole or in part contains or is derived from the Program or any
+    part thereof, to be licensed as a whole at no charge to all third
+    parties under the terms of this License.
+
+    c) If the modified program normally reads commands interactively
+    when run, you must cause it, when started running for such
+    interactive use in the most ordinary way, to print or display an
+    announcement including an appropriate copyright notice and a
+    notice that there is no warranty (or else, saying that you provide
+    a warranty) and that users may redistribute the program under
+    these conditions, and telling the user how to view a copy of this
+    License.  (Exception: if the Program itself is interactive but
+    does not normally print such an announcement, your work based on
+    the Program is not required to print an announcement.)
+
+These requirements apply to the modified work as a whole.  If
+identifiable sections of that work are not derived from the Program,
+and can be reasonably considered independent and separate works in
+themselves, then this License, and its terms, do not apply to those
+sections when you distribute them as separate works.  But when you
+distribute the same sections as part of a whole which is a work based
+on the Program, the distribution of the whole must be on the terms of
+this License, whose permissions for other licensees extend to the
+entire whole, and thus to each and every part regardless of who wrote it.
+
+Thus, it is not the intent of this section to claim rights or contest
+your rights to work written entirely by you; rather, the intent is to
+exercise the right to control the distribution of derivative or
+collective works based on the Program.
+
+In addition, mere aggregation of another work not based on the Program
+with the Program (or with a work based on the Program) on a volume of
+a storage or distribution medium does not bring the other work under
+the scope of this License.
+
+  3. You may copy and distribute the Program (or a work based on it,
+under Section 2) in object code or executable form under the terms of
+Sections 1 and 2 above provided that you also do one of the following:
+
+    a) Accompany it with the complete corresponding machine-readable
+    source code, which must be distributed under the terms of Sections
+    1 and 2 above on a medium customarily used for software interchange; or,
+
+    b) Accompany it with a written offer, valid for at least three
+    years, to give any third party, for a charge no more than your
+    cost of physically performing source distribution, a complete
+    machine-readable copy of the corresponding source code, to be
+    distributed under the terms of Sections 1 and 2 above on a medium
+    customarily used for software interchange; or,
+
+    c) Accompany it with the information you received as to the offer
+    to distribute corresponding source code.  (This alternative is
+    allowed only for noncommercial distribution and only if you
+    received the program in object code or executable form with such
+    an offer, in accord with Subsection b above.)
+
+The source code for a work means the preferred form of the work for
+making modifications to it.  For an executable work, complete source
+code means all the source code for all modules it contains, plus any
+associated interface definition files, plus the scripts used to
+control compilation and installation of the executable.  However, as a
+special exception, the source code distributed need not include
+anything that is normally distributed (in either source or binary
+form) with the major components (compiler, kernel, and so on) of the
+operating system on which the executable runs, unless that component
+itself accompanies the executable.
+
+If distribution of executable or object code is made by offering
+access to copy from a designated place, then offering equivalent
+access to copy the source code from the same place counts as
+distribution of the source code, even though third parties are not
+compelled to copy the source along with the object code.
+
+  4. You may not copy, modify, sublicense, or distribute the Program
+except as expressly provided under this License.  Any attempt
+otherwise to copy, modify, sublicense or distribute the Program is
+void, and will automatically terminate your rights under this License.
+However, parties who have received copies, or rights, from you under
+this License will not have their licenses terminated so long as such
+parties remain in full compliance.
+
+  5. You are not required to accept this License, since you have not
+signed it.  However, nothing else grants you permission to modify or
+distribute the Program or its derivative works.  These actions are
+prohibited by law if you do not accept this License.  Therefore, by
+modifying or distributing the Program (or any work based on the
+Program), you indicate your acceptance of this License to do so, and
+all its terms and conditions for copying, distributing or modifying
+the Program or works based on it.
+
+  6. Each time you redistribute the Program (or any work based on the
+Program), the recipient automatically receives a license from the
+original licensor to copy, distribute or modify the Program subject to
+these terms and conditions.  You may not impose any further
+restrictions on the recipients' exercise of the rights granted herein.
+You are not responsible for enforcing compliance by third parties to
+this License.
+
+  7. If, as a consequence of a court judgment or allegation of patent
+infringement or for any other reason (not limited to patent issues),
+conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
+excuse you from the conditions of this License.  If you cannot
+distribute so as to satisfy simultaneously your obligations under this
+License and any other pertinent obligations, then as a consequence you
+may not distribute the Program at all.  For example, if a patent
+license would not permit royalty-free redistribution of the Program by
+all those who receive copies directly or indirectly through you, then
+the only way you could satisfy both it and this License would be to
+refrain entirely from distribution of the Program.
+
+If any portion of this section is held invalid or unenforceable under
+any particular circumstance, the balance of the section is intended to
+apply and the section as a whole is intended to apply in other
+circumstances.
+
+It is not the purpose of this section to induce you to infringe any
+patents or other property right claims or to contest validity of any
+such claims; this section has the sole purpose of protecting the
+integrity of the free software distribution system, which is
+implemented by public license practices.  Many people have made
+generous contributions to the wide range of software distributed
+through that system in reliance on consistent application of that
+system; it is up to the author/donor to decide if he or she is willing
+to distribute software through any other system and a licensee cannot
+impose that choice.
+
+This section is intended to make thoroughly clear what is believed to
+be a consequence of the rest of this License.
+
+  8. If the distribution and/or use of the Program is restricted in
+certain countries either by patents or by copyrighted interfaces, the
+original copyright holder who places the Program under this License
+may add an explicit geographical distribution limitation excluding
+those countries, so that distribution is permitted only in or among
+countries not thus excluded.  In such case, this License incorporates
+the limitation as if written in the body of this License.
+
+  9. The Free Software Foundation may publish revised and/or new versions
+of the General Public License from time to time.  Such new versions will
+be similar in spirit to the present version, but may differ in detail to
+address new problems or concerns.
+
+Each version is given a distinguishing version number.  If the Program
+specifies a version number of this License which applies to it and "any
+later version", you have the option of following the terms and conditions
+either of that version or of any later version published by the Free
+Software Foundation.  If the Program does not specify a version number of
+this License, you may choose any version ever published by the Free Software
+Foundation.
+
+  10. If you wish to incorporate parts of the Program into other free
+programs whose distribution conditions are different, write to the author
+to ask for permission.  For software which is copyrighted by the Free
+Software Foundation, write to the Free Software Foundation; we sometimes
+make exceptions for this.  Our decision will be guided by the two goals
+of preserving the free status of all derivatives of our free software and
+of promoting the sharing and reuse of software generally.
+
+                            NO WARRANTY
+
+  11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY
+FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN
+OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES
+PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED
+OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS
+TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE
+PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING,
+REPAIR OR CORRECTION.
+
+  12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR
+REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES,
+INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING
+OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED
+TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY
+YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER
+PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE
+POSSIBILITY OF SUCH DAMAGES.
+
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+convey the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software; you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation; either version 2 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License along
+    with this program; if not, write to the Free Software Foundation, Inc.,
+    51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+Also add information on how to contact you by electronic and paper mail.
+
+If the program is interactive, make it output a short notice like this
+when it starts in an interactive mode:
+
+    Gnomovision version 69, Copyright (C) year name of author
+    Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, the commands you use may
+be called something other than `show w' and `show c'; they could even be
+mouse-clicks or menu items--whatever suits your program.
+
+You should also get your employer (if you work as a programmer) or your
+school, if any, to sign a "copyright disclaimer" for the program, if
+necessary.  Here is a sample; alter the names:
+
+  Yoyodyne, Inc., hereby disclaims all copyright interest in the program
+  `Gnomovision' (which makes passes at compilers) written by James Hacker.
+
+  <signature of Ty Coon>, 1 April 1989
+  Ty Coon, President of Vice
+
+This General Public License does not permit incorporating your program into
+proprietary programs.  If your program is a subroutine library, you may
+consider it more useful to permit linking proprietary applications with the
+library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.
diff --git a/LICENSES/MIT b/LICENSES/MIT
new file mode 100644
index 0000000..a348e30
--- /dev/null
+++ b/LICENSES/MIT
@@ -0,0 +1,20 @@
+MIT License
+
+Copyright (c) <year> <copyright holders>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software is furnished to do so,
+subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
diff --git a/LICENSES/OpenSSL b/LICENSES/OpenSSL
new file mode 100644
index 0000000..f86c523
--- /dev/null
+++ b/LICENSES/OpenSSL
@@ -0,0 +1,123 @@
+  LICENSE ISSUES
+  ==============
+
+  The OpenSSL toolkit stays under a double license, i.e. both the conditions of
+  the OpenSSL License and the original SSLeay license apply to the toolkit.
+  See below for the actual license texts.
+
+  OpenSSL License
+  ---------------
+
+/* ====================================================================
+ * Copyright (c) 1998-2019 The OpenSSL Project.  All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * 3. All advertising materials mentioning features or use of this
+ *    software must display the following acknowledgment:
+ *    "This product includes software developed by the OpenSSL Project
+ *    for use in the OpenSSL Toolkit. (http://www.openssl.org/)"
+ *
+ * 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to
+ *    endorse or promote products derived from this software without
+ *    prior written permission. For written permission, please contact
+ *    openssl-core@openssl.org.
+ *
+ * 5. Products derived from this software may not be called "OpenSSL"
+ *    nor may "OpenSSL" appear in their names without prior written
+ *    permission of the OpenSSL Project.
+ *
+ * 6. Redistributions of any form whatsoever must retain the following
+ *    acknowledgment:
+ *    "This product includes software developed by the OpenSSL Project
+ *    for use in the OpenSSL Toolkit (http://www.openssl.org/)"
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY
+ * EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE OpenSSL PROJECT OR
+ * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ * ====================================================================
+ *
+ * This product includes cryptographic software written by Eric Young
+ * (eay@cryptsoft.com).  This product includes software written by Tim
+ * Hudson (tjh@cryptsoft.com).
+ *
+ */
+
+ Original SSLeay License
+ -----------------------
+
+/* Copyright (C) 1995-1998 Eric Young (eay@cryptsoft.com)
+ * All rights reserved.
+ *
+ * This package is an SSL implementation written
+ * by Eric Young (eay@cryptsoft.com).
+ * The implementation was written so as to conform with Netscapes SSL.
+ *
+ * This library is free for commercial and non-commercial use as long as
+ * the following conditions are aheared to.  The following conditions
+ * apply to all code found in this distribution, be it the RC4, RSA,
+ * lhash, DES, etc., code; not just the SSL code.  The SSL documentation
+ * included with this distribution is covered by the same copyright terms
+ * except that the holder is Tim Hudson (tjh@cryptsoft.com).
+ *
+ * Copyright remains Eric Young's, and as such any Copyright notices in
+ * the code are not to be removed.
+ * If this package is used in a product, Eric Young should be given attribution
+ * as the author of the parts of the library used.
+ * This can be in the form of a textual message at program startup or
+ * in documentation (online or textual) provided with the package.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 3. All advertising materials mentioning features or use of this software
+ *    must display the following acknowledgement:
+ *    "This product includes cryptographic software written by
+ *     Eric Young (eay@cryptsoft.com)"
+ *    The word 'cryptographic' can be left out if the rouines from the library
+ *    being used are not cryptographic related :-).
+ * 4. If you include any Windows specific code (or a derivative thereof) from
+ *    the apps directory (application code) you must include an acknowledgement:
+ *    "This product includes software written by Tim Hudson (tjh@cryptsoft.com)"
+ *
+ * THIS SOFTWARE IS PROVIDED BY ERIC YOUNG ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * The licence and distribution terms for any publically available version or
+ * derivative of this code cannot be changed.  i.e. this code cannot simply be
+ * copied and put under another distribution licence
+ * [including the GNU Public Licence.]
+ */
diff --git a/LICENSES/exceptions/openvpn-openssl-exception b/LICENSES/exceptions/openvpn-openssl-exception
new file mode 100644
index 0000000..c63327f
--- /dev/null
+++ b/LICENSES/exceptions/openvpn-openssl-exception
@@ -0,0 +1,10 @@
+Special exception for linking `it` with OpenSSL:
+
+In addition, as a special exception, the copyright holders give permission to
+link the code of this program with the OpenSSL Library (or with modified
+versions of OpenSSL that use the same license as OpenSSL), and distribute linked
+combinations including the two. You must obey the GNU General Public License in
+all respects for all of the code used other than OpenSSL. If you modify this
+file, you may extend this exception to your version of the file, but you are not
+obligated to do so. If you do not wish to do so, delete this exception statement
+from your version.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..2b902f6
--- /dev/null
+++ b/README.md
@@ -0,0 +1,36 @@
+# it: zero-g git
+
+_it_ aims to augment git with primitives to build integrated, cryptographically
+verifiable collaboration workflows around source code. It maintains the
+distributed property of git, not requiring a central server. _it_ is transport
+agnostic, and permits data dissemination in client-server, federated, as well as
+peer-to-peer network topologies.
+
+
+## Status
+
+The [spec](./Documentation/spec.adoc) is thought of as being stable, in the
+sense that any amendments will consider backwards compatibility.
+
+The source code in this repository has been developed exploratively, in order to
+validate and refine the ideas laid out in above document. As such, it is
+incomplete, may occasionally malfunction, and does not yet provide the fine bone
+porcelain rendering it usable in anger. It's a prototype, if you wish.
+
+_it_ is actively developed on a volunteer basis.
+
+
+## Usage
+
+The _it_ implementation is written in Rust and can be installed from source
+using [cargo](https://doc.rust-lang.org/cargo/):
+
+    cargo install --git https://git.eagain.io/it
+
+To get an overview, see the [getting started](./Documentation/getting-started.adoc)
+document.
+
+
+## License
+
+GPL-2.0, see [COPYING](./COPYING)
diff --git a/TODO b/TODO
new file mode 100644
index 0000000..0c98462
--- /dev/null
+++ b/TODO
@@ -0,0 +1,55 @@
+- UI
+    - Fetch drop bundles
+        - git (from "dumb" mirrors)
+
+    - Apply patches from foreign drops
+    - Allow building drop state from foreign drop
+
+    - Improve $EDITOR use
+        - don't launch unless isatty
+        - for metadata json, use ft=javascript and add help text
+        - re-launch when edit error
+
+    - List auto-branches
+    - More topic traversals (DFS, BFS, ..)
+
+    - Handle $PAGER, like git does?
+    - Diff/apply patch?
+    - Teardown?
+
+- Internals
+    - Cache topic subjects
+    - Open repo eagerly at outermost command layer
+
+      So we can return actual iterators from commands. Do we need a repo always,
+      though?
+
+    - Index bundle hash -> header meta
+
+      For dynamic repacking
+
+    - Upstream bindings to git_indexer_*
+
+      So that we can resolve delta bases without committing to the odb
+      immediately. This would allow to deduplicate packfile storage, and
+      partition the odb as follows:
+
+      - store the bundle pack and associated index in a directory separate from
+        the repo objects (possibly scoped by drop)
+      - add this directory as an alternate to the repo, preventing compaction
+        initiated by git
+      - strip the pack data from the bundle and replace with a pointer to the
+        pack
+      - snapshots now become compactions of this alternate objects store,
+        controlled by `it`
+      - when a snapshot is taken, dangling bundle (-headers) may either be
+        pruned, resulting in a 404 from the HTTP server when the bundle is
+        requested, or the pointer be replaced by a marker indicating that the
+        bundle was made obsolete by the corresponding snapshot bundle. The HTTP
+        server may then send a redirect to the snapshot instead.
+
+- Protocol
+    - Implement automerge topics
+
+- HTTP
+    - Accept checkpoints if authorised
diff --git a/src/bin/it.rs b/src/bin/it.rs
new file mode 100644
index 0000000..e6d5d4c
--- /dev/null
+++ b/src/bin/it.rs
@@ -0,0 +1,181 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    io,
+    path::PathBuf,
+};
+
+use clap::ValueHint;
+use clap_complete::Shell;
+
+static OUTPUT: it::Output = it::Output;
+
+fn main() -> it::Result<()> {
+    use clap::Parser as _;
+
+    log::set_logger(&OUTPUT)?;
+    log::set_max_level(
+        std::env::var("RUST_LOG")
+            .ok()
+            .and_then(|v| v.parse().ok())
+            .unwrap_or(log::LevelFilter::Info),
+    );
+
+    let cli = It::parse();
+    match cli.cmd {
+        Cmd::Cmd(cmd) => cmd
+            .run()
+            .and_then(|o| render(o, cli.compact))
+            .or_else(|e| e.downcast::<it::cmd::Aborted>().map(|_aborted| ())),
+        Cmd::Hidden(cmd) => match cmd {
+            Hidden::Man { out } => hidden::mangen(&out),
+            Hidden::Completions { shell, out } => hidden::completions(shell, out.as_deref()),
+        },
+    }
+}
+
+/// it: zero-g git
+#[derive(Debug, clap::Parser)]
+#[clap(author, version, about, propagate_version = true, max_term_width = 100)]
+struct It {
+    /// Path to the git repository containing the drop state
+    #[clap(
+        long,
+        value_parser,
+        value_name = "DIR",
+        env = "GIT_DIR",
+        default_value_os_t = std::env::current_dir().unwrap(),
+        value_hint = ValueHint::DirPath,
+        global = true,
+    )]
+    git_dir: PathBuf,
+    /// Do not pretty-print the output
+    #[clap(long, value_parser, default_value_t = false, global = true)]
+    compact: bool,
+    #[clap(subcommand)]
+    cmd: Cmd,
+}
+
+fn render(output: it::cmd::Output, compact: bool) -> it::Result<()> {
+    use it::cmd::Output::*;
+
+    let go = |v| {
+        let out = io::stdout();
+        if compact {
+            serde_json::to_writer(out, &v)
+        } else {
+            serde_json::to_writer_pretty(out, &v)
+        }
+    };
+
+    match output {
+        Val(v) => go(v)?,
+        Iter(i) => {
+            for v in i {
+                let v = v?;
+                go(v)?;
+                println!();
+            }
+        },
+    }
+
+    Ok(())
+}
+
+#[derive(Debug, clap::Subcommand)]
+#[allow(clippy::large_enum_variant)]
+enum Cmd {
+    #[clap(flatten)]
+    Cmd(it::Cmd),
+    #[clap(flatten)]
+    Hidden(Hidden),
+}
+
+#[derive(Debug, clap::Subcommand)]
+#[clap(hide = true)]
+enum Hidden {
+    /// Generate man pages
+    #[clap(hide = true)]
+    Man {
+        /// Output to this directory
+        #[clap(
+            value_parser,
+            default_value = "man",
+            value_name = "DIR",
+            value_hint = ValueHint::DirPath,
+        )]
+        out: PathBuf,
+    },
+    /// Generate shell completions
+    #[clap(hide = true)]
+    Completions {
+        /// The shell to generate completions for
+        #[clap(value_parser)]
+        shell: Shell,
+        /// Output file (stdout if not set)
+        #[clap(value_parser, value_name = "FILE", value_hint = ValueHint::FilePath)]
+        out: Option<PathBuf>,
+    },
+}
+
+mod hidden {
+    use std::{
+        fs::File,
+        io,
+        path::Path,
+    };
+
+    use clap::CommandFactory as _;
+    use clap_complete::Shell;
+    use clap_mangen::Man;
+
+    pub fn mangen(out: &Path) -> it::Result<()> {
+        std::fs::create_dir_all(out)?;
+        let it = super::It::command();
+        for cmd in it.get_subcommands() {
+            if cmd.get_name() == "dev" {
+                continue;
+            }
+            for sub in cmd.get_subcommands() {
+                let name = format!("{}-{}-{}", it.get_name(), cmd.get_name(), sub.get_name());
+                let filename = out.join(&name).with_extension("1");
+
+                let the_cmd = sub.clone().name(&name);
+                let man = Man::new(the_cmd)
+                    .title(name.to_uppercase())
+                    .section("1")
+                    .manual("It Manual");
+
+                eprintln!("Generating {}...", filename.display());
+                man.render(
+                    &mut File::options()
+                        .write(true)
+                        .create(true)
+                        .truncate(true)
+                        .open(&filename)?,
+                )?;
+            }
+        }
+
+        Ok(())
+    }
+
+    pub fn completions(shell: Shell, out: Option<&Path>) -> it::Result<()> {
+        match out {
+            Some(path) => {
+                let mut out = File::options()
+                    .write(true)
+                    .create(true)
+                    .truncate(true)
+                    .open(path)?;
+                clap_complete::generate(shell, &mut super::It::command(), "it", &mut out);
+            },
+            None => {
+                clap_complete::generate(shell, &mut super::It::command(), "it", &mut io::stdout());
+            },
+        }
+
+        Ok(())
+    }
+}
diff --git a/src/bundle.rs b/src/bundle.rs
new file mode 100644
index 0000000..25eafd0
--- /dev/null
+++ b/src/bundle.rs
@@ -0,0 +1,114 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::io;
+
+use log::info;
+use sha2::{
+    Digest,
+    Sha256,
+};
+use url::Url;
+
+use crate::io::{
+    HashWriter,
+    LenWriter,
+};
+
+pub mod error;
+
+mod fetch;
+pub use fetch::{
+    Fetched,
+    Fetcher,
+};
+
+mod header;
+pub use header::{
+    Hash,
+    Header,
+    ObjectFormat,
+    ObjectId,
+    Version,
+};
+
+pub mod list;
+pub use list::{
+    List,
+    Location,
+    Uri,
+};
+
+pub const FILE_EXTENSION: &str = "bundle";
+pub const DOT_FILE_EXTENSION: &str = ".bundle";
+
+#[derive(Clone, Debug, serde::Serialize, serde::Deserialize)]
+pub struct Info {
+    pub len: u64,
+    pub hash: Hash,
+    #[serde(with = "hex::serde")]
+    pub checksum: [u8; 32],
+    #[serde(default, skip_serializing_if = "Vec::is_empty")]
+    pub uris: Vec<Url>,
+}
+
+#[derive(Clone, Copy)]
+pub struct Expect<'a> {
+    pub len: u64,
+    pub hash: &'a Hash,
+    pub checksum: Option<&'a [u8]>,
+}
+
+impl<'a> From<&'a Info> for Expect<'a> {
+    fn from(
+        Info {
+            len,
+            hash,
+            checksum,
+            ..
+        }: &'a Info,
+    ) -> Self {
+        Self {
+            len: *len,
+            hash,
+            checksum: Some(checksum),
+        }
+    }
+}
+
+pub fn create<W>(mut out: W, repo: &git2::Repository, header: &Header) -> crate::Result<Info>
+where
+    W: io::Write,
+{
+    let mut hasher = HashWriter::new(Sha256::new(), &mut out);
+    let mut writer = LenWriter::new(&mut hasher);
+    let mut pack = {
+        let mut pack = repo.packbuilder()?;
+        let mut walk = repo.revwalk()?;
+        for pre in &header.prerequisites {
+            walk.hide(pre.try_into()?)?;
+        }
+        for inc in header.references.values() {
+            walk.push(inc.try_into()?)?;
+        }
+        pack.insert_walk(&mut walk)?;
+        pack
+    };
+    header.to_writer(&mut writer)?;
+
+    info!("Packing objects...");
+    pack.foreach(|chunk| io::Write::write_all(&mut writer, chunk).is_ok())?;
+
+    let len = writer.bytes_written();
+    let hash = header.hash();
+    let checksum = hasher.hash().into();
+
+    info!("Created patch bundle {hash}");
+
+    Ok(Info {
+        len,
+        hash,
+        checksum,
+        uris: vec![],
+    })
+}
diff --git a/src/bundle/error.rs b/src/bundle/error.rs
new file mode 100644
index 0000000..41529c2
--- /dev/null
+++ b/src/bundle/error.rs
@@ -0,0 +1,31 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use thiserror::Error;
+
+use super::{
+    ObjectFormat,
+    ObjectId,
+};
+use crate::git::refs;
+
+#[derive(Debug, Error)]
+pub enum Header {
+    #[error("invalid header: {0}")]
+    Format(&'static str),
+
+    #[error("unrecognised header {0}")]
+    UnrecognisedHeader(String),
+
+    #[error("object id {oid} not valid for object-format {fmt}")]
+    ObjectFormat { fmt: ObjectFormat, oid: ObjectId },
+
+    #[error("invalid reference name")]
+    Refname(#[from] refs::error::RefFormat),
+
+    #[error("invalid hex oid")]
+    Oid(#[from] hex::FromHexError),
+
+    #[error(transparent)]
+    Io(#[from] std::io::Error),
+}
diff --git a/src/bundle/fetch.rs b/src/bundle/fetch.rs
new file mode 100644
index 0000000..4e58000
--- /dev/null
+++ b/src/bundle/fetch.rs
@@ -0,0 +1,130 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    fs,
+    io::{
+        self,
+        Read,
+        Seek,
+        SeekFrom,
+        Write,
+    },
+    path::{
+        Path,
+        PathBuf,
+    },
+};
+
+use anyhow::ensure;
+use either::Either::{
+    self,
+    Left,
+    Right,
+};
+use sha2::{
+    Digest,
+    Sha256,
+};
+use tempfile::NamedTempFile;
+use url::Url;
+
+use super::{
+    header,
+    Expect,
+    Header,
+};
+use crate::{
+    bundle,
+    fs::LockedFile,
+    git,
+    io::HashWriter,
+};
+
+const MAX_BUNDLE_URIS_BYTES: u64 = 50_000;
+
+pub struct Fetched {
+    path: PathBuf,
+    info: bundle::Info,
+}
+
+impl Fetched {
+    pub fn into_inner(self) -> (PathBuf, bundle::Info) {
+        (self.path, self.info)
+    }
+}
+
+pub struct Fetcher {
+    agent: ureq::Agent,
+}
+
+impl Default for Fetcher {
+    fn default() -> Self {
+        Self {
+            agent: ureq::agent(),
+        }
+    }
+}
+
+impl Fetcher {
+    pub fn fetch(
+        &self,
+        url: &Url,
+        out_dir: &Path,
+        expect: Expect,
+    ) -> crate::Result<Either<bundle::List, Fetched>> {
+        let resp = self.agent.request_url("GET", url).call()?;
+        let mut body = resp.into_reader();
+
+        let mut buf = [0; 16];
+        body.read_exact(&mut buf)?;
+        let is_bundle = buf.starts_with(header::SIGNATURE_V2.as_bytes())
+            || buf.starts_with(header::SIGNATURE_V3.as_bytes());
+        if is_bundle {
+            ensure!(
+                matches!(buf.last(), Some(b'\n')),
+                "malformed bundle header: trailing data"
+            )
+        }
+
+        if is_bundle {
+            let mut path = out_dir.join(expect.hash.to_string());
+            path.set_extension(bundle::FILE_EXTENSION);
+
+            let mut lck = {
+                fs::create_dir_all(out_dir)?;
+                LockedFile::atomic(&path, true, LockedFile::DEFAULT_PERMISSIONS)?
+            };
+
+            let mut out = HashWriter::new(Sha256::new(), &mut lck);
+            out.write_all(&buf)?;
+
+            let len = buf.len() as u64 + io::copy(&mut body.take(expect.len), &mut out)?;
+            let checksum = out.hash().into();
+            if let Some(chk) = expect.checksum {
+                ensure!(chk == checksum, "checksum mismatch");
+            }
+            lck.seek(SeekFrom::Start(0))?;
+            let header = Header::from_reader(&mut lck)?;
+            let hash = header.hash();
+
+            lck.persist()?;
+
+            let info = bundle::Info {
+                len,
+                hash,
+                checksum,
+                uris: vec![url.clone()],
+            };
+            Ok(Right(Fetched { path, info }))
+        } else {
+            let mut tmp = NamedTempFile::new()?;
+            tmp.write_all(&buf)?;
+            io::copy(&mut body.take(MAX_BUNDLE_URIS_BYTES), &mut tmp)?;
+            let cfg = git::config::Snapshot::try_from(git2::Config::open(tmp.path())?)?;
+            let list = bundle::List::from_config(cfg)?;
+
+            Ok(Left(list))
+        }
+    }
+}
diff --git a/src/bundle/header.rs b/src/bundle/header.rs
new file mode 100644
index 0000000..6f3dfe3
--- /dev/null
+++ b/src/bundle/header.rs
@@ -0,0 +1,365 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use core::fmt;
+use std::{
+    collections::{
+        BTreeMap,
+        BTreeSet,
+    },
+    io,
+    ops::Deref,
+    str::FromStr,
+};
+
+use hex::{
+    FromHex,
+    FromHexError,
+};
+use refs::Refname;
+use sha2::{
+    Digest,
+    Sha256,
+};
+
+use super::error;
+use crate::{
+    git::refs,
+    io::Lines,
+};
+
+pub const SIGNATURE_V2: &str = "# v2 git bundle";
+pub const SIGNATURE_V3: &str = "# v3 git bundle";
+
+#[derive(Debug, serde::Serialize, serde::Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum Version {
+    V2,
+    V3,
+}
+
+impl Default for Version {
+    fn default() -> Self {
+        Self::V2
+    }
+}
+
+#[derive(Debug, serde::Serialize, serde::Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum ObjectFormat {
+    Sha1,
+    Sha256,
+}
+
+impl Default for ObjectFormat {
+    fn default() -> Self {
+        Self::Sha1
+    }
+}
+
+impl fmt::Display for ObjectFormat {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(match self {
+            Self::Sha1 => "sha1",
+            Self::Sha256 => "sha256",
+        })
+    }
+}
+
+#[derive(Clone, Copy, Eq, Ord, PartialEq, PartialOrd, serde::Serialize, serde::Deserialize)]
+#[serde(untagged)]
+pub enum ObjectId {
+    Sha1(#[serde(with = "hex::serde")] [u8; 20]),
+    Sha2(#[serde(with = "hex::serde")] [u8; 32]),
+}
+
+impl ObjectId {
+    pub fn as_bytes(&self) -> &[u8] {
+        self.as_ref()
+    }
+}
+
+impl AsRef<[u8]> for ObjectId {
+    fn as_ref(&self) -> &[u8] {
+        match self {
+            Self::Sha1(b) => &b[..],
+            Self::Sha2(b) => &b[..],
+        }
+    }
+}
+
+impl fmt::Display for ObjectId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&hex::encode(self))
+    }
+}
+
+impl fmt::Debug for ObjectId {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Self::Sha1(x) => f.debug_tuple("Sha1").field(&hex::encode(x)).finish(),
+            Self::Sha2(x) => f.debug_tuple("Sha2").field(&hex::encode(x)).finish(),
+        }
+    }
+}
+
+impl FromHex for ObjectId {
+    type Error = hex::FromHexError;
+
+    #[inline]
+    fn from_hex<T: AsRef<[u8]>>(hex: T) -> Result<Self, Self::Error> {
+        match hex.as_ref().len() {
+            40 => Ok(Self::Sha1(<[u8; 20]>::from_hex(hex)?)),
+            64 => Ok(Self::Sha2(<[u8; 32]>::from_hex(hex)?)),
+            _ => Err(hex::FromHexError::InvalidStringLength),
+        }
+    }
+}
+
+impl From<&git2::Oid> for ObjectId {
+    fn from(oid: &git2::Oid) -> Self {
+        let bs = oid.as_bytes();
+        match bs.len() {
+            20 => Self::Sha1(bs.try_into().unwrap()),
+            32 => Self::Sha2(bs.try_into().unwrap()),
+            x => unreachable!("oid with strange hash size: {}", x),
+        }
+    }
+}
+
+impl TryFrom<&ObjectId> for git2::Oid {
+    type Error = git2::Error;
+
+    fn try_from(oid: &ObjectId) -> Result<Self, Self::Error> {
+        match oid {
+            ObjectId::Sha1(hash) => Self::from_bytes(hash),
+            ObjectId::Sha2(_) => Err(git2::Error::new(
+                git2::ErrorCode::Invalid,
+                git2::ErrorClass::Sha1,
+                "sha2 oids not yet supported",
+            )),
+        }
+    }
+}
+
+#[derive(Debug, Default, serde::Serialize, serde::Deserialize)]
+#[serde(rename_all = "kebab-case")]
+pub struct Header {
+    pub version: Version,
+    pub object_format: ObjectFormat,
+    pub prerequisites: BTreeSet<ObjectId>,
+    pub references: BTreeMap<Refname, ObjectId>,
+}
+
+impl Header {
+    /// Parse a [`Header`] from an IO stream.
+    ///
+    /// The stream will be buffered internally, and its position set to the
+    /// start of the packfile section.
+    pub fn from_reader<R>(mut io: R) -> Result<Self, error::Header>
+    where
+        R: io::Read + io::Seek,
+    {
+        use hex::FromHex as _;
+
+        let mut lines = Lines::new(io::BufReader::new(&mut io)).until_blank();
+
+        let mut version: Option<Version> = None;
+        let mut object_format: Option<ObjectFormat> = None;
+        let mut prerequisites = BTreeSet::new();
+        let mut references = BTreeMap::new();
+
+        match lines
+            .next()
+            .ok_or(error::Header::Format("empty input"))??
+            .as_str()
+        {
+            SIGNATURE_V2 => {
+                version = Some(Version::V2);
+                object_format = Some(ObjectFormat::Sha1);
+                Ok(())
+            },
+
+            SIGNATURE_V3 => {
+                version = Some(Version::V2);
+                Ok(())
+            },
+
+            _ => Err(error::Header::Format("invalid signature")),
+        }?;
+
+        if let Some(Version::V3) = version {
+            for capability in lines.by_ref() {
+                let capability = capability?;
+
+                if !capability.starts_with('@') {
+                    return Err(error::Header::Format("expected capabilities"));
+                }
+
+                if capability.starts_with("@filter") {
+                    return Err(error::Header::Format("object filters are not supported"));
+                }
+
+                match capability.strip_prefix("@object-format=") {
+                    Some("sha1") => {
+                        object_format = Some(ObjectFormat::Sha1);
+                    },
+
+                    Some("sha256") => {
+                        object_format = Some(ObjectFormat::Sha256);
+                    },
+
+                    _ => return Err(error::Header::Format("unrecognised capability")),
+                }
+
+                if object_format.is_some() {
+                    break;
+                }
+            }
+        }
+
+        let version = version.unwrap();
+        let object_format = object_format.ok_or(error::Header::Format("missing object-format"))?;
+
+        for tip in lines.by_ref() {
+            let mut tip = tip?;
+            let oid_off = usize::from(tip.starts_with('-'));
+            let oid_hexsz = match object_format {
+                ObjectFormat::Sha1 => 40,
+                ObjectFormat::Sha256 => 64,
+            };
+
+            let oid = ObjectId::from_hex(&tip[oid_off..oid_hexsz + oid_off])?;
+            if matches!(
+                (&object_format, &oid),
+                (ObjectFormat::Sha1, ObjectId::Sha2(_)) | (ObjectFormat::Sha256, ObjectId::Sha1(_))
+            ) {
+                return Err(error::Header::ObjectFormat {
+                    fmt: object_format,
+                    oid,
+                });
+            }
+            if !matches!(tip.chars().nth(oid_off + oid_hexsz), None | Some(' ')) {
+                return Err(error::Header::UnrecognisedHeader(tip));
+            }
+
+            if oid_off > 0 {
+                prerequisites.insert(oid);
+            } else {
+                let refname = tip.split_off(oid_off + oid_hexsz + 1);
+                if !refname.starts_with("refs/") {
+                    return Err(error::Header::Format("shorthand refname"));
+                }
+                if references.insert(refname.parse()?, oid).is_some() {
+                    return Err(error::Header::Format("duplicate refname"));
+                }
+            }
+        }
+
+        if references.is_empty() {
+            return Err(error::Header::Format("empty references"));
+        }
+
+        let pos = io::Seek::stream_position(&mut lines)?;
+        drop(lines);
+        io.seek(io::SeekFrom::Start(pos))?;
+
+        Ok(Header {
+            version,
+            object_format,
+            prerequisites,
+            references,
+        })
+    }
+
+    pub fn to_writer<W>(&self, mut io: W) -> io::Result<()>
+    where
+        W: io::Write,
+    {
+        match self.version {
+            Version::V2 => writeln!(&mut io, "{}", SIGNATURE_V2)?,
+            Version::V3 => {
+                writeln!(&mut io, "{}", SIGNATURE_V3)?;
+                match self.object_format {
+                    ObjectFormat::Sha1 => writeln!(&mut io, "@object-format=sha1")?,
+                    ObjectFormat::Sha256 => writeln!(&mut io, "@object-format=sha256")?,
+                }
+            },
+        }
+        for pre in &self.prerequisites {
+            writeln!(&mut io, "-{}", pre)?;
+        }
+        for (name, oid) in &self.references {
+            writeln!(&mut io, "{} {}", oid, name)?;
+        }
+
+        writeln!(&mut io)
+    }
+
+    pub fn add_prerequisite<O>(&mut self, oid: O) -> bool
+    where
+        O: Into<ObjectId>,
+    {
+        self.prerequisites.insert(oid.into())
+    }
+
+    pub fn add_reference<O>(&mut self, name: Refname, oid: O) -> Option<ObjectId>
+    where
+        O: Into<ObjectId>,
+    {
+        self.references.insert(name, oid.into())
+    }
+
+    pub fn hash(&self) -> Hash {
+        let mut ids: BTreeSet<&ObjectId> = BTreeSet::new();
+        ids.extend(self.prerequisites.iter());
+        ids.extend(self.references.values());
+
+        let mut sha = Sha256::new();
+        for id in ids {
+            sha.update(id);
+        }
+        Hash(sha.finalize().into())
+    }
+}
+
+#[derive(Clone, Copy, Eq, Ord, PartialEq, PartialOrd, serde::Serialize, serde::Deserialize)]
+pub struct Hash(#[serde(with = "hex::serde")] [u8; 32]);
+
+impl Hash {
+    pub fn as_bytes(&self) -> &[u8] {
+        self.deref()
+    }
+
+    pub fn is_valid(hex: &str) -> bool {
+        Self::from_str(hex).is_ok()
+    }
+}
+
+impl Deref for Hash {
+    type Target = [u8; 32];
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl fmt::Display for Hash {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        f.write_str(&hex::encode(self.0))
+    }
+}
+
+impl fmt::Debug for Hash {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&hex::encode(self.0))
+    }
+}
+
+impl FromStr for Hash {
+    type Err = FromHexError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        <[u8; 32]>::from_hex(s).map(Self)
+    }
+}
diff --git a/src/bundle/list.rs b/src/bundle/list.rs
new file mode 100644
index 0000000..21753fa
--- /dev/null
+++ b/src/bundle/list.rs
@@ -0,0 +1,335 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+//! Bundle Lists in git config format, as per [`bundle-uri`].
+//!
+//! [`bundle-uri`]: https://git.kernel.org/pub/scm/git/git.git/tree/Documentation/technical/bundle-uri.txt
+
+use std::{
+    borrow::Cow,
+    cmp::Ordering,
+    collections::HashMap,
+    fmt,
+    io,
+    str::FromStr,
+    time::{
+        SystemTime,
+        UNIX_EPOCH,
+    },
+};
+
+use anyhow::anyhow;
+use once_cell::sync::Lazy;
+use sha2::{
+    Digest,
+    Sha256,
+};
+use url::Url;
+
+use crate::git::{
+    self,
+    if_not_found_none,
+};
+
+pub const FILE_EXTENSION: &str = "uris";
+pub const DOT_FILE_EXTENSION: &str = ".uris";
+
+#[derive(Clone, Copy, Debug)]
+pub enum Mode {
+    All,
+    Any,
+}
+
+impl Mode {
+    pub fn as_str(&self) -> &str {
+        match self {
+            Self::All => "all",
+            Self::Any => "any",
+        }
+    }
+}
+
+impl fmt::Display for Mode {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self.as_str())
+    }
+}
+
+impl FromStr for Mode {
+    type Err = crate::Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s {
+            "all" => Ok(Self::All),
+            "any" => Ok(Self::Any),
+            x => Err(anyhow!("unknown bundle list mode: {x}")),
+        }
+    }
+}
+
+#[derive(Debug)]
+pub enum Uri {
+    Absolute(Url),
+    Relative(String),
+}
+
+impl Uri {
+    pub fn as_str(&self) -> &str {
+        match self {
+            Self::Absolute(url) => url.as_str(),
+            Self::Relative(path) => path.as_str(),
+        }
+    }
+
+    pub fn abs(&self, base: &Url) -> Result<Cow<Url>, url::ParseError> {
+        match self {
+            Self::Absolute(url) => Ok(Cow::Borrowed(url)),
+            Self::Relative(path) => base.join(path).map(Cow::Owned),
+        }
+    }
+}
+
+impl From<Url> for Uri {
+    fn from(url: Url) -> Self {
+        Self::Absolute(url)
+    }
+}
+
+impl FromStr for Uri {
+    type Err = url::ParseError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        static DUMMY_BASE: Lazy<Url> =
+            Lazy::new(|| Url::parse("https://bundles.example.com").unwrap());
+
+        Url::parse(s).map(Self::Absolute).or_else(|e| match e {
+            url::ParseError::RelativeUrlWithoutBase => {
+                let url = Url::options().base_url(Some(&DUMMY_BASE)).parse(s)?;
+
+                let path = if s.starts_with('/') {
+                    url.path()
+                } else {
+                    url.path().trim_start_matches('/')
+                };
+
+                Ok(Self::Relative(path.to_owned()))
+            },
+            other => Err(other),
+        })
+    }
+}
+
+#[derive(Debug)]
+pub struct Location {
+    pub id: String,
+    pub uri: Uri,
+    pub filter: Option<String>,
+    pub creation_token: Option<u64>,
+    pub location: Option<String>,
+}
+
+impl Location {
+    pub fn new(id: String, uri: Uri) -> Self {
+        Self {
+            id,
+            uri,
+            filter: None,
+            creation_token: None,
+            location: None,
+        }
+    }
+
+    pub fn to_config(&self, cfg: &mut git2::Config) -> crate::Result<()> {
+        let section = format!("bundle.{}", self.id);
+
+        cfg.set_str(&format!("{section}.uri"), self.uri.as_str())?;
+        if let Some(filter) = self.filter.as_deref() {
+            cfg.set_str(&format!("{section}.filter"), filter)?;
+        }
+        if let Some(token) = &self.creation_token {
+            cfg.set_str(&format!("{section}.creationToken"), &token.to_string())?;
+        }
+        if let Some(loc) = self.location.as_deref() {
+            cfg.set_str(&format!("{section}.location"), loc)?;
+        }
+
+        Ok(())
+    }
+
+    pub fn to_writer<W: io::Write>(&self, mut out: W) -> io::Result<()> {
+        writeln!(&mut out, "[bundle \"{}\"]", self.id)?;
+        writeln!(&mut out, "\turi = {}", self.uri.as_str())?;
+        if let Some(filter) = self.filter.as_deref() {
+            writeln!(&mut out, "\tfilter = {}", filter)?;
+        }
+        if let Some(token) = &self.creation_token {
+            writeln!(&mut out, "\tcreationToken = {}", token)?;
+        }
+        if let Some(loc) = self.location.as_deref() {
+            writeln!(&mut out, "\tlocation = {}", loc)?;
+        }
+
+        Ok(())
+    }
+}
+
+impl From<Url> for Location {
+    fn from(url: Url) -> Self {
+        let id = hex::encode(Sha256::digest(url.as_str()));
+        let now = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .expect("backwards system clock")
+            .as_secs();
+        Self {
+            id,
+            uri: url.into(),
+            filter: None,
+            creation_token: Some(now),
+            location: None,
+        }
+    }
+}
+
+#[derive(Debug)]
+pub struct List {
+    pub mode: Mode,
+    pub heuristic: Option<String>,
+    pub bundles: Vec<Location>,
+}
+
+impl List {
+    pub fn any() -> Self {
+        Self {
+            mode: Mode::Any,
+            heuristic: Some("creationToken".into()),
+            bundles: Vec::new(),
+        }
+    }
+
+    /// Parse a bundle list from a [`git2::Config`]
+    ///
+    /// The config is expected to contain the list config keys `bundle.mode` and
+    /// optionally `bundle.heuristic`. `bundle.version` is currently ignored.
+    ///
+    /// A bundle [`Location`] is yielded if at least `bundle.<id>.uri` is set
+    /// and a valid [`Url`]. The `base` [`Url`] must be provided to resolve
+    /// relative uris in the file.
+    ///
+    /// The [`Location`] list is sorted by creation token in descending order
+    /// (entries without a token sort last). The sort is unstable.
+    pub fn from_config(cfg: git::config::Snapshot) -> crate::Result<Self> {
+        // nb. ignoring version
+        let mode = cfg.get_str("bundle.mode")?.parse()?;
+        let heuristic = if_not_found_none(cfg.get_string("bundle.heuristic"))?;
+
+        #[derive(Default)]
+        struct Info {
+            uri: Option<Uri>,
+            filter: Option<String>,
+            creation_token: Option<u64>,
+            location: Option<String>,
+        }
+
+        let mut bundles: HashMap<String, Info> = HashMap::new();
+        let mut iter = cfg.entries(Some("bundle\\.[^.]+\\.[^.]+$"))?;
+        while let Some(entry) = iter.next() {
+            let entry = entry?;
+            if let Some(("bundle", id, key)) = entry
+                .name()
+                .and_then(|name| name.split_once('.'))
+                .and_then(|(a, b)| b.split_once('.').map(|(c, d)| (a, c, d)))
+            {
+                let value = entry
+                    .value()
+                    .ok_or_else(|| anyhow!("value for bundle.{id}.{key} not utf8"))?;
+                let info = bundles.entry(id.to_owned()).or_default();
+                match key {
+                    "uri" => {
+                        let uri = value.parse()?;
+                        info.uri = Some(uri);
+                    },
+
+                    "filter" => {
+                        info.filter = Some(value.to_owned());
+                    },
+
+                    "creationToken" | "creationtoken" => {
+                        let token = value.parse()?;
+                        info.creation_token = Some(token);
+                    },
+
+                    "location" => {
+                        info.location = Some(value.to_owned());
+                    },
+
+                    _ => {},
+                }
+            }
+        }
+        let mut bundles = bundles
+            .into_iter()
+            .filter_map(|(id, info)| {
+                info.uri.map(|uri| Location {
+                    id,
+                    uri,
+                    filter: info.filter,
+                    creation_token: info.creation_token,
+                    location: info.location,
+                })
+            })
+            .collect::<Vec<_>>();
+        bundles.sort_unstable_by(|a, b| match (&a.creation_token, &b.creation_token) {
+            (Some(x), Some(y)) => y.cmp(x),
+            (Some(_), None) => Ordering::Less,
+            (None, Some(_)) => Ordering::Greater,
+            (None, None) => Ordering::Equal,
+        });
+
+        Ok(Self {
+            mode,
+            heuristic,
+            bundles,
+        })
+    }
+
+    pub fn to_config(&self, cfg: &mut git2::Config) -> crate::Result<()> {
+        cfg.set_i32("bundle.version", 1)?;
+        cfg.set_str("bundle.mode", self.mode.as_str())?;
+        if let Some(heuristic) = self.heuristic.as_deref() {
+            cfg.set_str("bundle.heuristic", heuristic)?;
+        }
+        self.bundles.iter().try_for_each(|loc| loc.to_config(cfg))?;
+
+        Ok(())
+    }
+
+    pub fn to_writer<W: io::Write>(&self, mut out: W) -> io::Result<()> {
+        writeln!(&mut out, "[bundle]")?;
+        writeln!(&mut out, "\tversion = 1")?;
+        writeln!(&mut out, "\tmode = {}", self.mode)?;
+        if let Some(heuristic) = self.heuristic.as_deref() {
+            writeln!(&mut out, "\theuristic = {}", heuristic)?;
+        }
+        for loc in &self.bundles {
+            writeln!(&mut out)?;
+            loc.to_writer(&mut out)?;
+        }
+
+        Ok(())
+    }
+
+    pub fn to_str(&self) -> String {
+        let mut buf = Vec::new();
+        self.to_writer(&mut buf).unwrap();
+        unsafe { String::from_utf8_unchecked(buf) }
+    }
+}
+
+impl Extend<Location> for List {
+    fn extend<T>(&mut self, iter: T)
+    where
+        T: IntoIterator<Item = Location>,
+    {
+        self.bundles.extend(iter)
+    }
+}
diff --git a/src/cfg.rs b/src/cfg.rs
new file mode 100644
index 0000000..b6a74da
--- /dev/null
+++ b/src/cfg.rs
@@ -0,0 +1,180 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+pub mod paths {
+    use directories::ProjectDirs;
+    use std::path::{
+        Path,
+        PathBuf,
+    };
+
+    pub fn ids() -> PathBuf {
+        project_dirs().data_dir().join("ids")
+    }
+
+    /// Default path where to store bundles.
+    ///
+    /// This is a relative path, to be treated as relative to GIT_DIR.
+    pub fn bundles() -> &'static Path {
+        Path::new("it/bundles")
+    }
+
+    fn project_dirs() -> ProjectDirs {
+        ProjectDirs::from("io", "eagain", "it").expect("no valid $HOME")
+    }
+}
+
+pub mod git {
+    use std::path::Path;
+
+    use anyhow::{
+        anyhow,
+        bail,
+        ensure,
+    };
+    use zeroize::Zeroizing;
+
+    use crate::{
+        git::{
+            self,
+            if_not_found_none,
+            Refname,
+        },
+        keys::{
+            Agent,
+            Signer,
+        },
+        metadata::IdentityId,
+        ssh::{
+            self,
+            agent,
+        },
+    };
+
+    /// Last resort to override the signing key, if neither [`USER_SIGNING_KEY`]
+    /// nor [`SSH_KEY_COMMAND`] will cut it.
+    pub const IT_SIGNING_KEY: &str = "it.signingKey";
+    /// The default `it` identity to use.
+    pub const IT_ID: &str = "it.id";
+    /// Command to dynamically set the signing key, see
+    /// [`gpg.ssh.defaultKeyCommand`]
+    ///
+    /// [`gpg.ssh.defaultKeyCommand`]: https://git-scm.com/docs/git-config#Documentation/git-config.txt-gpgsshdefaultKeyCommand
+    pub const SSH_KEY_COMMAND: &str = "gpg.ssh.defaultKeyCommand";
+    /// The key to sign git and it objects with, see [`user.signingKey`]
+    ///
+    /// [`user.signingKey`]: https://git-scm.com/docs/git-config#Documentation/git-config.txt-usersigningKey
+    pub const USER_SIGNING_KEY: &str = "user.signingKey";
+    /// The default branch name, see [`init.defaultBranch`]
+    ///
+    /// If not set, the default branch is "master".
+    ///
+    /// [`init.defaultBranch`]: https://git-scm.com/docs/git-config#Documentation/git-config.txt-initdefaultBranch
+    pub const DEFAULT_BRANCH: &str = "init.defaultBranch";
+
+    #[allow(clippy::large_enum_variant)]
+    pub enum Key {
+        Secret(ssh::PrivateKey),
+        Public(ssh::PublicKey),
+    }
+
+    impl Key {
+        pub fn public(&self) -> &ssh::PublicKey {
+            match self {
+                Self::Secret(sk) => sk.public_key(),
+                Self::Public(pk) => pk,
+            }
+        }
+    }
+
+    pub fn signing_key(c: &git2::Config) -> crate::Result<Option<Key>> {
+        match if_not_found_none(c.get_string(IT_SIGNING_KEY))? {
+            Some(v) => ssh_signing_key_from_config_value(v).map(Some),
+            None => ssh_signing_key(c)
+                .transpose()
+                .or_else(|| ssh_key_command(c).transpose())
+                .transpose(),
+        }
+    }
+
+    pub fn signer<F>(c: &git2::Config, askpass: F) -> crate::Result<Box<dyn Signer>>
+    where
+        F: Fn(&str) -> crate::Result<Zeroizing<Vec<u8>>>,
+    {
+        let key = signing_key(c)?.ok_or_else(|| anyhow!("no signing key in git config"))?;
+        match key {
+            Key::Public(pk) => {
+                let client = agent::Client::from_env()?;
+                Ok(Box::new(Agent::new(client, pk.into())))
+            },
+            Key::Secret(sk) => {
+                if sk.is_encrypted() {
+                    let prompt = format!(
+                        "`it` wants to use the key {}. Please provide a passphrase to decrypt it",
+                        sk.public_key().to_openssh()?
+                    );
+                    for _ in 0..3 {
+                        let pass = askpass(&prompt)?;
+                        if let Ok(key) = sk.decrypt(pass) {
+                            return Ok(Box::new(key));
+                        }
+                    }
+                    bail!("unable to decrypt secret key");
+                } else {
+                    Ok(Box::new(sk))
+                }
+            },
+        }
+    }
+
+    pub fn identity(c: &git2::Config) -> crate::Result<Option<IdentityId>> {
+        if_not_found_none(c.get_string(IT_ID))?
+            .map(IdentityId::try_from)
+            .transpose()
+            .map_err(Into::into)
+    }
+
+    pub fn ssh_signing_key(cfg: &git2::Config) -> crate::Result<Option<Key>> {
+        if_not_found_none(cfg.get_string(USER_SIGNING_KEY))?
+            .map(ssh_signing_key_from_config_value)
+            .transpose()
+    }
+
+    pub(crate) fn ssh_signing_key_from_config_value<V: AsRef<str>>(v: V) -> crate::Result<Key> {
+        match v.as_ref().strip_prefix("key::") {
+            Some(lit) => {
+                let key = ssh::PublicKey::from_openssh(lit)?;
+                Ok(Key::Public(key))
+            },
+            None => {
+                let path = Path::new(v.as_ref());
+                ensure!(
+                    path.exists(),
+                    "{} is not a valid path to an SSH private key",
+                    path.display()
+                );
+                let key = ssh::PrivateKey::read_openssh_file(path)?;
+                Ok(Key::Secret(key))
+            },
+        }
+    }
+
+    pub fn ssh_key_command(cfg: &git2::Config) -> crate::Result<Option<Key>> {
+        let out = git::config_command(cfg, SSH_KEY_COMMAND)?;
+        let key = out
+            .as_deref()
+            .map(ssh::PublicKey::from_openssh)
+            .transpose()?
+            .map(Key::Public);
+
+        Ok(key)
+    }
+
+    pub fn default_branch(cfg: &git2::Config) -> crate::Result<Refname> {
+        if_not_found_none(cfg.get_string(DEFAULT_BRANCH))?
+            .unwrap_or_else(|| String::from("master"))
+            .try_into()
+            .map_err(Into::into)
+    }
+}
+pub use git::signer;
diff --git a/src/cmd.rs b/src/cmd.rs
new file mode 100644
index 0000000..85669f9
--- /dev/null
+++ b/src/cmd.rs
@@ -0,0 +1,117 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use crate::metadata::git::{
+    find_parent,
+    FromGit,
+    GitAlternates,
+    GitDrop,
+    GitIdentity,
+    GitMirrors,
+};
+
+mod util;
+use util::args;
+
+pub mod drop;
+pub mod id;
+pub mod mergepoint;
+pub mod patch;
+pub mod topic;
+pub mod ui;
+
+pub use crate::{
+    Error,
+    Result,
+};
+
+/// Error indicating that the command was cancelled at the user's request, eg.
+/// by pressing ESC in an interactive prompt.
+///
+/// By means of [`anyhow::Error::downcast`], this allows for exiting the program
+/// with a zero exit status, even though the invocation returned an `Err`.
+#[derive(Debug, thiserror::Error)]
+#[error("command aborted")]
+pub struct Aborted;
+
+/// Shortcut to return early from a command with an [`Aborted`] error.
+macro_rules! abort {
+    () => {
+        return Err(crate::Error::from(Aborted))
+    };
+}
+pub(crate) use abort;
+
+pub enum Output {
+    Val(Box<dyn erased_serde::Serialize>),
+    Iter(Box<dyn Iterator<Item = Result<Box<dyn erased_serde::Serialize>>>>),
+}
+
+impl Output {
+    pub fn val<T>(v: T) -> Self
+    where
+        T: serde::Serialize + 'static,
+    {
+        Self::Val(Box::new(v))
+    }
+
+    pub fn iter<T, U>(v: T) -> Self
+    where
+        T: IntoIterator<Item = Result<U>> + 'static,
+        U: serde::Serialize + 'static,
+    {
+        let iter = v
+            .into_iter()
+            .map(|x| x.map(|i| Box::new(i) as Box<dyn erased_serde::Serialize>));
+
+        Self::Iter(Box::new(iter))
+    }
+}
+
+trait IntoOutput {
+    fn into_output(self) -> Output;
+}
+
+impl<T> IntoOutput for T
+where
+    T: serde::Serialize + 'static,
+{
+    fn into_output(self) -> Output {
+        Output::Val(Box::new(self))
+    }
+}
+
+#[derive(Debug, clap::Subcommand)]
+pub enum Cmd {
+    /// Drop management
+    #[clap(subcommand)]
+    Drop(drop::Cmd),
+
+    /// Identity management
+    #[clap(subcommand)]
+    Id(id::Cmd),
+
+    /// Patches
+    #[clap(subcommand)]
+    Patch(patch::Cmd),
+
+    /// Merge points
+    #[clap(subcommand)]
+    MergePoint(mergepoint::Cmd),
+
+    /// Topics
+    #[clap(subcommand)]
+    Topic(topic::Cmd),
+}
+
+impl Cmd {
+    pub fn run(self) -> Result<Output> {
+        match self {
+            Self::Drop(cmd) => cmd.run(),
+            Self::Id(cmd) => cmd.run(),
+            Self::Patch(cmd) => cmd.run(),
+            Self::MergePoint(cmd) => cmd.run(),
+            Self::Topic(cmd) => cmd.run(),
+        }
+    }
+}
diff --git a/src/cmd/drop.rs b/src/cmd/drop.rs
new file mode 100644
index 0000000..208dbd6
--- /dev/null
+++ b/src/cmd/drop.rs
@@ -0,0 +1,205 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    ops::Deref,
+    path::PathBuf,
+};
+
+use anyhow::{
+    ensure,
+    Context,
+};
+use clap::ValueHint;
+use either::Either::Left;
+
+use crate::{
+    cmd,
+    metadata::{
+        self,
+        git::{
+            FromGit,
+            META_FILE_ALTERNATES,
+            META_FILE_MIRRORS,
+        },
+        IdentityId,
+        Signed,
+    },
+    patches::REF_HEADS_PATCHES,
+};
+
+mod bundles;
+pub use bundles::{
+    sync,
+    Bundles,
+    Sync,
+};
+
+mod edit;
+pub use edit::{
+    edit,
+    Edit,
+};
+
+mod init;
+pub use init::{
+    init,
+    Init,
+};
+
+mod serve;
+pub use serve::{
+    serve,
+    Serve,
+};
+
+mod snapshot;
+pub use snapshot::{
+    snapshot,
+    Snapshot,
+};
+
+mod show;
+pub use show::{
+    show,
+    Show,
+};
+
+mod unbundle;
+pub use unbundle::{
+    unbundle,
+    Unbundle,
+};
+
+#[derive(Debug, clap::Subcommand)]
+#[allow(clippy::large_enum_variant)]
+pub enum Cmd {
+    /// Initialise a drop
+    Init(Init),
+    /// Display the drop metadata
+    Show(Show),
+    /// Serve bundles and patch submission over HTTP
+    Serve(Serve),
+    /// Edit the drop metadata
+    Edit(Edit),
+    /// Manage patch bundles
+    #[clap(subcommand)]
+    Bundles(Bundles),
+    /// Take a snapshot of the patches received so far
+    Snapshot(Snapshot),
+    /// Unbundle the entire drop history
+    Unbundle(Unbundle),
+}
+
+impl Cmd {
+    pub fn run(self) -> cmd::Result<cmd::Output> {
+        match self {
+            Self::Init(args) => init(args).map(cmd::IntoOutput::into_output),
+            Self::Show(args) => show(args).map(cmd::IntoOutput::into_output),
+            Self::Serve(args) => serve(args).map(cmd::IntoOutput::into_output),
+            Self::Edit(args) => edit(args).map(cmd::IntoOutput::into_output),
+            Self::Bundles(cmd) => cmd.run(),
+            Self::Snapshot(args) => snapshot(args).map(cmd::IntoOutput::into_output),
+            Self::Unbundle(args) => unbundle(args).map(cmd::IntoOutput::into_output),
+        }
+    }
+}
+
+#[derive(Debug, clap::Args)]
+struct Common {
+    /// Path to the drop repository
+    #[clap(from_global)]
+    git_dir: PathBuf,
+    /// A list of paths to search for identity repositories
+    #[clap(
+        long,
+        value_parser,
+        value_name = "PATH",
+        env = "IT_ID_PATH",
+        default_value_t,
+        value_hint = ValueHint::DirPath,
+    )]
+    id_path: cmd::util::args::IdSearchPath,
+}
+
+fn find_id(
+    repo: &git2::Repository,
+    id_path: &[git2::Repository],
+    id: &IdentityId,
+) -> cmd::Result<Signed<metadata::Identity>> {
+    let signed = metadata::Identity::from_search_path(id_path, cmd::id::identity_ref(Left(id))?)?
+        .meta
+        .signed;
+
+    let verified_id = signed
+        .verify(cmd::find_parent(repo))
+        .with_context(|| format!("invalid identity {id}"))?;
+    ensure!(
+        &verified_id == id,
+        "ids do not match after verification: expected {id}, found {verified_id}",
+    );
+
+    Ok(signed)
+}
+
+#[derive(serde::Serialize, serde::Deserialize)]
+struct Editable {
+    description: metadata::drop::Description,
+    roles: metadata::drop::Roles,
+    custom: metadata::Custom,
+}
+
+impl From<metadata::Drop> for Editable {
+    fn from(
+        metadata::Drop {
+            description,
+            roles,
+            custom,
+            ..
+        }: metadata::Drop,
+    ) -> Self {
+        Self {
+            description,
+            roles,
+            custom,
+        }
+    }
+}
+
+impl TryFrom<Editable> for metadata::Drop {
+    type Error = crate::Error;
+
+    fn try_from(
+        Editable {
+            description,
+            roles,
+            custom,
+        }: Editable,
+    ) -> Result<Self, Self::Error> {
+        ensure!(!roles.root.ids.is_empty(), "drop role cannot be empty");
+        ensure!(
+            !roles.snapshot.ids.is_empty(),
+            "snapshot roles cannot be empty"
+        );
+        ensure!(
+            !roles.branches.is_empty(),
+            "at least one branch role is required"
+        );
+        for (name, ann) in &roles.branches {
+            ensure!(
+                !ann.role.ids.is_empty(),
+                "branch role {name} cannot be empty"
+            );
+            ensure!(name.starts_with("refs/heads/"), "not a branch {name}");
+            ensure!(name.deref() != REF_HEADS_PATCHES, "reserved branch {name}");
+        }
+
+        Ok(Self {
+            spec_version: crate::SPEC_VERSION,
+            description,
+            prev: None,
+            roles,
+            custom,
+        })
+    }
+}
diff --git a/src/cmd/drop/bundles.rs b/src/cmd/drop/bundles.rs
new file mode 100644
index 0000000..7c3e726
--- /dev/null
+++ b/src/cmd/drop/bundles.rs
@@ -0,0 +1,32 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use crate::cmd;
+
+mod prune;
+pub use prune::{
+    prune,
+    Prune,
+};
+
+mod sync;
+pub use sync::{
+    sync,
+    Sync,
+};
+
+#[derive(Debug, clap::Subcommand)]
+#[allow(clippy::large_enum_variant)]
+pub enum Bundles {
+    Sync(Sync),
+    Prune(Prune),
+}
+
+impl Bundles {
+    pub fn run(self) -> cmd::Result<cmd::Output> {
+        match self {
+            Self::Sync(args) => sync(args).map(cmd::IntoOutput::into_output),
+            Self::Prune(args) => prune(args).map(cmd::IntoOutput::into_output),
+        }
+    }
+}
diff --git a/src/cmd/drop/bundles/prune.rs b/src/cmd/drop/bundles/prune.rs
new file mode 100644
index 0000000..6bd984d
--- /dev/null
+++ b/src/cmd/drop/bundles/prune.rs
@@ -0,0 +1,113 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    collections::BTreeSet,
+    fs,
+    path::PathBuf,
+    str::FromStr,
+};
+
+use clap::ValueHint;
+
+use crate::{
+    bundle,
+    cfg,
+    cmd::{
+        self,
+        ui::{
+            info,
+            warn,
+        },
+    },
+    git,
+    patches::iter::dropped,
+};
+
+// TODO:
+//
+// - option to prune bundles made obsolete by snapshots
+
+#[derive(Debug, clap::Args)]
+pub struct Prune {
+    /// Path to the drop repository
+    #[clap(from_global)]
+    git_dir: PathBuf,
+    /// The directory where to write the bundle to
+    ///
+    /// Unless this is an absolute path, it is treated as relative to $GIT_DIR.
+    #[clap(
+        long,
+        value_parser,
+        value_name = "DIR",
+        default_value_os_t = cfg::paths::bundles().to_owned(),
+        value_hint = ValueHint::DirPath,
+    )]
+    bundle_dir: PathBuf,
+    /// Name of a git ref holding the drop metadata history
+    ///
+    /// All locally tracked drops should be given, otherwise bundles might get
+    /// pruned which are still being referred to.
+    #[clap(long = "drop", value_parser, value_name = "REF")]
+    drop_refs: Vec<String>,
+    /// Pretend to unlink, but don't
+    #[clap(long, value_parser)]
+    dry_run: bool,
+    /// Also remove location files (.uris)
+    #[clap(long, value_parser)]
+    remove_locations: bool,
+}
+
+pub fn prune(args: Prune) -> cmd::Result<Vec<bundle::Hash>> {
+    let repo = git::repo::open_bare(&args.git_dir)?;
+    let bundle_dir = if args.bundle_dir.is_relative() {
+        repo.path().join(args.bundle_dir)
+    } else {
+        args.bundle_dir
+    };
+
+    let mut seen = BTreeSet::new();
+    for short in &args.drop_refs {
+        let drop_ref = repo.resolve_reference_from_short_name(short)?;
+        let ref_name = drop_ref.name().expect("drop references to be valid utf8");
+        info!("Collecting bundle hashes from {ref_name} ...");
+        for record in dropped::records(&repo, ref_name) {
+            let record = record?;
+            seen.insert(*record.bundle_hash());
+        }
+    }
+
+    info!("Traversing bundle dir {} ...", bundle_dir.display());
+    let mut pruned = Vec::new();
+    for entry in fs::read_dir(&bundle_dir)? {
+        let entry = entry?;
+        let path = entry.path();
+        match path.extension() {
+            Some(ext) if ext == bundle::FILE_EXTENSION => {
+                let name = path.file_stem();
+                match name
+                    .and_then(|n| n.to_str())
+                    .and_then(|s| bundle::Hash::from_str(s).ok())
+                {
+                    Some(hash) => {
+                        if !seen.contains(&hash) {
+                            if !args.dry_run {
+                                fs::remove_file(&path)?;
+                            }
+                            pruned.push(hash);
+                        }
+                    },
+                    None => warn!("Ignoring {}: file name not a bundle hash", path.display()),
+                }
+            },
+            Some(ext) if ext == bundle::list::FILE_EXTENSION => {
+                if args.remove_locations {
+                    fs::remove_file(&path)?;
+                }
+            },
+            _ => warn!("Ignoring {}: missing .bundle", path.display()),
+        }
+    }
+
+    Ok(pruned)
+}
diff --git a/src/cmd/drop/bundles/sync.rs b/src/cmd/drop/bundles/sync.rs
new file mode 100644
index 0000000..21fd58b
--- /dev/null
+++ b/src/cmd/drop/bundles/sync.rs
@@ -0,0 +1,276 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    borrow::Cow,
+    mem,
+    num::NonZeroUsize,
+    path::PathBuf,
+    sync::{
+        Arc,
+        Mutex,
+    },
+    time::{
+        SystemTime,
+        UNIX_EPOCH,
+    },
+};
+
+use anyhow::anyhow;
+use clap::ValueHint;
+use either::Either::{
+    Left,
+    Right,
+};
+use threadpool::ThreadPool;
+use url::Url;
+
+use crate::{
+    bundle,
+    cfg,
+    cmd::{
+        self,
+        drop::Common,
+        ui::{
+            debug,
+            info,
+            warn,
+        },
+    },
+    git::{
+        self,
+        if_not_found_none,
+    },
+    patches::{
+        self,
+        iter::dropped,
+        record,
+        REF_IT_PATCHES,
+    },
+};
+
+/// Max number of locations to store from the remote for which we don't know if
+/// they'd succeed or not.
+pub const MAX_UNTRIED_LOCATIONS: usize = 3;
+
+#[derive(Debug, clap::Args)]
+pub struct Sync {
+    #[clap(flatten)]
+    common: Common,
+    /// The directory where to write the bundle to
+    ///
+    /// Unless this is an absolute path, it is treated as relative to $GIT_DIR.
+    #[clap(
+        long,
+        value_parser,
+        value_name = "DIR",
+        default_value_os_t = cfg::paths::bundles().to_owned(),
+        value_hint = ValueHint::DirPath,
+    )]
+    bundle_dir: PathBuf,
+    /// Name of the git ref holding the drop metadata history
+    #[clap(long = "drop", value_parser, value_name = "REF")]
+    drop_ref: Option<String>,
+    /// Base URL to fetch from
+    #[clap(long, value_parser, value_name = "URL", value_hint = ValueHint::Url)]
+    url: Url,
+    /// Fetch via IPFS
+    #[clap(
+        long,
+        value_parser,
+        value_name = "URL",
+        value_hint = ValueHint::Url,
+        env = "IPFS_GATEWAY",
+        default_value_t = Url::parse("https://ipfs.io").unwrap(),
+    )]
+    ipfs_gateway: Url,
+    /// Fetch even if the bundle already exists locally
+    #[clap(long, value_parser)]
+    overwrite: bool,
+    /// Ignore snapshots if encountered
+    #[clap(long, value_parser)]
+    no_snapshots: bool,
+    /// Maximum number of concurrent downloads. Default is the number of
+    /// available cores.
+    #[clap(short, long, value_parser, default_value_t = def_jobs())]
+    jobs: NonZeroUsize,
+}
+
+fn def_jobs() -> NonZeroUsize {
+    NonZeroUsize::new(num_cpus::get()).unwrap_or_else(|| NonZeroUsize::new(1).unwrap())
+}
+
+pub fn sync(args: Sync) -> cmd::Result<Vec<bundle::Info>> {
+    let repo = git::repo::open_bare(&args.common.git_dir)?;
+    let bundle_dir = if args.bundle_dir.is_relative() {
+        repo.path().join(args.bundle_dir)
+    } else {
+        args.bundle_dir
+    };
+    let drop_ref = match args.drop_ref {
+        Some(rev) => if_not_found_none(repo.resolve_reference_from_short_name(&rev))?
+            .ok_or_else(|| anyhow!("no ref matching {rev} found"))?
+            .name()
+            .ok_or_else(|| anyhow!("invalid drop"))?
+            .to_owned(),
+        None => REF_IT_PATCHES.to_owned(),
+    };
+    let base_url = args.url.join("bundles/")?;
+    let fetcher = Arc::new(Fetcher {
+        fetcher: bundle::Fetcher::default(),
+        bundle_dir,
+        base_url: base_url.clone(),
+        ipfs_gateway: args.ipfs_gateway,
+    });
+
+    let pool = ThreadPool::new(args.jobs.get());
+
+    let fetched = Arc::new(Mutex::new(Vec::new()));
+    let mut chasing_snaphots = false;
+    for record in dropped::records(&repo, &drop_ref) {
+        let record = record?;
+        let hexdig = record.bundle_hash().to_string();
+
+        if record.is_snapshot() {
+            if args.no_snapshots {
+                info!("Skipping snapshot bundle {hexdig}");
+                continue;
+            } else {
+                chasing_snaphots = true;
+            }
+        } else if chasing_snaphots && !record.is_mergepoint() {
+            info!("Skipping non-snapshot bundle {hexdig}");
+            continue;
+        }
+
+        if !args.overwrite && record.bundle_path(&fetcher.bundle_dir).exists() {
+            info!("Skipping existing bundle {hexdig}");
+            continue;
+        }
+
+        let record::BundleInfo {
+            info: bundle::Info { len, hash, .. },
+            prerequisites,
+            ..
+        } = record.bundle_info();
+        let url = base_url.join(&hexdig)?;
+
+        pool.execute({
+            let len = *len;
+            let hash = *hash;
+            let fetched = Arc::clone(&fetched);
+            let fetcher = Arc::clone(&fetcher);
+            move || match fetcher.try_fetch(url, len, &hash) {
+                Ok(hash) => fetched.lock().unwrap().push(hash),
+                Err(e) => warn!("Download failed: {e}"),
+            }
+        });
+
+        if record.is_snapshot() && prerequisites.is_empty() {
+            info!("Full snapshot encountered, stopping here");
+            break;
+        }
+    }
+
+    pool.join();
+    let fetched = {
+        let mut guard = fetched.lock().unwrap();
+        mem::take(&mut *guard)
+    };
+
+    Ok(fetched)
+}
+
+struct Fetcher {
+    fetcher: bundle::Fetcher,
+    bundle_dir: PathBuf,
+    base_url: Url,
+    ipfs_gateway: Url,
+}
+
+impl Fetcher {
+    fn try_fetch(&self, url: Url, len: u64, hash: &bundle::Hash) -> cmd::Result<bundle::Info> {
+        info!("Fetching {url} ...");
+
+        let expect = bundle::Expect {
+            len,
+            hash,
+            checksum: None,
+        };
+        let mut locations = Vec::new();
+        let (fetched, origin) = self
+            .fetcher
+            .fetch(&url, &self.bundle_dir, expect)
+            .and_then(|resp| match resp {
+                Right(fetched) => Ok((fetched, url)),
+                Left(lst) => {
+                    info!("{url}: response was a bundle list, trying alternate locations");
+
+                    let mut iter = lst.bundles.into_iter();
+                    let mut found = None;
+
+                    for bundle::Location { uri, .. } in &mut iter {
+                        if let Some(url) = self.url_from_uri(uri) {
+                            if let Ok(Right(info)) =
+                                self.fetcher.fetch(&url, &self.bundle_dir, expect)
+                            {
+                                found = Some((info, url));
+                                break;
+                            }
+                        }
+                    }
+
+                    // If there are bundle uris left, remember a few
+                    let now = SystemTime::now()
+                        .duration_since(UNIX_EPOCH)
+                        .expect("backwards system clock")
+                        .as_secs();
+                    locations.extend(
+                        iter
+                            // Don't let the remote inflate the priority of
+                            // unverified locations
+                            .filter(|loc| loc.creation_token.map(|t| t < now).unwrap_or(true))
+                            // Only known protocols, relative to base url
+                            .filter_map(|loc| {
+                                let url = loc.uri.abs(&self.base_url).ok()?;
+                                matches!(url.scheme(), "http" | "https" | "ipfs").then(|| {
+                                    bundle::Location {
+                                        uri: url.into_owned().into(),
+                                        ..loc
+                                    }
+                                })
+                            })
+                            .take(MAX_UNTRIED_LOCATIONS),
+                    );
+
+                    found.ok_or_else(|| anyhow!("{url}: no reachable location found"))
+                },
+            })?;
+
+        info!("Downloaded {hash} from {origin}");
+        let bundle = patches::Bundle::from_fetched(fetched)?;
+        bundle.write_bundle_list(locations)?;
+
+        Ok(bundle.into())
+    }
+
+    fn url_from_uri(&self, uri: bundle::Uri) -> Option<Url> {
+        uri.abs(&self.base_url)
+            .map_err(Into::into)
+            .and_then(|url: Cow<Url>| -> cmd::Result<Url> {
+                match url.scheme() {
+                    "http" | "https" => Ok(url.into_owned()),
+                    "ipfs" => {
+                        let cid = url
+                            .host_str()
+                            .ok_or_else(|| anyhow!("{url}: host part not an IPFS CID"))?;
+                        let url = self.ipfs_gateway.join(&format!("/ipfs/{cid}"))?;
+                        Ok(url)
+                    },
+                    _ => Err(anyhow!("{url}: unsupported protocol")),
+                }
+            })
+            .map_err(|e| debug!("discarding {}: {}", uri.as_str(), e))
+            .ok()
+    }
+}
diff --git a/src/cmd/drop/edit.rs b/src/cmd/drop/edit.rs
new file mode 100644
index 0000000..9103819
--- /dev/null
+++ b/src/cmd/drop/edit.rs
@@ -0,0 +1,368 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    iter,
+    path::PathBuf,
+};
+
+use anyhow::{
+    anyhow,
+    ensure,
+};
+
+use super::{
+    find_id,
+    Common,
+    Editable,
+};
+use crate::{
+    cfg,
+    cmd::{
+        self,
+        ui::{
+            self,
+            edit_commit_message,
+            edit_metadata,
+            info,
+        },
+        Aborted,
+    },
+    git::{
+        self,
+        refs,
+        Refname,
+    },
+    json,
+    keys::Signer,
+    metadata::{
+        self,
+        git::{
+            FromGit,
+            GitDrop,
+            META_FILE_ALTERNATES,
+            META_FILE_DROP,
+            META_FILE_MIRRORS,
+        },
+        IdentityId,
+        Metadata,
+    },
+    patches::{
+        self,
+        REF_HEADS_PATCHES,
+        REF_IT_PATCHES,
+    },
+};
+
+#[derive(Debug, clap::Args)]
+pub struct Edit {
+    #[clap(flatten)]
+    common: Common,
+    /// Commit message for this edit
+    ///
+    /// Like git, $EDITOR will be invoked if not specified.
+    #[clap(short, long, value_parser)]
+    message: Option<String>,
+
+    #[clap(subcommand)]
+    cmd: Option<Cmd>,
+}
+
+#[derive(Debug, clap::Subcommand)]
+enum Cmd {
+    /// Edit the mirrors file
+    Mirrors,
+    /// Edit the alternates file
+    Alternates,
+}
+
+#[derive(serde::Serialize)]
+pub struct Output {
+    repo: PathBuf,
+    #[serde(rename = "ref")]
+    refname: Refname,
+    #[serde(with = "crate::git::serde::oid")]
+    commit: git2::Oid,
+}
+
+pub fn edit(args: Edit) -> cmd::Result<Output> {
+    let Common { git_dir, id_path } = args.common;
+
+    let repo = git::repo::open(git_dir)?;
+    let drop_ref = if repo.is_bare() {
+        REF_HEADS_PATCHES
+    } else {
+        REF_IT_PATCHES
+    }
+    .parse()
+    .unwrap();
+
+    let id_path = id_path.open_git();
+    git::add_alternates(&repo, &id_path)?;
+    let cfg = repo.config()?.snapshot()?;
+    let signer = cfg::signer(&cfg, ui::askpass)?;
+    let signer_id = SignerIdentity::new(&signer, &repo, &cfg, &id_path)?;
+    let meta = metadata::Drop::from_tip(&repo, &drop_ref)?;
+
+    let s = EditState {
+        repo,
+        id_path,
+        signer,
+        signer_id,
+        drop_ref,
+        meta,
+    };
+
+    match args.cmd {
+        None => s.edit_drop(args.message),
+        Some(Cmd::Mirrors) => s.edit_mirrors(args.message),
+        Some(Cmd::Alternates) => s.edit_alternates(args.message),
+    }
+}
+
+struct EditState<S> {
+    repo: git2::Repository,
+    id_path: Vec<git2::Repository>,
+    signer: S,
+    signer_id: SignerIdentity,
+    drop_ref: Refname,
+    meta: GitDrop,
+}
+
+impl<S: Signer + 'static> EditState<S> {
+    fn edit_drop(mut self, message: Option<String>) -> cmd::Result<Output> {
+        let GitDrop {
+            hash: parent_hash,
+            signed: metadata::Signed { signed: parent, .. },
+        } = self.meta;
+
+        ensure!(
+            self.signer_id.can_edit_drop(&parent),
+            "signer identity not allowed to edit the drop metadata"
+        );
+
+        let mut meta: metadata::Drop = edit_metadata(Editable::from(parent.clone()))?.try_into()?;
+        if meta.canonicalise()? == parent.canonicalise()? {
+            info!("Document unchanged");
+            cmd::abort!();
+        }
+        meta.prev = Some(parent_hash);
+
+        let signed = Metadata::drop(&meta).sign(iter::once(&mut self.signer as &mut dyn Signer))?;
+
+        let mut tx = refs::Transaction::new(&self.repo)?;
+        let drop_ref = tx.lock_ref(self.drop_ref)?;
+
+        let parent = self
+            .repo
+            .find_reference(drop_ref.name())?
+            .peel_to_commit()?;
+        let parent_tree = parent.tree()?;
+        let mut root = self.repo.treebuilder(Some(&parent_tree))?;
+        patches::Record::remove_from(&mut root)?;
+
+        let mut ids = self
+            .repo
+            .treebuilder(get_tree(&self.repo, &root, "ids")?.as_ref())?;
+        let identities = meta
+            .roles
+            .ids()
+            .into_iter()
+            .map(|id| find_id(&self.repo, &self.id_path, &id).map(|signed| (id, signed)))
+            .collect::<Result<Vec<_>, _>>()?;
+        for (iid, id) in identities {
+            let iid = iid.to_string();
+            let mut tb = self
+                .repo
+                .treebuilder(get_tree(&self.repo, &ids, &iid)?.as_ref())?;
+            metadata::identity::fold_to_tree(&self.repo, &mut tb, id)?;
+            ids.insert(&iid, tb.write()?, git2::FileMode::Tree.into())?;
+        }
+        root.insert("ids", ids.write()?, git2::FileMode::Tree.into())?;
+
+        root.insert(
+            META_FILE_DROP,
+            json::to_blob(&self.repo, &signed)?,
+            git2::FileMode::Blob.into(),
+        )?;
+        let tree = self.repo.find_tree(root.write()?)?;
+
+        let msg = message.map(Ok).unwrap_or_else(|| {
+            edit_commit_message(&self.repo, drop_ref.name(), &parent_tree, &tree)
+        })?;
+        let commit = git::commit_signed(&mut self.signer, &self.repo, msg, &tree, &[&parent])?;
+        drop_ref.set_target(commit, "it: metadata edit");
+
+        tx.commit()?;
+
+        Ok(Output {
+            repo: self.repo.path().to_owned(),
+            refname: drop_ref.into(),
+            commit,
+        })
+    }
+
+    pub fn edit_mirrors(mut self, message: Option<String>) -> cmd::Result<Output> {
+        ensure!(
+            self.signer_id.can_edit_mirrors(&self.meta.signed.signed),
+            "signer identity not allowed to edit mirrors"
+        );
+
+        let prev = metadata::Mirrors::from_tip(&self.repo, &self.drop_ref)
+            .map(|m| m.signed.signed)
+            .or_else(|e| {
+                if e.is::<metadata::git::error::FileNotFound>() {
+                    Ok(Default::default())
+                } else {
+                    Err(e)
+                }
+            })?;
+        let prev_canonical = prev.canonicalise()?;
+        let meta = edit_metadata(prev)?;
+        if meta.canonicalise()? == prev_canonical {
+            info!("Document unchanged");
+            cmd::abort!();
+        }
+
+        let signed =
+            Metadata::mirrors(meta).sign(iter::once(&mut self.signer as &mut dyn Signer))?;
+
+        let mut tx = refs::Transaction::new(&self.repo)?;
+        let drop_ref = tx.lock_ref(self.drop_ref)?;
+
+        let parent = self
+            .repo
+            .find_reference(drop_ref.name())?
+            .peel_to_commit()?;
+        let parent_tree = parent.tree()?;
+        let mut root = self.repo.treebuilder(Some(&parent_tree))?;
+        patches::Record::remove_from(&mut root)?;
+        root.insert(
+            META_FILE_MIRRORS,
+            json::to_blob(&self.repo, &signed)?,
+            git2::FileMode::Blob.into(),
+        )?;
+        let tree = self.repo.find_tree(root.write()?)?;
+
+        let msg = message.map(Ok).unwrap_or_else(|| {
+            edit_commit_message(&self.repo, drop_ref.name(), &parent_tree, &tree)
+        })?;
+        let commit = git::commit_signed(&mut self.signer, &self.repo, msg, &tree, &[&parent])?;
+        drop_ref.set_target(commit, "it: mirrors edit");
+
+        tx.commit()?;
+
+        Ok(Output {
+            repo: self.repo.path().to_owned(),
+            refname: drop_ref.into(),
+            commit,
+        })
+    }
+
+    pub fn edit_alternates(mut self, message: Option<String>) -> cmd::Result<Output> {
+        ensure!(
+            self.signer_id.can_edit_mirrors(&self.meta.signed.signed),
+            "signer identity not allowed to edit alternates"
+        );
+
+        let prev = metadata::Alternates::from_tip(&self.repo, &self.drop_ref)
+            .map(|m| m.signed.signed)
+            .or_else(|e| {
+                if e.is::<metadata::git::error::FileNotFound>() {
+                    Ok(Default::default())
+                } else {
+                    Err(e)
+                }
+            })?;
+        let prev_canonical = prev.canonicalise()?;
+        let meta = edit_metadata(prev)?;
+        if meta.canonicalise()? == prev_canonical {
+            info!("Document unchanged");
+            cmd::abort!();
+        }
+
+        let signed =
+            Metadata::alternates(meta).sign(iter::once(&mut self.signer as &mut dyn Signer))?;
+
+        let mut tx = refs::Transaction::new(&self.repo)?;
+        let drop_ref = tx.lock_ref(self.drop_ref)?;
+
+        let parent = self
+            .repo
+            .find_reference(drop_ref.name())?
+            .peel_to_commit()?;
+        let parent_tree = parent.tree()?;
+        let mut root = self.repo.treebuilder(Some(&parent_tree))?;
+        patches::Record::remove_from(&mut root)?;
+        root.insert(
+            META_FILE_ALTERNATES,
+            json::to_blob(&self.repo, &signed)?,
+            git2::FileMode::Blob.into(),
+        )?;
+        let tree = self.repo.find_tree(root.write()?)?;
+
+        let msg = message.map(Ok).unwrap_or_else(|| {
+            edit_commit_message(&self.repo, drop_ref.name(), &parent_tree, &tree)
+        })?;
+        let commit = git::commit_signed(&mut self.signer, &self.repo, msg, &tree, &[&parent])?;
+        drop_ref.set_target(commit, "it: alternates edit");
+
+        tx.commit()?;
+
+        Ok(Output {
+            repo: self.repo.path().to_owned(),
+            refname: drop_ref.into(),
+            commit,
+        })
+    }
+}
+
+fn get_tree<'a>(
+    repo: &'a git2::Repository,
+    builder: &git2::TreeBuilder,
+    name: &str,
+) -> cmd::Result<Option<git2::Tree<'a>>> {
+    if let Some(entry) = builder.get(name)? {
+        return Ok(Some(
+            entry
+                .to_object(repo)?
+                .into_tree()
+                .map_err(|_| anyhow!("{name} is not a tree"))?,
+        ));
+    }
+
+    Ok(None)
+}
+
+struct SignerIdentity {
+    id: IdentityId,
+}
+
+impl SignerIdentity {
+    pub fn new<S: Signer>(
+        signer: &S,
+        repo: &git2::Repository,
+        cfg: &git2::Config,
+        id_path: &[git2::Repository],
+    ) -> cmd::Result<Self> {
+        let id =
+            cfg::git::identity(cfg)?.ok_or_else(|| anyhow!("signer identity not in gitconfig"))?;
+        let meta = find_id(repo, id_path, &id)?;
+        let keyid = metadata::KeyId::from(signer.ident());
+
+        ensure!(
+            meta.signed.keys.contains_key(&keyid),
+            "signing key {keyid} is not in identity {id}"
+        );
+
+        Ok(Self { id })
+    }
+
+    pub fn can_edit_drop(&self, parent: &metadata::Drop) -> bool {
+        parent.roles.root.ids.contains(&self.id)
+    }
+
+    pub fn can_edit_mirrors(&self, parent: &metadata::Drop) -> bool {
+        parent.roles.mirrors.ids.contains(&self.id)
+    }
+}
diff --git a/src/cmd/drop/init.rs b/src/cmd/drop/init.rs
new file mode 100644
index 0000000..b843255
--- /dev/null
+++ b/src/cmd/drop/init.rs
@@ -0,0 +1,194 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    iter,
+    num::NonZeroUsize,
+    path::PathBuf,
+};
+
+use anyhow::{
+    anyhow,
+    ensure,
+};
+
+use super::{
+    find_id,
+    Common,
+    Editable,
+};
+use crate::{
+    cfg,
+    cmd::{
+        self,
+        args::Refname,
+        ui::{
+            self,
+            edit_metadata,
+        },
+    },
+    git::{
+        self,
+        if_not_found_none,
+        refs,
+    },
+    json,
+    metadata::{
+        self,
+        git::META_FILE_DROP,
+        Metadata,
+    },
+    patches::{
+        REF_HEADS_PATCHES,
+        REF_IT_PATCHES,
+    },
+};
+
+#[derive(Debug, clap::Args)]
+pub struct Init {
+    #[clap(flatten)]
+    common: Common,
+    /// A description for this drop instance, max. 128 characters
+    #[clap(long, value_parser, value_name = "STRING")]
+    description: metadata::drop::Description,
+    /// If the repository does not already exist, initialise it as non-bare
+    ///
+    /// A drop is usually initialised inside an already existing git repository,
+    /// or as a standalone drop repository. The latter is advisable for serving
+    /// over the network.
+    ///
+    /// When init is given a directory which does not already exist, it is
+    /// assumed that a standalone drop should be created, and thus the
+    /// repository is initialised as bare. This behaviour can be overridden
+    /// by --no-bare.
+    #[clap(long, value_parser)]
+    no_bare: bool,
+}
+
+#[derive(serde::Serialize)]
+pub struct Output {
+    repo: PathBuf,
+    #[serde(rename = "ref")]
+    refname: Refname,
+    #[serde(with = "crate::git::serde::oid")]
+    commit: git2::Oid,
+}
+
+pub fn init(args: Init) -> cmd::Result<Output> {
+    let Common { git_dir, id_path } = args.common;
+    let drop_ref: Refname = REF_IT_PATCHES.parse().unwrap();
+
+    let repo = git::repo::open_or_init(
+        git_dir,
+        git::repo::InitOpts {
+            bare: !args.no_bare,
+            description: "`it` drop",
+            initial_head: &drop_ref,
+        },
+    )?;
+
+    let mut tx = refs::Transaction::new(&repo)?;
+    let drop_ref = tx.lock_ref(drop_ref)?;
+    ensure!(
+        if_not_found_none(repo.refname_to_id(drop_ref.name()))?.is_none(),
+        "{} already exists",
+        drop_ref
+    );
+
+    let id_path = id_path.open_git();
+    git::add_alternates(&repo, &id_path)?;
+
+    let cfg = repo.config()?.snapshot()?;
+    let mut signer = cfg::signer(&cfg, ui::askpass)?;
+    let signer_id = {
+        let iid =
+            cfg::git::identity(&cfg)?.ok_or_else(|| anyhow!("signer identity not in gitconfig"))?;
+        let id = find_id(&repo, &id_path, &iid)?;
+        let keyid = metadata::KeyId::from(signer.ident());
+        ensure!(
+            id.signed.keys.contains_key(&keyid),
+            "signing key {keyid} is not in identity {iid}"
+        );
+
+        iid
+    };
+
+    let default = {
+        let default_role = metadata::drop::Role {
+            ids: [signer_id].into(),
+            threshold: NonZeroUsize::new(1).unwrap(),
+        };
+        let default_branch = cfg::git::default_branch(&cfg)?;
+
+        metadata::Drop {
+            spec_version: crate::SPEC_VERSION,
+            description: args.description,
+            prev: None,
+            custom: Default::default(),
+            roles: metadata::drop::Roles {
+                root: default_role.clone(),
+                snapshot: default_role.clone(),
+                mirrors: default_role.clone(),
+                branches: [(
+                    default_branch,
+                    metadata::drop::Annotated {
+                        role: default_role,
+                        description: metadata::drop::Description::try_from(
+                            "the default branch".to_owned(),
+                        )
+                        .unwrap(),
+                    },
+                )]
+                .into(),
+            },
+        }
+    };
+    let meta: metadata::Drop = edit_metadata(Editable::from(default))?.try_into()?;
+    ensure!(
+        meta.roles.root.ids.contains(&signer_id),
+        "signing identity {signer_id} is lacking the drop role required to sign the metadata"
+    );
+    let signed = Metadata::drop(&meta).sign(iter::once(&mut signer))?;
+
+    let mut root = repo.treebuilder(None)?;
+    let mut ids = repo.treebuilder(None)?;
+    let identities = meta
+        .roles
+        .ids()
+        .into_iter()
+        .map(|id| find_id(&repo, &id_path, &id).map(|signed| (id, signed)))
+        .collect::<Result<Vec<_>, _>>()?;
+    for (iid, id) in identities {
+        let iid = iid.to_string();
+        let mut tb = repo.treebuilder(None)?;
+        metadata::identity::fold_to_tree(&repo, &mut tb, id)?;
+        ids.insert(&iid, tb.write()?, git2::FileMode::Tree.into())?;
+    }
+    root.insert("ids", ids.write()?, git2::FileMode::Tree.into())?;
+    root.insert(
+        META_FILE_DROP,
+        json::to_blob(&repo, &signed)?,
+        git2::FileMode::Blob.into(),
+    )?;
+    let tree = repo.find_tree(root.write()?)?;
+    let msg = format!("Create drop '{}'", meta.description);
+    let commit = git::commit_signed(&mut signer, &repo, msg, &tree, &[])?;
+
+    if repo.is_bare() {
+        // Arrange refs to be `git-clone`-friendly
+        let heads_patches = tx.lock_ref(REF_HEADS_PATCHES.parse()?)?;
+        heads_patches.set_target(commit, "it: create");
+        drop_ref.set_symbolic_target(heads_patches.name().clone(), String::new());
+        repo.set_head(heads_patches.name())?;
+    } else {
+        drop_ref.set_target(commit, "it: create");
+    }
+
+    tx.commit()?;
+
+    Ok(Output {
+        repo: repo.path().to_owned(),
+        refname: drop_ref.into(),
+        commit,
+    })
+}
diff --git a/src/cmd/drop/serve.rs b/src/cmd/drop/serve.rs
new file mode 100644
index 0000000..7540d58
--- /dev/null
+++ b/src/cmd/drop/serve.rs
@@ -0,0 +1,140 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    fs::File,
+    io::Read,
+    path::PathBuf,
+    str::FromStr,
+};
+
+use clap::ValueHint;
+use url::Url;
+
+use super::Common;
+use crate::{
+    cfg,
+    cmd::{
+        self,
+        args::Refname,
+    },
+    http,
+    patches::{
+        REF_IT_BUNDLES,
+        REF_IT_PATCHES,
+        REF_IT_SEEN,
+    },
+};
+
+#[derive(Debug, clap::Args)]
+pub struct Serve {
+    #[clap(flatten)]
+    common: Common,
+    /// The directory where to write the bundle to
+    ///
+    /// Unless this is an absolute path, it is treated as relative to $GIT_DIR.
+    #[clap(
+        long,
+        value_parser,
+        value_name = "DIR",
+        default_value_os_t = cfg::paths::bundles().to_owned(),
+        value_hint = ValueHint::DirPath,
+    )]
+    bundle_dir: PathBuf,
+    /// Ref prefix under which to store the refs contained in patch bundles
+    #[clap(
+        long,
+        value_parser,
+        value_name = "REF",
+        default_value_t = Refname::from_str(REF_IT_BUNDLES).unwrap()
+    )]
+    unbundle_prefix: Refname,
+    /// The refname anchoring the seen objects tree
+    #[clap(
+        long,
+        value_parser,
+        value_name = "REF",
+        default_value_t = Refname::from_str(REF_IT_SEEN).unwrap()
+    )]
+    seen_ref: Refname,
+    /// 'host:port' to listen on
+    #[clap(
+        long,
+        value_parser,
+        value_name = "HOST:PORT",
+        default_value = "127.0.0.1:8084"
+    )]
+    listen: String,
+    /// Number of threads to use for the server
+    ///
+    /// If not set, the number of available cores is used.
+    #[clap(long, value_parser, value_name = "INT")]
+    threads: Option<usize>,
+    /// PEM-encoded TLS certificate
+    ///
+    /// Requires 'tls-key'. If not set (the default), the server will not use
+    /// TLS.
+    #[clap(
+        long,
+        value_parser,
+        value_name = "FILE",
+        requires = "tls_key",
+        value_hint = ValueHint::FilePath
+    )]
+    tls_cert: Option<PathBuf>,
+    /// PEM-encoded TLS private key
+    ///
+    /// Requires 'tls-cert'. If not set (the default), the server will not use
+    /// TLS.
+    #[clap(
+        long,
+        value_parser,
+        value_name = "FILE",
+        requires = "tls_cert",
+        value_hint = ValueHint::FilePath
+    )]
+    tls_key: Option<PathBuf>,
+    /// IPFS API to publish received patch bundle to
+    #[clap(
+        long,
+        value_parser,
+        value_name = "URL",
+        value_hint = ValueHint::Url,
+    )]
+    ipfs_api: Option<Url>,
+}
+
+#[derive(serde::Serialize)]
+pub struct Output;
+
+pub fn serve(args: Serve) -> cmd::Result<Output> {
+    let tls = args
+        .tls_cert
+        .map(|cert_path| -> cmd::Result<http::SslConfig> {
+            let mut certificate = Vec::new();
+            let mut private_key = Vec::new();
+            File::open(cert_path)?.read_to_end(&mut certificate)?;
+            File::open(args.tls_key.expect("presence of 'tls-key' ensured by clap"))?
+                .read_to_end(&mut private_key)?;
+
+            Ok(http::SslConfig {
+                certificate,
+                private_key,
+            })
+        })
+        .transpose()?;
+
+    http::serve(
+        args.listen,
+        http::Options {
+            git_dir: args.common.git_dir,
+            bundle_dir: args.bundle_dir,
+            unbundle_prefix: args.unbundle_prefix.into(),
+            drop_ref: REF_IT_PATCHES.into(),
+            seen_ref: args.seen_ref.into(),
+            threads: args.threads,
+            tls,
+            ipfs_api: args.ipfs_api,
+        },
+    )
+}
diff --git a/src/cmd/drop/show.rs b/src/cmd/drop/show.rs
new file mode 100644
index 0000000..e3fdcfc
--- /dev/null
+++ b/src/cmd/drop/show.rs
@@ -0,0 +1,208 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    collections::BTreeMap,
+    io,
+    path::PathBuf,
+};
+
+use anyhow::Context;
+
+use super::{
+    Common,
+    META_FILE_ALTERNATES,
+    META_FILE_MIRRORS,
+};
+use crate::{
+    cmd::{
+        self,
+        util::args::Refname,
+        FromGit as _,
+        GitAlternates,
+        GitDrop,
+        GitMirrors,
+    },
+    git,
+    metadata::{
+        self,
+        ContentHash,
+        IdentityId,
+        KeySet,
+    },
+    patches::REF_IT_PATCHES,
+};
+
+#[derive(Debug, clap::Args)]
+pub struct Show {
+    #[clap(flatten)]
+    common: Common,
+    /// Name of the git ref holding the drop metadata history
+    #[clap(
+        long = "drop",
+        value_parser,
+        value_name = "REF",
+        default_value_t = REF_IT_PATCHES.parse().unwrap(),
+    )]
+    drop_ref: Refname,
+}
+
+#[derive(serde::Serialize)]
+pub struct Output {
+    repo: PathBuf,
+    refname: Refname,
+    drop: Data<metadata::Drop>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    mirrors: Option<Data<metadata::Mirrors>>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    alternates: Option<Data<metadata::Alternates>>,
+}
+
+#[derive(serde::Serialize)]
+pub struct Data<T> {
+    hash: ContentHash,
+    status: Status,
+    json: T,
+}
+
+#[derive(serde::Serialize)]
+#[serde(rename_all = "UPPERCASE")]
+pub enum Status {
+    Verified,
+    #[serde(with = "crate::serde::display")]
+    Invalid(metadata::error::Verification),
+}
+
+impl From<Result<(), metadata::error::Verification>> for Status {
+    fn from(r: Result<(), metadata::error::Verification>) -> Self {
+        r.map(|()| Self::Verified).unwrap_or_else(Self::Invalid)
+    }
+}
+
+pub fn show(args: Show) -> cmd::Result<Output> {
+    let Common { git_dir, .. } = args.common;
+    let drop_ref = args.drop_ref;
+
+    let repo = git::repo::open(git_dir)?;
+
+    let GitDrop {
+        hash,
+        signed: metadata::Signed {
+            signed: drop,
+            signatures,
+        },
+    } = metadata::Drop::from_tip(&repo, &drop_ref)?;
+
+    let mut signer_cache = SignerCache::new(&repo, &drop_ref)?;
+    let status = drop
+        .verify(
+            &signatures,
+            cmd::find_parent(&repo),
+            find_signer(&mut signer_cache),
+        )
+        .into();
+
+    let mut mirrors = None;
+    let mut alternates = None;
+
+    let tree = repo.find_reference(&drop_ref)?.peel_to_commit()?.tree()?;
+    if let Some(entry) = tree.get_name(META_FILE_MIRRORS) {
+        let blob = entry.to_object(&repo)?.peel_to_blob()?;
+        let GitMirrors { hash, signed } = metadata::Mirrors::from_blob(&blob)?;
+        let status = drop
+            .verify_mirrors(&signed, find_signer(&mut signer_cache))
+            .into();
+
+        mirrors = Some(Data {
+            hash,
+            status,
+            json: signed.signed,
+        });
+    }
+
+    if let Some(entry) = tree.get_name(META_FILE_ALTERNATES) {
+        let blob = entry.to_object(&repo)?.peel_to_blob()?;
+        let GitAlternates { hash, signed } = metadata::Alternates::from_blob(&blob)?;
+        let status = drop
+            .verify_alternates(&signed, find_signer(&mut signer_cache))
+            .into();
+
+        alternates = Some(Data {
+            hash,
+            status,
+            json: signed.signed,
+        });
+    }
+
+    Ok(Output {
+        repo: repo.path().to_owned(),
+        refname: drop_ref,
+        drop: Data {
+            hash,
+            status,
+            json: drop,
+        },
+        mirrors,
+        alternates,
+    })
+}
+
+struct SignerCache<'a> {
+    repo: &'a git2::Repository,
+    root: git2::Tree<'a>,
+    keys: BTreeMap<IdentityId, KeySet<'static>>,
+}
+
+impl<'a> SignerCache<'a> {
+    pub(self) fn new(repo: &'a git2::Repository, refname: &Refname) -> git::Result<Self> {
+        let root = {
+            let id = repo
+                .find_reference(refname)?
+                .peel_to_tree()?
+                .get_name("ids")
+                .ok_or_else(|| {
+                    git2::Error::new(
+                        git2::ErrorCode::NotFound,
+                        git2::ErrorClass::Tree,
+                        "'ids' tree not found",
+                    )
+                })?
+                .id();
+            repo.find_tree(id)?
+        };
+        let keys = BTreeMap::new();
+
+        Ok(Self { repo, root, keys })
+    }
+}
+
+fn find_signer<'a>(
+    cache: &'a mut SignerCache,
+) -> impl FnMut(&IdentityId) -> io::Result<KeySet<'static>> + 'a {
+    fn go(
+        repo: &git2::Repository,
+        root: &git2::Tree,
+        keys: &mut BTreeMap<IdentityId, KeySet<'static>>,
+        id: &IdentityId,
+    ) -> cmd::Result<KeySet<'static>> {
+        match keys.get(id) {
+            Some(keys) => Ok(keys.clone()),
+            None => {
+                let (id, verified) = metadata::identity::find_in_tree(repo, root, id)
+                    .with_context(|| format!("identity {id} failed to verify"))?
+                    .into_parts();
+                keys.insert(id, verified.keys.clone());
+                Ok(verified.keys)
+            },
+        }
+    }
+
+    |id| go(cache.repo, &cache.root, &mut cache.keys, id).map_err(as_io)
+}
+
+fn as_io<E>(e: E) -> io::Error
+where
+    E: Into<Box<dyn std::error::Error + Send + Sync>>,
+{
+    io::Error::new(io::ErrorKind::Other, e)
+}
diff --git a/src/cmd/drop/snapshot.rs b/src/cmd/drop/snapshot.rs
new file mode 100644
index 0000000..b1348d3
--- /dev/null
+++ b/src/cmd/drop/snapshot.rs
@@ -0,0 +1,20 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use crate::{
+    cmd::{
+        self,
+        patch,
+    },
+    patches,
+};
+
+#[derive(Debug, clap::Args)]
+pub struct Snapshot {
+    #[clap(flatten)]
+    common: patch::Common,
+}
+
+pub fn snapshot(Snapshot { common }: Snapshot) -> cmd::Result<patches::Record> {
+    patch::create(patch::Kind::Snapshot { common })
+}
diff --git a/src/cmd/drop/unbundle.rs b/src/cmd/drop/unbundle.rs
new file mode 100644
index 0000000..a9c9f77
--- /dev/null
+++ b/src/cmd/drop/unbundle.rs
@@ -0,0 +1,93 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    collections::BTreeMap,
+    path::PathBuf,
+};
+
+use anyhow::anyhow;
+use clap::ValueHint;
+
+use crate::{
+    cmd,
+    git::{
+        self,
+        if_not_found_none,
+        refs,
+        Refname,
+    },
+    patches::{
+        self,
+        iter::dropped,
+        Bundle,
+        REF_IT_BUNDLES,
+        REF_IT_PATCHES,
+    },
+    paths,
+};
+
+// TODO:
+//
+// - require drop metadata verification
+// - abort if existing ref would be set to a different target (or --force)
+// - honour snapshots
+//
+
+#[derive(Debug, clap::Args)]
+pub struct Unbundle {
+    #[clap(from_global)]
+    git_dir: PathBuf,
+    /// The directory where to write the bundle to
+    ///
+    /// Unless this is an absolute path, it is treated as relative to $GIT_DIR.
+    #[clap(
+        long,
+        value_parser,
+        value_name = "DIR",
+        default_value_os_t = paths::bundles().to_owned(),
+        value_hint = ValueHint::DirPath,
+    )]
+    bundle_dir: PathBuf,
+    /// The drop history to find the topic in
+    #[clap(value_parser)]
+    drop: Option<String>,
+}
+
+#[derive(serde::Serialize)]
+pub struct Output {
+    updated: BTreeMap<Refname, git::serde::oid::Oid>,
+}
+
+pub fn unbundle(args: Unbundle) -> cmd::Result<Output> {
+    let repo = git::repo::open(&args.git_dir)?;
+    let bundle_dir = if args.bundle_dir.is_relative() {
+        repo.path().join(args.bundle_dir)
+    } else {
+        args.bundle_dir
+    };
+    let drop = match args.drop {
+        Some(rev) => if_not_found_none(repo.resolve_reference_from_short_name(&rev))?
+            .ok_or_else(|| anyhow!("no ref matching {rev} found"))?
+            .name()
+            .ok_or_else(|| anyhow!("invalid drop"))?
+            .to_owned(),
+        None => REF_IT_PATCHES.to_owned(),
+    };
+
+    let odb = repo.odb()?;
+    let mut tx = refs::Transaction::new(&repo)?;
+    let mut up = BTreeMap::new();
+    for rec in dropped::records_rev(&repo, &drop) {
+        let rec = rec?;
+        let bundle = Bundle::from_stored(&bundle_dir, rec.bundle_info().as_expect())?;
+        bundle.packdata()?.index(&odb)?;
+        let updated = patches::unbundle(&odb, &mut tx, REF_IT_BUNDLES, &rec)?;
+        for (name, oid) in updated {
+            up.insert(name, oid.into());
+        }
+    }
+    tx.commit()?;
+
+    Ok(Output { updated: up })
+}
diff --git a/src/cmd/id.rs b/src/cmd/id.rs
new file mode 100644
index 0000000..7504489
--- /dev/null
+++ b/src/cmd/id.rs
@@ -0,0 +1,188 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    collections::BTreeSet,
+    num::NonZeroUsize,
+    path::PathBuf,
+};
+
+use anyhow::{
+    anyhow,
+    ensure,
+};
+use clap::ValueHint;
+use either::{
+    Either,
+    Left,
+    Right,
+};
+use url::Url;
+
+use crate::{
+    cfg,
+    cmd::{
+        self,
+        args::Refname,
+    },
+    git,
+    metadata::{
+        self,
+        git::META_FILE_ID,
+        IdentityId,
+    },
+    paths,
+};
+
+mod edit;
+pub use edit::{
+    edit,
+    Edit,
+};
+
+mod init;
+pub use init::{
+    init,
+    Init,
+};
+
+mod show;
+pub use show::{
+    show,
+    Show,
+};
+
+mod sign;
+pub use sign::{
+    sign,
+    Sign,
+};
+
+#[derive(Debug, clap::Subcommand)]
+#[allow(clippy::large_enum_variant)]
+pub enum Cmd {
+    /// Initialise a fresh identity
+    Init(Init),
+    /// Display the identity docment
+    Show(Show),
+    /// Edit the identity document
+    Edit(Edit),
+    /// Sign a proposed identity document
+    Sign(Sign),
+}
+
+impl Cmd {
+    pub fn run(self) -> cmd::Result<cmd::Output> {
+        match self {
+            Self::Init(args) => init(args).map(cmd::IntoOutput::into_output),
+            Self::Show(args) => show(args).map(cmd::IntoOutput::into_output),
+            Self::Edit(args) => edit(args).map(cmd::IntoOutput::into_output),
+            Self::Sign(args) => sign(args).map(cmd::IntoOutput::into_output),
+        }
+    }
+}
+
+#[derive(Clone, Debug, clap::Args)]
+pub struct Common {
+    /// Path to the 'keyring' repository
+    // nb. not using from_global here -- current_dir doesn't make sense here as
+    // the default
+    #[clap(
+        long,
+        value_parser,
+        value_name = "DIR",
+        env = "GIT_DIR",
+        default_value_os_t = paths::ids(),
+        value_hint = ValueHint::DirPath,
+    )]
+    git_dir: PathBuf,
+    /// Identity to operate on
+    ///
+    /// If not set as an option nor in the environment, the value of `it.id` in
+    /// the git config is tried.
+    #[clap(short = 'I', long = "identity", value_name = "ID", env = "IT_ID")]
+    id: Option<IdentityId>,
+}
+
+impl Common {
+    pub fn resolve(&self) -> cmd::Result<(git2::Repository, Refname)> {
+        let repo = git::repo::open(&self.git_dir)?;
+        let refname = identity_ref(
+            match self.id {
+                Some(id) => Left(id),
+                None => Right(repo.config()?),
+            }
+            .as_ref(),
+        )?;
+
+        Ok((repo, refname))
+    }
+}
+
+pub fn identity_ref(id: Either<&IdentityId, &git2::Config>) -> cmd::Result<Refname> {
+    let id = id.either(
+        |iid| Ok(iid.to_string()),
+        |cfg| {
+            cfg::git::identity(cfg)?
+                .ok_or_else(|| anyhow!("'{}' not set", cfg::git::IT_ID))
+                .map(|iid| iid.to_string())
+        },
+    )?;
+    Ok(Refname::try_from(format!("refs/heads/it/ids/{id}"))?)
+}
+
+#[derive(serde::Serialize, serde::Deserialize)]
+struct Editable {
+    keys: metadata::KeySet<'static>,
+    threshold: NonZeroUsize,
+    mirrors: BTreeSet<Url>,
+    expires: Option<metadata::DateTime>,
+    custom: metadata::Custom,
+}
+
+impl From<metadata::Identity> for Editable {
+    fn from(
+        metadata::Identity {
+            keys,
+            threshold,
+            mirrors,
+            expires,
+            custom,
+            ..
+        }: metadata::Identity,
+    ) -> Self {
+        Self {
+            keys,
+            threshold,
+            mirrors,
+            expires,
+            custom,
+        }
+    }
+}
+
+impl TryFrom<Editable> for metadata::Identity {
+    type Error = crate::Error;
+
+    fn try_from(
+        Editable {
+            keys,
+            threshold,
+            mirrors,
+            expires,
+            custom,
+        }: Editable,
+    ) -> Result<Self, Self::Error> {
+        ensure!(!keys.is_empty(), "keys cannot be empty");
+
+        Ok(Self {
+            spec_version: crate::SPEC_VERSION,
+            prev: None,
+            keys,
+            threshold,
+            mirrors,
+            expires,
+            custom,
+        })
+    }
+}
diff --git a/src/cmd/id/edit.rs b/src/cmd/id/edit.rs
new file mode 100644
index 0000000..02687b8
--- /dev/null
+++ b/src/cmd/id/edit.rs
@@ -0,0 +1,209 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    fs::File,
+    iter,
+    path::Path,
+};
+
+use anyhow::{
+    anyhow,
+    bail,
+    ensure,
+    Context,
+};
+
+use super::{
+    Common,
+    Editable,
+    META_FILE_ID,
+};
+use crate::{
+    cfg,
+    cmd::{
+        self,
+        args::Refname,
+        ui::{
+            self,
+            edit_commit_message,
+            edit_metadata,
+            info,
+            warn,
+        },
+        Aborted,
+        FromGit as _,
+        GitIdentity,
+    },
+    git::{
+        self,
+        refs,
+    },
+    json,
+    metadata::{
+        self,
+        Metadata,
+    },
+};
+
+#[derive(Debug, clap::Args)]
+#[allow(rustdoc::bare_urls)]
+pub struct Edit {
+    #[clap(flatten)]
+    common: Common,
+    /// Commit to this branch to propose the update
+    ///
+    /// If not given, the edit is performed in-place if the signature threshold
+    /// is met using the supplied keys.
+    #[clap(long, value_parser)]
+    propose_as: Option<Refname>,
+    /// Check out the committed changes
+    ///
+    /// Only has an effect if the repository is non-bare.
+    #[clap(long, value_parser)]
+    checkout: bool,
+    /// Don't commit anything to disk
+    #[clap(long, value_parser)]
+    dry_run: bool,
+    /// Commit message for this edit
+    ///
+    /// Like git, $EDITOR will be invoked if not specified.
+    #[clap(short, long, value_parser)]
+    message: Option<String>,
+}
+
+#[derive(serde::Serialize)]
+pub struct Output {
+    #[serde(rename = "ref")]
+    refname: Refname,
+    #[serde(with = "crate::git::serde::oid")]
+    commit: git2::Oid,
+}
+
+pub fn edit(args: Edit) -> cmd::Result<Output> {
+    let (repo, refname) = args.common.resolve()?;
+
+    let GitIdentity {
+        hash: parent_hash,
+        signed: metadata::Signed { signed: parent, .. },
+    } = metadata::Identity::from_tip(&repo, &refname)?;
+
+    let mut id: metadata::Identity = edit_metadata(Editable::from(parent.clone()))?.try_into()?;
+    if id.canonicalise()? == parent.canonicalise()? {
+        info!("Document unchanged");
+        cmd::abort!();
+    }
+    id.prev = Some(parent_hash.clone());
+
+    let cfg = repo.config()?;
+    let mut signer = cfg::signer(&cfg, ui::askpass)?;
+    let keyid = metadata::KeyId::from(signer.ident());
+    ensure!(
+        parent.keys.contains_key(&keyid) || id.keys.contains_key(&keyid),
+        "signing key {keyid} is not eligible to sign the document"
+    );
+    let signed = Metadata::identity(&id).sign(iter::once(&mut signer))?;
+
+    let commit_to = match id.verify(&signed.signatures, cmd::find_parent(&repo)) {
+        Ok(_) => args.propose_as.as_ref().unwrap_or(&refname),
+        Err(metadata::error::Verification::SignatureThreshold) => match &args.propose_as {
+            None => bail!("cannot update {refname} in place as signature threshold is not met"),
+            Some(tgt) => {
+                warn!("Signature threshold is not met");
+                tgt
+            },
+        },
+        Err(e) => bail!(e),
+    };
+
+    let mut tx = refs::Transaction::new(&repo)?;
+
+    let _tip = tx.lock_ref(refname.clone())?;
+    let tip = repo.find_reference(_tip.name())?;
+    let parent_commit = tip.peel_to_commit()?;
+    let parent_tree = parent_commit.tree()?;
+    // check that parent is valid
+    {
+        let entry = parent_tree.get_name(META_FILE_ID).ok_or_else(|| {
+            anyhow!("{refname} was modified concurrently, {META_FILE_ID} not found in tree")
+        })?;
+        ensure!(
+            parent_hash == entry.to_object(&repo)?.peel_to_blob()?.id(),
+            "{refname} was modified concurrently",
+        );
+    }
+    let commit_to = tx.lock_ref(commit_to.clone())?;
+    let on_head =
+        !repo.is_bare() && git2::Branch::wrap(repo.find_reference(commit_to.name())?).is_head();
+
+    let tree = if on_head {
+        write_tree(&repo, &signed)
+    } else {
+        write_tree_bare(&repo, &signed, Some(&parent_tree))
+    }?;
+    let msg = args
+        .message
+        .map(Ok)
+        .unwrap_or_else(|| edit_commit_message(&repo, commit_to.name(), &parent_tree, &tree))?;
+    let commit = git::commit_signed(&mut signer, &repo, msg, &tree, &[&parent_commit])?;
+    commit_to.set_target(commit, "it: edit identity");
+
+    tx.commit()?;
+
+    if args.checkout && repo.is_bare() {
+        bail!("repository is bare, refusing checkout");
+    }
+    if args.checkout || on_head {
+        repo.checkout_tree(
+            tree.as_object(),
+            Some(git2::build::CheckoutBuilder::new().safe()),
+        )?;
+        repo.set_head(commit_to.name())?;
+        info!("Switched to branch '{commit_to}'");
+    }
+
+    Ok(Output {
+        refname: commit_to.into(),
+        commit,
+    })
+}
+
+pub(super) fn write_tree<'a>(
+    repo: &'a git2::Repository,
+    meta: &metadata::Signed<metadata::Metadata>,
+) -> crate::Result<git2::Tree<'a>> {
+    ensure!(
+        repo.statuses(None)?.is_empty(),
+        "uncommitted changes in working tree. Please commit or stash them before proceeding"
+    );
+    let id_json = repo
+        .workdir()
+        .expect("non-bare repo ought to have a workdir")
+        .join(META_FILE_ID);
+    let out = File::options()
+        .write(true)
+        .truncate(true)
+        .open(&id_json)
+        .with_context(|| format!("error opening {} for writing", id_json.display()))?;
+    serde_json::to_writer_pretty(&out, meta)
+        .with_context(|| format!("serialising to {} failed", id_json.display()))?;
+
+    let mut index = repo.index()?;
+    index.add_path(Path::new(META_FILE_ID))?;
+    let oid = index.write_tree()?;
+
+    Ok(repo.find_tree(oid)?)
+}
+
+pub(super) fn write_tree_bare<'a>(
+    repo: &'a git2::Repository,
+    meta: &metadata::Signed<metadata::Metadata>,
+    from: Option<&git2::Tree>,
+) -> crate::Result<git2::Tree<'a>> {
+    let blob = json::to_blob(repo, meta)?;
+    let mut bld = repo.treebuilder(from)?;
+    bld.insert(META_FILE_ID, blob, git2::FileMode::Blob.into())?;
+    let oid = bld.write()?;
+
+    Ok(repo.find_tree(oid)?)
+}
diff --git a/src/cmd/id/init.rs b/src/cmd/id/init.rs
new file mode 100644
index 0000000..a0ed119
--- /dev/null
+++ b/src/cmd/id/init.rs
@@ -0,0 +1,230 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use core::{
+    iter,
+    num::NonZeroUsize,
+};
+use std::path::PathBuf;
+
+use anyhow::ensure;
+use clap::ValueHint;
+use url::Url;
+
+use super::{
+    Editable,
+    META_FILE_ID,
+};
+use crate::{
+    cfg::{
+        self,
+        paths,
+    },
+    cmd::{
+        self,
+        args::Refname,
+        ui::{
+            self,
+            edit_metadata,
+            info,
+        },
+    },
+    git::{
+        self,
+        if_not_found_none,
+        refs,
+    },
+    json,
+    metadata::{
+        self,
+        DateTime,
+        Key,
+        KeySet,
+    },
+};
+
+#[derive(Debug, clap::Args)]
+pub struct Init {
+    /// Path to the 'keyring' repository
+    #[clap(
+        long,
+        value_parser,
+        value_name = "DIR",
+        env = "GIT_DIR",
+        default_value_os_t = paths::ids(),
+        value_hint = ValueHint::DirPath,
+    )]
+    git_dir: PathBuf,
+    /// If the repository does not already exist, initialise it as non-bare
+    ///
+    /// Having the identity files checked out into a work tree may make it
+    /// easier to manipulate them with external tooling. Note, however, that
+    /// only committed files are considered by `it`.
+    #[clap(long, value_parser)]
+    no_bare: bool,
+    /// Set this identity as the default in the user git config
+    #[clap(long, value_parser)]
+    set_default: bool,
+    /// Additional public key to add to the identity; may be given multiple
+    /// times
+    #[clap(short, long, value_parser)]
+    public: Vec<Key<'static>>,
+    /// Threshold of keys required to sign the next revision
+    #[clap(long, value_parser)]
+    threshold: Option<NonZeroUsize>,
+    /// Alternate location where the identity history is published to; may be
+    /// given multiple times
+    #[clap(
+        long = "mirror",
+        value_parser,
+        value_name = "URL",
+        value_hint = ValueHint::Url,
+    )]
+    mirrors: Vec<Url>,
+    /// Optional date/time after which the current revision of the identity
+    /// should no longer be considered valid
+    #[clap(long, value_parser, value_name = "DATETIME")]
+    expires: Option<DateTime>,
+    /// Custom data
+    ///
+    /// The data must be parseable as canonical JSON, ie. not contain any
+    /// floating point values.
+    #[clap(
+        long,
+        value_parser,
+        value_name = "FILE",
+        value_hint = ValueHint::FilePath,
+    )]
+    custom: Option<PathBuf>,
+    /// Stop for editing the metadata in $EDITOR
+    #[clap(long, value_parser)]
+    edit: bool,
+    /// Don't commit anything to disk
+    #[clap(long, value_parser)]
+    dry_run: bool,
+}
+
+#[derive(serde::Serialize)]
+pub struct Output {
+    #[serde(skip_serializing_if = "Option::is_none")]
+    committed: Option<Committed>,
+    data: metadata::Signed<metadata::Metadata<'static>>,
+}
+
+#[derive(serde::Serialize)]
+pub struct Committed {
+    repo: PathBuf,
+    #[serde(rename = "ref")]
+    refname: Refname,
+    #[serde(with = "crate::git::serde::oid")]
+    commit: git2::Oid,
+}
+
+pub fn init(args: Init) -> cmd::Result<Output> {
+    let git_dir = args.git_dir;
+    info!("Initialising fresh identity at {}", git_dir.display());
+
+    let custom = args.custom.map(json::load).transpose()?.unwrap_or_default();
+    let cfg = git2::Config::open_default()?;
+    let mut signer = cfg::signer(&cfg, ui::askpass)?;
+    let threshold = match args.threshold {
+        None => NonZeroUsize::new(1)
+            .unwrap()
+            .saturating_add(args.public.len() / 2),
+        Some(t) => {
+            ensure!(
+                t.get() < args.public.len(),
+                "threshold must be smaller than the number of keys"
+            );
+            t
+        },
+    };
+
+    let signer_id = signer.ident().to_owned();
+    let keys = iter::once(signer_id.clone())
+        .map(metadata::Key::from)
+        .chain(args.public)
+        .collect::<KeySet>();
+
+    let meta = {
+        let id = metadata::Identity {
+            spec_version: crate::SPEC_VERSION,
+            prev: None,
+            keys,
+            threshold,
+            mirrors: args.mirrors.into_iter().collect(),
+            expires: args.expires,
+            custom,
+        };
+
+        if args.edit {
+            edit_metadata(Editable::from(id))?.try_into()?
+        } else {
+            id
+        }
+    };
+    let sigid = metadata::IdentityId::try_from(&meta).unwrap();
+    let signed = metadata::Metadata::identity(meta).sign(iter::once(&mut signer))?;
+
+    let out = if !args.dry_run {
+        let id_ref = Refname::try_from(format!("refs/heads/it/ids/{}", sigid)).unwrap();
+        let repo = git::repo::open_or_init(
+            git_dir,
+            git::repo::InitOpts {
+                bare: !args.no_bare,
+                description: "`it` keyring",
+                initial_head: &id_ref,
+            },
+        )?;
+
+        let mut tx = refs::Transaction::new(&repo)?;
+        let id_ref = tx.lock_ref(id_ref)?;
+        ensure!(
+            if_not_found_none(repo.refname_to_id(id_ref.name()))?.is_none(),
+            "{id_ref} already exists",
+        );
+
+        let blob = json::to_blob(&repo, &signed)?;
+        let tree = {
+            let mut bld = repo.treebuilder(None)?;
+            bld.insert(META_FILE_ID, blob, git2::FileMode::Blob.into())?;
+            let oid = bld.write()?;
+            repo.find_tree(oid)?
+        };
+        let msg = format!("Create identity {}", sigid);
+        let oid = git::commit_signed(&mut signer, &repo, msg, &tree, &[])?;
+        id_ref.set_target(oid, "it: create");
+
+        let mut cfg = repo.config()?;
+        cfg.set_str(
+            cfg::git::USER_SIGNING_KEY,
+            &format!("key::{}", signer_id.to_openssh()?),
+        )?;
+        let idstr = sigid.to_string();
+        cfg.set_str(cfg::git::IT_ID, &idstr)?;
+        if args.set_default {
+            cfg.open_global()?.set_str(cfg::git::IT_ID, &idstr)?;
+        }
+
+        tx.commit()?;
+        if !repo.is_bare() {
+            repo.checkout_head(None).ok();
+        }
+
+        Output {
+            committed: Some(Committed {
+                repo: repo.path().to_owned(),
+                refname: id_ref.into(),
+                commit: oid,
+            }),
+            data: signed,
+        }
+    } else {
+        Output {
+            committed: None,
+            data: signed,
+        }
+    };
+
+    Ok(out)
+}
diff --git a/src/cmd/id/show.rs b/src/cmd/id/show.rs
new file mode 100644
index 0000000..4a25455
--- /dev/null
+++ b/src/cmd/id/show.rs
@@ -0,0 +1,75 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::path::PathBuf;
+
+use super::Common;
+use crate::{
+    cmd::{
+        self,
+        args::Refname,
+        FromGit as _,
+        GitIdentity,
+    },
+    metadata::{
+        self,
+        ContentHash,
+    },
+};
+
+#[derive(Debug, clap::Args)]
+pub struct Show {
+    #[clap(flatten)]
+    common: Common,
+    /// Blob hash to show
+    ///
+    /// Instead of looking for an id.json in the tree --ref points to, load a
+    /// particular id.json by hash. If given, --ref is ignored.
+    #[clap(long = "hash", value_parser, value_name = "OID")]
+    blob_hash: Option<git2::Oid>,
+}
+
+#[derive(serde::Serialize)]
+pub struct Output {
+    repo: PathBuf,
+    #[serde(rename = "ref")]
+    refname: Refname,
+    hash: ContentHash,
+    status: Status,
+    data: metadata::Signed<metadata::Identity>,
+}
+
+#[derive(serde::Serialize)]
+#[serde(rename_all = "UPPERCASE")]
+pub enum Status {
+    Verified {
+        id: metadata::IdentityId,
+    },
+    #[serde(with = "crate::serde::display")]
+    Invalid(metadata::error::Verification),
+}
+
+impl From<Result<metadata::IdentityId, metadata::error::Verification>> for Status {
+    fn from(r: Result<metadata::IdentityId, metadata::error::Verification>) -> Self {
+        r.map(|id| Self::Verified { id })
+            .unwrap_or_else(Self::Invalid)
+    }
+}
+
+pub fn show(args: Show) -> cmd::Result<Output> {
+    let (repo, refname) = args.common.resolve()?;
+
+    let GitIdentity { hash, signed } = match args.blob_hash {
+        None => metadata::Identity::from_tip(&repo, &refname)?,
+        Some(oid) => metadata::Identity::from_blob(&repo.find_blob(oid)?)?,
+    };
+    let status = signed.verify(cmd::find_parent(&repo)).into();
+
+    Ok(Output {
+        repo: repo.path().to_owned(),
+        refname,
+        hash,
+        status,
+        data: signed,
+    })
+}
diff --git a/src/cmd/id/sign.rs b/src/cmd/id/sign.rs
new file mode 100644
index 0000000..b63ef94
--- /dev/null
+++ b/src/cmd/id/sign.rs
@@ -0,0 +1,221 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::collections::BTreeMap;
+
+use anyhow::{
+    anyhow,
+    bail,
+    ensure,
+    Context as _,
+};
+
+use super::{
+    edit,
+    Common,
+};
+use crate::{
+    cfg,
+    cmd::{
+        self,
+        args::Refname,
+        id::META_FILE_ID,
+        ui::{
+            self,
+            edit_commit_message,
+            info,
+        },
+        FromGit as _,
+        GitIdentity,
+    },
+    git::{
+        self,
+        if_not_found_none,
+        refs,
+    },
+    metadata,
+};
+
+#[derive(Debug, clap::Args)]
+pub struct Sign {
+    #[clap(flatten)]
+    common: Common,
+    /// Commit to this branch if the signature threshold is met
+    #[clap(short = 'b', long, value_parser, value_name = "REF")]
+    commit_to: Refname,
+    /// Check out the committed changes
+    ///
+    /// Only has an effect if the repository is non-bare.
+    #[clap(long, value_parser)]
+    checkout: bool,
+    /// Don't commit anything to disk
+    #[clap(long, value_parser)]
+    dry_run: bool,
+    /// Commit message for this edit
+    ///
+    /// Like git, $EDITOR will be invoked if not specified.
+    #[clap(short, long, value_parser)]
+    message: Option<String>,
+}
+
+#[derive(serde::Serialize)]
+pub struct Output {
+    #[serde(rename = "ref")]
+    refname: Refname,
+    #[serde(with = "crate::git::serde::oid")]
+    commit: git2::Oid,
+}
+
+pub fn sign(args: Sign) -> cmd::Result<Output> {
+    let (repo, refname) = args.common.resolve()?;
+    let mut tx = refs::Transaction::new(&repo)?;
+    let _tip = tx.lock_ref(refname.clone())?;
+
+    let GitIdentity {
+        signed:
+            metadata::Signed {
+                signed: proposed,
+                signatures: proposed_signatures,
+            },
+        ..
+    } = metadata::Identity::from_tip(&repo, &refname)?;
+    let prev_hash: git2::Oid = proposed
+        .prev
+        .as_ref()
+        .ok_or_else(|| anyhow!("cannot sign a genesis revision"))?
+        .into();
+    let (parent, target_ref) = if refname == args.commit_to {
+        // Signing in-place is only legal if the proposed update already
+        // meets the signature threshold
+        let _ = proposed
+            .verify(&proposed_signatures, cmd::find_parent(&repo))
+            .context("proposed update does not meet the signature threshold")?;
+        (proposed.clone(), repo.find_reference(&args.commit_to)?)
+    } else {
+        let target_ref = if_not_found_none(repo.find_reference(&args.commit_to))?;
+        match target_ref {
+            // If the target ref exists, it must yield a verified id.json whose
+            // blob hash equals the 'prev' hash of the proposed update
+            Some(tgt) => {
+                let parent_commit = tgt.peel_to_commit()?;
+                let GitIdentity {
+                    hash: parent_hash,
+                    signed:
+                        metadata::Signed {
+                            signed: parent,
+                            signatures: parent_signatures,
+                        },
+                } = metadata::Identity::from_commit(&repo, &parent_commit).with_context(|| {
+                    format!("failed to load {} from {}", META_FILE_ID, &args.commit_to)
+                })?;
+                let _ = parent
+                    .verify(&parent_signatures, cmd::find_parent(&repo))
+                    .with_context(|| format!("target {} could not be verified", &args.commit_to))?;
+                ensure!(
+                    parent_hash == prev_hash,
+                    "parent hash (.prev) doesn't match"
+                );
+
+                (parent, tgt)
+            },
+
+            // If the target ref is unborn, the proposed's parent commit must
+            // yield a verified id.json, as we will create the target from
+            // HEAD^1
+            None => {
+                let parent_commit = repo
+                    .find_reference(&refname)?
+                    .peel_to_commit()?
+                    .parents()
+                    .next()
+                    .ok_or_else(|| anyhow!("cannot sign an initial commit"))?;
+                let GitIdentity {
+                    hash: parent_hash,
+                    signed:
+                        metadata::Signed {
+                            signed: parent,
+                            signatures: parent_signatures,
+                        },
+                } = metadata::Identity::from_commit(&repo, &parent_commit)?;
+                let _ = parent
+                    .verify(&parent_signatures, cmd::find_parent(&repo))
+                    .with_context(|| {
+                        format!(
+                            "parent commit {} of {} could not be verified",
+                            parent_commit.id(),
+                            refname
+                        )
+                    })?;
+                ensure!(
+                    parent_hash == prev_hash,
+                    "parent hash (.prev) doesn't match"
+                );
+
+                let tgt = repo.reference(
+                    &args.commit_to,
+                    parent_commit.id(),
+                    false,
+                    &format!("branch: Created from {}^1", refname),
+                )?;
+
+                (parent, tgt)
+            },
+        }
+    };
+    let commit_to = tx.lock_ref(args.commit_to)?;
+
+    let canonical = proposed.canonicalise()?;
+    let mut signer = cfg::signer(&repo.config()?, ui::askpass)?;
+    let mut signatures = BTreeMap::new();
+    let keyid = metadata::KeyId::from(signer.ident());
+    if !parent.keys.contains_key(&keyid) && !proposed.keys.contains_key(&keyid) {
+        bail!("key {} is not eligible to sign the document", keyid);
+    }
+    if proposed_signatures.contains_key(&keyid) {
+        bail!("proposed update is already signed with key {}", keyid);
+    }
+
+    let signature = signer.sign(&canonical)?;
+    signatures.insert(keyid, metadata::Signature::from(signature));
+    signatures.extend(proposed_signatures);
+
+    let _ = proposed
+        .verify(&signatures, cmd::find_parent(&repo))
+        .context("proposal could not be verified after signing")?;
+
+    let signed = metadata::Signed {
+        signed: metadata::Metadata::identity(proposed),
+        signatures,
+    };
+
+    let parent_commit = target_ref.peel_to_commit()?;
+    let parent_tree = parent_commit.tree()?;
+    let on_head = !repo.is_bare() && git2::Branch::wrap(target_ref).is_head();
+
+    let tree = if on_head {
+        edit::write_tree(&repo, &signed)
+    } else {
+        edit::write_tree_bare(&repo, &signed, Some(&parent_tree))
+    }?;
+    let msg = args
+        .message
+        .map(Ok)
+        .unwrap_or_else(|| edit_commit_message(&repo, commit_to.name(), &parent_tree, &tree))?;
+    let commit = git::commit_signed(&mut signer, &repo, msg, &tree, &[&parent_commit])?;
+    commit_to.set_target(commit, "it: identity signoff");
+
+    tx.commit()?;
+
+    if on_head {
+        repo.checkout_tree(
+            tree.as_object(),
+            Some(git2::build::CheckoutBuilder::new().safe()),
+        )?;
+        info!("Checked out tree {}", tree.id());
+    }
+
+    Ok(Output {
+        refname: commit_to.into(),
+        commit,
+    })
+}
diff --git a/src/cmd/mergepoint.rs b/src/cmd/mergepoint.rs
new file mode 100644
index 0000000..2bf4f79
--- /dev/null
+++ b/src/cmd/mergepoint.rs
@@ -0,0 +1,75 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use crate::{
+    cmd::{
+        self,
+        patch,
+    },
+    patches,
+};
+
+#[derive(Debug, clap::Subcommand)]
+pub enum Cmd {
+    /// Record a mergepoint in a local repository
+    Record(Record),
+    /// Submit a mergepoint to a remote drop
+    Submit(Submit),
+}
+
+impl Cmd {
+    pub fn run(self) -> cmd::Result<cmd::Output> {
+        match self {
+            Self::Record(args) => record(args),
+            Self::Submit(args) => submit(args),
+        }
+        .map(cmd::IntoOutput::into_output)
+    }
+}
+
+#[derive(Debug, clap::Args)]
+pub struct Record {
+    #[clap(flatten)]
+    common: patch::Common,
+    /// Allow branches to be uneven with their upstream (if any)
+    #[clap(long, visible_alias = "force", value_parser)]
+    ignore_upstream: bool,
+}
+
+#[derive(Debug, clap::Args)]
+pub struct Submit {
+    #[clap(flatten)]
+    common: patch::Common,
+    #[clap(flatten)]
+    remote: patch::Remote,
+    /// Allow branches to be uneven with their upstream (if any)
+    #[clap(long, visible_alias = "force", value_parser)]
+    ignore_upstream: bool,
+}
+
+pub fn record(
+    Record {
+        common,
+        ignore_upstream,
+    }: Record,
+) -> cmd::Result<patches::Record> {
+    patch::create(patch::Kind::Merges {
+        common,
+        remote: None,
+        force: ignore_upstream,
+    })
+}
+
+pub fn submit(
+    Submit {
+        common,
+        remote,
+        ignore_upstream,
+    }: Submit,
+) -> cmd::Result<patches::Record> {
+    patch::create(patch::Kind::Merges {
+        common,
+        remote: Some(remote),
+        force: ignore_upstream,
+    })
+}
diff --git a/src/cmd/patch.rs b/src/cmd/patch.rs
new file mode 100644
index 0000000..a1b781d
--- /dev/null
+++ b/src/cmd/patch.rs
@@ -0,0 +1,77 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use crate::{
+    cmd,
+    patches,
+};
+
+mod create;
+mod prepare;
+
+pub use create::{
+    create,
+    Comment,
+    Common,
+    Kind,
+    Patch,
+    Remote,
+};
+
+#[derive(Debug, clap::Subcommand)]
+pub enum Cmd {
+    /// Record a patch in a local drop history
+    Record(Record),
+    /// Submit a patch to a remote drop
+    Submit(Submit),
+}
+
+impl Cmd {
+    pub fn run(self) -> cmd::Result<cmd::Output> {
+        match self {
+            Self::Record(args) => record(args),
+            Self::Submit(args) => submit(args),
+        }
+        .map(cmd::IntoOutput::into_output)
+    }
+}
+
+#[derive(Debug, clap::Args)]
+pub struct Record {
+    #[clap(flatten)]
+    common: Common,
+    #[clap(flatten)]
+    patch: Patch,
+}
+
+#[derive(Debug, clap::Args)]
+pub struct Submit {
+    #[clap(flatten)]
+    common: Common,
+    #[clap(flatten)]
+    patch: Patch,
+    #[clap(flatten)]
+    remote: Remote,
+}
+
+pub fn record(Record { common, patch }: Record) -> cmd::Result<patches::Record> {
+    create(Kind::Patch {
+        common,
+        remote: None,
+        patch,
+    })
+}
+
+pub fn submit(
+    Submit {
+        common,
+        patch,
+        remote,
+    }: Submit,
+) -> cmd::Result<patches::Record> {
+    create(Kind::Patch {
+        common,
+        remote: Some(remote),
+        patch,
+    })
+}
diff --git a/src/cmd/patch/create.rs b/src/cmd/patch/create.rs
new file mode 100644
index 0000000..7527364
--- /dev/null
+++ b/src/cmd/patch/create.rs
@@ -0,0 +1,483 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    borrow::Cow,
+    collections::BTreeMap,
+    env,
+    path::PathBuf,
+};
+
+use anyhow::anyhow;
+use clap::ValueHint;
+use globset::{
+    GlobSet,
+    GlobSetBuilder,
+};
+use once_cell::sync::Lazy;
+use url::Url;
+
+use super::prepare;
+use crate::{
+    cfg,
+    cmd::{
+        self,
+        ui::{
+            self,
+            debug,
+            info,
+        },
+        util::args::IdSearchPath,
+        Aborted,
+    },
+    git::{
+        self,
+        Refname,
+    },
+    metadata::IdentityId,
+    patches::{
+        self,
+        iter,
+        DropHead,
+        Topic,
+        TrackingBranch,
+        GLOB_IT_BUNDLES,
+        GLOB_IT_IDS,
+        GLOB_IT_TOPICS,
+        REF_HEADS_PATCHES,
+        REF_IT_BUNDLES,
+        REF_IT_PATCHES,
+        REF_IT_SEEN,
+    },
+    paths,
+};
+
+#[derive(Debug, clap::Args)]
+pub struct Common {
+    /// Path to the drop repository
+    #[clap(from_global)]
+    git_dir: PathBuf,
+    /// Path to the source repository
+    ///
+    /// If set, the patch bundle will be created from objects residing in an
+    /// external repository. The main use case for this is to allow a bare
+    /// drop to pull in checkpoints from a local repo with a regular layout
+    /// (ie. non it-aware).
+    #[clap(
+        long = "source-dir",
+        alias = "src-dir",
+        value_parser,
+        value_name = "DIR",
+        value_hint = ValueHint::DirPath,
+    )]
+    src_dir: Option<PathBuf>,
+    /// Identity to assume
+    ///
+    /// If not set as an option nor in the environment, the value of `it.id` in
+    /// the git config is tried.
+    #[clap(short = 'I', long = "identity", value_name = "ID", env = "IT_ID")]
+    id: Option<IdentityId>,
+    /// A list of paths to search for identity repositories
+    #[clap(
+        long,
+        value_parser,
+        value_name = "PATH",
+        env = "IT_ID_PATH",
+        default_value_t,
+        value_hint = ValueHint::DirPath,
+    )]
+    id_path: IdSearchPath,
+    /// The directory where to write the bundle to
+    ///
+    /// Unless this is an absolute path, it is treated as relative to $GIT_DIR.
+    #[clap(
+        long,
+        value_parser,
+        value_name = "DIR",
+        default_value_os_t = paths::bundles().to_owned(),
+        value_hint = ValueHint::DirPath,
+    )]
+    bundle_dir: PathBuf,
+    /// IPFS API to publish the patch bundle to
+    ///
+    /// Currently has no effect when submitting a patch to a remote drop. When
+    /// running `ipfs daemon`, the default API address is 'http://127.0.0.1:5001'.
+    #[clap(
+        long,
+        value_parser,
+        value_name = "URL",
+        value_hint = ValueHint::Url,
+    )]
+    ipfs_api: Option<Url>,
+    /// Additional identities to include, eg. to allow commit verification
+    #[clap(long = "add-id", value_parser, value_name = "ID")]
+    ids: Vec<IdentityId>,
+    /// Message to attach to the patch (cover letter, comment)
+    ///
+    /// If not set, $EDITOR will be invoked to author one.
+    #[clap(short, long, value_parser, value_name = "STRING")]
+    message: Option<String>,
+    /// Create the patch, but stop short of submitting / recording it
+    #[clap(long, value_parser)]
+    dry_run: bool,
+}
+
+#[derive(Debug, clap::Args)]
+pub struct Remote {
+    /// Url to submit the patch to
+    ///
+    /// Usually one of the alternates from the drop metadata. If not set,
+    /// GIT_DIR is assumed to contain a drop with which the patch can be
+    /// recorded without any network access.
+    #[clap(long, visible_alias = "submit-to", value_parser, value_name = "URL")]
+    url: Url,
+    /// Refname of the drop to record the patch with
+    ///
+    /// We need to pick a local (remote-tracking) drop history in order to
+    /// compute delta bases for the patch. The value is interpreted
+    /// according to "DWIM" rules, i.e. shorthand forms like 'it/patches',
+    /// 'origin/patches' are attempted to be resolved.
+    #[clap(long = "drop", value_parser, value_name = "STRING")]
+    drop_ref: String,
+}
+
+#[derive(Debug, clap::Args)]
+pub struct Patch {
+    /// Base branch the patch is against
+    ///
+    /// If --topic is given, the branch must exist in the patch bundle
+    /// --reply-to refers to, or the default entry to reply to on that
+    /// topic. Otherwise, the branch must exist in the drop
+    /// metadata. Shorthand branch names are accepted.
+    ///
+    /// If not given, "main" or "master" is tried, in that order.
+    #[clap(long = "base", value_parser, value_name = "REF")]
+    base: Option<String>,
+    /// Head revision of the patch, in 'git rev-parse' syntax
+    #[clap(
+        long = "head",
+        value_parser,
+        value_name = "REVSPEC",
+        default_value = "HEAD"
+    )]
+    head: String,
+    /// Post the patch to a previously recorded topic
+    #[clap(long, value_parser, value_name = "TOPIC")]
+    topic: Option<Topic>,
+    /// Reply to a particular entry within a topic
+    ///
+    /// Only considered if --topic is given.
+    #[clap(long, value_parser, value_name = "ID")]
+    reply_to: Option<git2::Oid>,
+}
+
+#[derive(Debug, clap::Args)]
+pub struct Comment {
+    /// The topic to comment on
+    #[clap(value_parser, value_name = "TOPIC")]
+    topic: Topic,
+    /// Reply to a particular entry within the topic
+    #[clap(long, value_parser, value_name = "ID")]
+    reply_to: Option<git2::Oid>,
+}
+
+pub enum Kind {
+    Merges {
+        common: Common,
+        remote: Option<Remote>,
+        force: bool,
+    },
+    Snapshot {
+        common: Common,
+    },
+    Comment {
+        common: Common,
+        remote: Option<Remote>,
+        comment: Comment,
+    },
+    Patch {
+        common: Common,
+        remote: Option<Remote>,
+        patch: Patch,
+    },
+}
+
+impl Kind {
+    fn common(&self) -> &Common {
+        match self {
+            Self::Merges { common, .. }
+            | Self::Snapshot { common }
+            | Self::Comment { common, .. }
+            | Self::Patch { common, .. } => common,
+        }
+    }
+
+    fn remote(&self) -> Option<&Remote> {
+        match self {
+            Self::Merges { remote, .. }
+            | Self::Comment { remote, .. }
+            | Self::Patch { remote, .. } => remote.as_ref(),
+            Self::Snapshot { .. } => None,
+        }
+    }
+
+    fn accept_options(&self, drop: &DropHead) -> patches::AcceptOptions {
+        let mut options = patches::AcceptOptions::default();
+        match self {
+            Self::Merges { common, .. } => {
+                options.allow_fat_pack = true;
+                options.max_branches = drop.meta.roles.branches.len();
+                options.max_refs = options.max_branches + common.ids.len() + 1;
+                options.max_commits = 100_000;
+            },
+            Self::Snapshot { .. } => {
+                options.allow_fat_pack = true;
+                options.allowed_refs = SNAPSHOT_REFS.clone();
+                options.max_branches = usize::MAX;
+                options.max_refs = usize::MAX;
+                options.max_commits = usize::MAX;
+                options.max_notes = usize::MAX;
+                options.max_tags = usize::MAX;
+            },
+
+            _ => {},
+        }
+
+        options
+    }
+}
+
+struct Resolved {
+    repo: prepare::Repo,
+    signer_id: IdentityId,
+    bundle_dir: PathBuf,
+}
+
+impl Common {
+    fn resolve(&self) -> cmd::Result<Resolved> {
+        let drp = git::repo::open(&self.git_dir)?;
+        let ids = self.id_path.open_git();
+        let src = match self.src_dir.as_ref() {
+            None => {
+                let cwd = env::current_dir()?;
+                (cwd != self.git_dir).then_some(cwd)
+            },
+            Some(dir) => Some(dir.to_owned()),
+        }
+        .as_deref()
+        .map(git::repo::open_bare)
+        .transpose()?;
+
+        debug!(
+            "drop: {}, src: {:?}, ids: {:?}",
+            drp.path().display(),
+            src.as_ref().map(|r| r.path().display()),
+            env::join_paths(ids.iter().map(|r| r.path()))
+        );
+
+        // IT_ID_PATH could differ from what was used at initialisation
+        git::add_alternates(&drp, &ids)?;
+
+        let repo = prepare::Repo::new(drp, ids, src);
+        let signer_id = match self.id {
+            Some(id) => id,
+            None => cfg::git::identity(&repo.source().config()?)?
+                .ok_or_else(|| anyhow!("no identity configured for signer"))?,
+        };
+        let bundle_dir = if self.bundle_dir.is_absolute() {
+            self.bundle_dir.clone()
+        } else {
+            repo.target().path().join(&self.bundle_dir)
+        };
+
+        Ok(Resolved {
+            repo,
+            signer_id,
+            bundle_dir,
+        })
+    }
+}
+
+static SNAPSHOT_REFS: Lazy<GlobSet> = Lazy::new(|| {
+    GlobSetBuilder::new()
+        .add(GLOB_IT_TOPICS.clone())
+        .add(GLOB_IT_BUNDLES.clone())
+        .add(GLOB_IT_IDS.clone())
+        .build()
+        .unwrap()
+});
+
+pub fn create(args: Kind) -> cmd::Result<patches::Record> {
+    let Resolved {
+        repo,
+        signer_id,
+        bundle_dir,
+    } = args.common().resolve()?;
+    let drop_ref: Cow<str> = match args.remote() {
+        Some(remote) => {
+            let full = repo
+                .source()
+                .resolve_reference_from_short_name(&remote.drop_ref)?;
+            full.name()
+                .ok_or_else(|| anyhow!("invalid drop ref"))?
+                .to_owned()
+                .into()
+        },
+        None if repo.target().is_bare() => REF_HEADS_PATCHES.into(),
+        None => REF_IT_PATCHES.into(),
+    };
+
+    let mut signer = cfg::git::signer(&repo.source().config()?, ui::askpass)?;
+    let drop = patches::DropHead::from_refname(repo.target(), &drop_ref)?;
+
+    let spec = match &args {
+        Kind::Merges { force, .. } => prepare::Kind::Mergepoint { force: *force },
+        Kind::Snapshot { .. } => prepare::Kind::Snapshot { incremental: true },
+        Kind::Comment { comment, .. } => prepare::Kind::Comment {
+            topic: comment.topic.clone(),
+            reply: comment.reply_to,
+        },
+        Kind::Patch { patch, .. } => {
+            let (name, base_ref) = dwim_base(
+                repo.target(),
+                &drop,
+                patch.topic.as_ref(),
+                patch.reply_to,
+                patch.base.as_deref(),
+            )?
+            .ok_or_else(|| anyhow!("unable to determine base branch"))?;
+            let base = repo
+                .target()
+                .find_reference(&base_ref)?
+                .peel_to_commit()?
+                .id();
+            let head = repo
+                .source()
+                .revparse_single(&patch.head)?
+                .peel_to_commit()?
+                .id();
+
+            prepare::Kind::Patch {
+                head,
+                base,
+                name,
+                re: patch.topic.as_ref().map(|t| (t.clone(), patch.reply_to)),
+            }
+        },
+    };
+
+    let mut patch = prepare::Preparator::new(
+        &repo,
+        &drop,
+        prepare::Submitter {
+            signer: &mut signer,
+            id: signer_id,
+        },
+    )
+    .prepare_patch(
+        &bundle_dir,
+        spec,
+        args.common().message.clone(),
+        &args.common().ids,
+    )?;
+
+    if args.common().dry_run {
+        info!("--dry-run given, stopping here");
+        cmd::abort!();
+    }
+
+    match args.remote() {
+        Some(remote) => patch.submit(remote.url.clone()),
+        None => patch.try_accept(patches::AcceptArgs {
+            unbundle_prefix: REF_IT_BUNDLES,
+            drop_ref: &drop_ref,
+            seen_ref: REF_IT_SEEN,
+            repo: repo.target(),
+            signer: &mut signer,
+            ipfs_api: args.common().ipfs_api.as_ref(),
+            options: args.accept_options(&drop),
+        }),
+    }
+}
+
+fn dwim_base(
+    repo: &git2::Repository,
+    drop: &DropHead,
+    topic: Option<&Topic>,
+    reply_to: Option<git2::Oid>,
+    base: Option<&str>,
+) -> cmd::Result<Option<(Refname, Refname)>> {
+    let mut candidates = BTreeMap::new();
+    match topic {
+        Some(topic) => {
+            let reply_to = reply_to.map(Ok).unwrap_or_else(|| {
+                iter::topic::default_reply_to(repo, topic)?
+                    .ok_or_else(|| anyhow!("topic {topic} not found"))
+            })?;
+            let mut patch_id = None;
+            for note in iter::topic(repo, topic) {
+                let note = note?;
+                if note.header.id == reply_to {
+                    patch_id = Some(note.header.patch.id);
+                    break;
+                }
+            }
+            let patch_id = patch_id.ok_or_else(|| {
+                anyhow!("no patch found corresponding to topic: {topic}, reply-to: {reply_to}")
+            })?;
+
+            let prefix = format!("{REF_IT_BUNDLES}/{patch_id}/");
+            let mut iter = repo.references_glob(&format!("{prefix}**"))?;
+            for candidate in iter.names() {
+                let candidate = candidate?;
+                if let Some(suf) = candidate.strip_prefix(&prefix) {
+                    if !suf.starts_with("it/") {
+                        candidates.insert(format!("refs/{suf}"), candidate.parse()?);
+                    }
+                }
+            }
+        },
+
+        None => candidates.extend(
+            drop.meta
+                .roles
+                .branches
+                .keys()
+                .cloned()
+                .map(|name| (name.to_string(), name)),
+        ),
+    };
+
+    const FMTS: &[fn(&str) -> String] = &[
+        |s| s.to_owned(),
+        |s| format!("refs/{}", s),
+        |s| format!("refs/heads/{}", s),
+        |s| format!("refs/tags/{}", s),
+    ];
+
+    debug!("dwim candidates: {candidates:#?}");
+
+    match base {
+        Some(base) => {
+            for (virt, act) in candidates {
+                for f in FMTS {
+                    let name = f(base);
+                    if name == virt {
+                        let refname = name.parse()?;
+                        return Ok(Some((refname, act)));
+                    }
+                }
+            }
+            Ok(None)
+        },
+
+        // nb. biased towards "main" because we use a BTreeMap
+        None => Ok(candidates.into_iter().find_map(|(k, _)| match k.as_str() {
+            "refs/heads/main" => Some((Refname::main(), TrackingBranch::main().into_refname())),
+            "refs/heads/master" => {
+                Some((Refname::master(), TrackingBranch::master().into_refname()))
+            },
+            _ => None,
+        })),
+    }
+}
diff --git a/src/cmd/patch/prepare.rs b/src/cmd/patch/prepare.rs
new file mode 100644
index 0000000..06d5ec9
--- /dev/null
+++ b/src/cmd/patch/prepare.rs
@@ -0,0 +1,615 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::path::{
+    Path,
+    PathBuf,
+};
+
+use anyhow::{
+    anyhow,
+    bail,
+    ensure,
+};
+use either::Either::Left;
+use sha2::{
+    Digest,
+    Sha256,
+};
+
+use crate::{
+    bundle,
+    cmd::{
+        self,
+        ui::{
+            debug,
+            edit_comment,
+            edit_cover_letter,
+            info,
+            warn,
+        },
+    },
+    git::{
+        self,
+        if_not_found_none,
+        Refname,
+    },
+    keys::Signer,
+    metadata::{
+        self,
+        git::{
+            FromGit,
+            GitMeta,
+            META_FILE_ID,
+        },
+        identity::{
+            self,
+            IdentityId,
+        },
+        ContentHash,
+        KeyId,
+    },
+    patches::{
+        self,
+        iter::{
+            dropped,
+            topic,
+        },
+        notes,
+        record,
+        Topic,
+        REF_IT_BUNDLES,
+        REF_IT_PATCHES,
+        TOPIC_MERGES,
+        TOPIC_SNAPSHOTS,
+    },
+};
+
+pub enum Kind {
+    Mergepoint {
+        force: bool,
+    },
+    Snapshot {
+        incremental: bool,
+    },
+    Patch {
+        head: git2::Oid,
+        base: git2::Oid,
+        name: Refname,
+        re: Option<(Topic, Option<git2::Oid>)>,
+    },
+    Comment {
+        topic: Topic,
+        reply: Option<git2::Oid>,
+    },
+}
+
+pub struct Submitter<'a, S: ?Sized> {
+    pub signer: &'a mut S,
+    pub id: IdentityId,
+}
+
+pub struct Repo {
+    drp: git2::Repository,
+    src: Option<git2::Repository>,
+    ids: Vec<git2::Repository>,
+}
+
+impl Repo {
+    pub fn new(
+        drp: git2::Repository,
+        ids: Vec<git2::Repository>,
+        src: Option<git2::Repository>,
+    ) -> Self {
+        Self { drp, ids, src }
+    }
+
+    /// Repository containing the patch objects
+    pub fn source(&self) -> &git2::Repository {
+        self.src.as_ref().unwrap_or(&self.drp)
+    }
+
+    /// Repository containing the drop state
+    pub fn target(&self) -> &git2::Repository {
+        &self.drp
+    }
+
+    /// Repositories containing identity histories
+    pub fn id_path(&self) -> &[git2::Repository] {
+        &self.ids
+    }
+}
+
+pub struct Preparator<'a, S: ?Sized> {
+    repo: &'a Repo,
+    drop: &'a patches::DropHead<'a>,
+    submitter: Submitter<'a, S>,
+}
+
+impl<'a, S: Signer> Preparator<'a, S> {
+    pub fn new(
+        repo: &'a Repo,
+        drop: &'a patches::DropHead<'a>,
+        submitter: Submitter<'a, S>,
+    ) -> Self {
+        Self {
+            repo,
+            drop,
+            submitter,
+        }
+    }
+
+    pub fn prepare_patch(
+        &mut self,
+        bundle_dir: &Path,
+        kind: Kind,
+        message: Option<String>,
+        additional_ids: &[IdentityId],
+    ) -> cmd::Result<patches::Submission> {
+        let mut header = bundle::Header::default();
+
+        match kind {
+            Kind::Mergepoint { force } => {
+                mergepoint(self.repo, &self.drop.meta, &mut header, force)?;
+                ensure!(
+                    !header.references.is_empty(),
+                    "refusing to create empty checkpoint"
+                );
+                self.annotate_checkpoint(&mut header, &TOPIC_MERGES, message)?;
+            },
+            Kind::Snapshot { incremental } => {
+                snapshot(self.repo, &mut header, incremental)?;
+                ensure!(
+                    !header.references.is_empty(),
+                    "refusing to create empty snapshot"
+                );
+                self.annotate_checkpoint(&mut header, &TOPIC_SNAPSHOTS, message)?;
+            },
+            Kind::Patch {
+                head,
+                base,
+                name,
+                re,
+            } => {
+                ensure!(base != head, "refusing to create empty patch");
+                ensure!(
+                    if_not_found_none(self.repo.source().merge_base(base, head))?.is_some(),
+                    "{base} is not reachable from {head}"
+                );
+                info!("Adding patch for {name}: {base}..{head}");
+                header.add_prerequisite(&base);
+                header.add_reference(name, &head);
+                self.annotate_patch(&mut header, message, re)?;
+            },
+            Kind::Comment { topic, reply } => {
+                self.annotate_comment(&mut header, topic, message, reply)?;
+            },
+        }
+
+        for id in additional_ids {
+            Identity::find(
+                self.repo.target(),
+                &self.drop.ids,
+                self.repo.id_path(),
+                cmd::id::identity_ref(Left(id))?,
+            )?
+            .update(&mut header);
+        }
+
+        let signer_hash = {
+            let keyid = self.submitter.signer.ident().keyid();
+            let id_ref = cmd::id::identity_ref(Left(&self.submitter.id))?;
+            let id = Identity::find(
+                self.repo.target(),
+                &self.drop.ids,
+                self.repo.id_path(),
+                id_ref,
+            )?;
+            ensure!(
+                id.contains(&keyid),
+                "signing key {keyid} not in identity {}",
+                id.id()
+            );
+            id.update(&mut header);
+
+            id.hash().clone()
+        };
+
+        let bundle = patches::Bundle::create(bundle_dir, self.repo.source(), header)?;
+        let signature = bundle
+            .sign(self.submitter.signer)
+            .map(|signature| patches::Signature {
+                signer: signer_hash,
+                signature: signature.into(),
+            })?;
+
+        Ok(patches::Submission { signature, bundle })
+    }
+
+    fn annotate_checkpoint(
+        &mut self,
+        bundle: &mut bundle::Header,
+        topic: &Topic,
+        message: Option<String>,
+    ) -> cmd::Result<()> {
+        let kind = if topic == &*TOPIC_MERGES {
+            notes::CheckpointKind::Merge
+        } else if topic == &*TOPIC_SNAPSHOTS {
+            notes::CheckpointKind::Snapshot
+        } else {
+            bail!("not a checkpoint topic: {topic}")
+        };
+        let note = notes::Simple::checkpoint(kind, bundle.references.clone(), message);
+        let parent = topic::default_reply_to(self.repo.target(), topic)?
+            .map(|id| self.repo.source().find_commit(id))
+            .transpose()?;
+
+        self.annotate(bundle, topic, parent, &note)
+    }
+
+    fn annotate_patch(
+        &mut self,
+        bundle: &mut bundle::Header,
+        cover: Option<String>,
+        re: Option<(Topic, Option<git2::Oid>)>,
+    ) -> cmd::Result<()> {
+        let cover = cover
+            .map(notes::Simple::new)
+            .map(Ok)
+            .unwrap_or_else(|| edit_cover_letter(self.repo.source()))?;
+        let (topic, parent) = match re {
+            Some((topic, reply_to)) => {
+                let parent = find_reply_to(self.repo, &topic, reply_to)?;
+                (topic, Some(parent))
+            },
+            None => {
+                // This is pretty arbitrary -- just use a random string instead?
+                let topic = {
+                    let mut hasher = Sha256::new();
+                    hasher.update(record::Heads::from(bundle as &bundle::Header));
+                    serde_json::to_writer(&mut hasher, &cover)?;
+                    hasher.update(self.submitter.signer.ident().keyid());
+                    Topic::from(hasher.finalize())
+                };
+                let parent = topic::default_reply_to(self.repo.target(), &topic)?
+                    .map(|id| self.repo.source().find_commit(id))
+                    .transpose()?;
+
+                (topic, parent)
+            },
+        };
+
+        self.annotate(bundle, &topic, parent, &cover)
+    }
+
+    fn annotate_comment(
+        &mut self,
+        bundle: &mut bundle::Header,
+        topic: Topic,
+        message: Option<String>,
+        reply_to: Option<git2::Oid>,
+    ) -> cmd::Result<()> {
+        let parent = find_reply_to(self.repo, &topic, reply_to)?;
+        let edit = || -> cmd::Result<notes::Simple> {
+            let re = notes::Simple::from_commit(self.repo.target(), &parent)?;
+            edit_comment(self.repo.source(), Some(&re))
+        };
+        let comment = message
+            .map(notes::Simple::new)
+            .map(Ok)
+            .unwrap_or_else(edit)?;
+
+        self.annotate(bundle, &topic, Some(parent), &comment)
+    }
+
+    fn annotate(
+        &mut self,
+        bundle: &mut bundle::Header,
+        topic: &Topic,
+        parent: Option<git2::Commit>,
+        note: &notes::Simple,
+    ) -> cmd::Result<()> {
+        let repo = self.repo.source();
+        let topic_ref = topic.as_refname();
+        let tree = {
+            let mut tb = repo.treebuilder(None)?;
+            patches::to_tree(repo, &mut tb, note)?;
+            repo.find_tree(tb.write()?)?
+        };
+        let msg = match note.subject() {
+            Some(s) => format!("{}\n\n{}", s, topic.as_trailer()),
+            None => topic.as_trailer(),
+        };
+        let commit = git::commit_signed(
+            self.submitter.signer,
+            repo,
+            &msg,
+            &tree,
+            parent.as_ref().into_iter().collect::<Vec<_>>().as_slice(),
+        )?;
+
+        if let Some(commit) = parent {
+            bundle.add_prerequisite(&commit.id());
+        }
+        bundle.add_reference(topic_ref, &commit);
+
+        Ok(())
+    }
+}
+
+fn mergepoint(
+    repos: &Repo,
+    meta: &metadata::drop::Verified,
+    bundle: &mut bundle::Header,
+    force: bool,
+) -> git::Result<()> {
+    for branch in meta.roles.branches.keys() {
+        let sandboxed = match patches::TrackingBranch::try_from(branch) {
+            Ok(tracking) => tracking,
+            Err(e) => {
+                warn!("Skipping invalid branch {branch}: {e}");
+                continue;
+            },
+        };
+        let head = {
+            let local = repos.source().find_reference(branch)?;
+            let head = local.peel_to_commit()?.id();
+            if !force {
+                if let Some(upstream) = if_not_found_none(git2::Branch::wrap(local).upstream())? {
+                    let upstream_head = upstream.get().peel_to_commit()?.id();
+                    if head != upstream_head {
+                        warn!(
+                            "Upstream {} is not even with {branch}; you may want to push first",
+                            String::from_utf8_lossy(upstream.name_bytes()?)
+                        );
+                        info!("Skipping {branch}");
+                        continue;
+                    }
+                }
+            }
+
+            head
+        };
+        match if_not_found_none(repos.target().find_reference(&sandboxed))? {
+            Some(base) => {
+                let base = base.peel_to_commit()?.id();
+                if base == head {
+                    info!("Skipping empty checkpoint");
+                } else if if_not_found_none(repos.source().merge_base(base, head))?.is_some() {
+                    info!("Adding thin checkpoint for branch {branch}: {base}..{head}");
+                    bundle.add_prerequisite(&base);
+                    bundle.add_reference(branch.clone(), &head);
+                } else {
+                    warn!(
+                        "{branch} diverges from drop state: no merge base between {base}..{head}"
+                    );
+                }
+            },
+
+            None => {
+                info!("Adding full checkpoint for branch {branch}: {head}");
+                bundle.add_reference(branch.clone(), &head);
+            },
+        }
+    }
+
+    Ok(())
+}
+
+fn snapshot(repo: &Repo, bundle: &mut bundle::Header, incremental: bool) -> cmd::Result<()> {
+    for record in dropped::records(repo.target(), REF_IT_PATCHES) {
+        let record = record?;
+        let bundle_hash = record.bundle_hash();
+        if record.is_encrypted() {
+            warn!("Skipping encrypted patch bundle {bundle_hash}",);
+            continue;
+        }
+
+        if record.topic == *TOPIC_SNAPSHOTS {
+            if !incremental {
+                debug!("Full snapshot: skipping previous snapshot {bundle_hash}");
+                continue;
+            } else {
+                info!("Incremental snapshot: found previous snapshot {bundle_hash}");
+                for oid in record.meta.bundle.references.values().copied() {
+                    info!("Adding prerequisite {oid} from {bundle_hash}");
+                    bundle.add_prerequisite(oid);
+                }
+                break;
+            }
+        }
+
+        info!("Including {bundle_hash} in snapshot");
+        for (name, oid) in &record.meta.bundle.references {
+            info!("Adding {oid} {name}");
+            let name = patches::unbundled_ref(REF_IT_BUNDLES, &record, name)?;
+            bundle.add_reference(name, *oid);
+        }
+    }
+
+    Ok(())
+}
+
+fn find_reply_to<'a>(
+    repo: &'a Repo,
+    topic: &Topic,
+    reply_to: Option<git2::Oid>,
+) -> cmd::Result<git2::Commit<'a>> {
+    let tip = if_not_found_none(repo.target().refname_to_id(&topic.as_refname()))?
+        .ok_or_else(|| anyhow!("topic {topic} does not exist"))?;
+    let id = match reply_to {
+        Some(id) => {
+            ensure!(
+                repo.target().graph_descendant_of(tip, id)?,
+                "{id} not found in topic {topic}, cannot reply"
+            );
+            id
+        },
+        None => topic::default_reply_to(repo.target(), topic)?.expect("impossible: empty topic"),
+    };
+
+    Ok(repo.source().find_commit(id)?)
+}
+
+struct Identity {
+    hash: ContentHash,
+    verified: identity::Verified,
+    update: Option<Range>,
+}
+
+impl Identity {
+    pub fn find(
+        repo: &git2::Repository,
+        ids: &git2::Tree,
+        id_path: &[git2::Repository],
+        refname: Refname,
+    ) -> cmd::Result<Self> {
+        let find_parent = metadata::git::find_parent(repo);
+
+        struct Meta {
+            hash: ContentHash,
+            id: identity::Verified,
+        }
+
+        impl Meta {
+            fn identity(&self) -> &metadata::Identity {
+                self.id.identity()
+            }
+        }
+
+        let (ours_in, ours) =
+            metadata::Identity::from_search_path(id_path, &refname).and_then(|data| {
+                let signer = data.meta.signed.verified(&find_parent)?;
+                Ok((
+                    data.repo,
+                    Meta {
+                        hash: data.meta.hash,
+                        id: signer,
+                    },
+                ))
+            })?;
+
+        let tree_path = PathBuf::from(ours.id.id().to_string()).join(META_FILE_ID);
+        let newer = match if_not_found_none(ids.get_path(&tree_path))? {
+            None => {
+                let start = ours_in.refname_to_id(&refname)?;
+                let range = Range {
+                    refname,
+                    start,
+                    end: None,
+                };
+                Self {
+                    hash: ours.hash,
+                    verified: ours.id,
+                    update: Some(range),
+                }
+            },
+            Some(in_tree) if ours.hash == in_tree.id() => Self {
+                hash: ours.hash,
+                verified: ours.id,
+                update: None,
+            },
+            Some(in_tree) => {
+                let theirs = metadata::Identity::from_blob(&repo.find_blob(in_tree.id())?)
+                    .and_then(|GitMeta { hash, signed }| {
+                        let signer = signed.verified(&find_parent)?;
+                        Ok(Meta { hash, id: signer })
+                    })?;
+
+                if ours.identity().has_ancestor(&theirs.hash, &find_parent)? {
+                    let range = Range::compute(ours_in, refname, theirs.hash.as_oid())?;
+                    Self {
+                        hash: ours.hash,
+                        verified: ours.id,
+                        update: range,
+                    }
+                } else if theirs.identity().has_ancestor(&ours.hash, &find_parent)? {
+                    Self {
+                        hash: theirs.hash,
+                        verified: theirs.id,
+                        update: None,
+                    }
+                } else {
+                    bail!(
+                        "provided identity at {} diverges from in-tree at {}",
+                        ours.hash,
+                        theirs.hash,
+                    )
+                }
+            },
+        };
+
+        Ok(newer)
+    }
+
+    pub fn id(&self) -> &IdentityId {
+        self.verified.id()
+    }
+
+    pub fn hash(&self) -> &ContentHash {
+        &self.hash
+    }
+
+    pub fn contains(&self, key: &KeyId) -> bool {
+        self.verified.identity().keys.contains_key(key)
+    }
+
+    pub fn update(&self, bundle: &mut bundle::Header) {
+        if let Some(range) = &self.update {
+            range.add_to_bundle(bundle);
+        }
+    }
+}
+
+struct Range {
+    refname: Refname,
+    start: git2::Oid,
+    end: Option<git2::Oid>,
+}
+
+impl Range {
+    fn compute(
+        repo: &git2::Repository,
+        refname: Refname,
+        known: git2::Oid,
+    ) -> cmd::Result<Option<Self>> {
+        let start = repo.refname_to_id(&refname)?;
+
+        let mut walk = repo.revwalk()?;
+        walk.push(start)?;
+        for oid in walk {
+            let oid = oid?;
+            let blob_id = repo
+                .find_commit(oid)?
+                .tree()?
+                .get_name(META_FILE_ID)
+                .ok_or_else(|| anyhow!("corrupt identity: missing {META_FILE_ID}"))?
+                .id();
+
+            if blob_id == known {
+                return Ok(if oid == start {
+                    None
+                } else {
+                    Some(Self {
+                        refname,
+                        start,
+                        end: Some(oid),
+                    })
+                });
+            }
+        }
+
+        Ok(Some(Self {
+            refname,
+            start,
+            end: None,
+        }))
+    }
+
+    fn add_to_bundle(&self, header: &mut bundle::Header) {
+        header.add_reference(self.refname.clone(), &self.start);
+        if let Some(end) = self.end {
+            header.add_prerequisite(&end);
+        }
+    }
+}
diff --git a/src/cmd/topic.rs b/src/cmd/topic.rs
new file mode 100644
index 0000000..fe4e2df
--- /dev/null
+++ b/src/cmd/topic.rs
@@ -0,0 +1,58 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::path::PathBuf;
+
+use crate::cmd;
+
+pub mod comment;
+
+mod ls;
+pub use ls::{
+    ls,
+    Ls,
+};
+
+mod show;
+pub use show::{
+    show,
+    Show,
+};
+
+mod unbundle;
+pub use unbundle::{
+    unbundle,
+    Unbundle,
+};
+
+#[derive(Debug, clap::Subcommand)]
+#[allow(clippy::large_enum_variant)]
+pub enum Cmd {
+    /// List the recorded topics
+    Ls(Ls),
+    /// Show a topic
+    Show(Show),
+    /// Comment on a topic
+    #[clap(subcommand)]
+    Comment(comment::Cmd),
+    /// Unbundle a topic
+    Unbundle(Unbundle),
+}
+
+impl Cmd {
+    pub fn run(self) -> cmd::Result<cmd::Output> {
+        match self {
+            Self::Ls(args) => ls(args).map(cmd::Output::iter),
+            Self::Show(args) => show(args).map(cmd::Output::iter),
+            Self::Comment(cmd) => cmd.run(),
+            Self::Unbundle(args) => unbundle(args).map(cmd::Output::val),
+        }
+    }
+}
+
+#[derive(Debug, clap::Args)]
+struct Common {
+    /// Path to the drop repository
+    #[clap(from_global)]
+    git_dir: PathBuf,
+}
diff --git a/src/cmd/topic/comment.rs b/src/cmd/topic/comment.rs
new file mode 100644
index 0000000..121dabb
--- /dev/null
+++ b/src/cmd/topic/comment.rs
@@ -0,0 +1,68 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use crate::{
+    cmd::{
+        self,
+        patch,
+    },
+    patches,
+};
+
+#[derive(Debug, clap::Subcommand)]
+pub enum Cmd {
+    /// Record the comment with a local drop history
+    Record(Record),
+    /// Submit the comment to a remote drop
+    Submit(Submit),
+}
+
+impl Cmd {
+    pub fn run(self) -> cmd::Result<cmd::Output> {
+        match self {
+            Self::Record(args) => record(args),
+            Self::Submit(args) => submit(args),
+        }
+        .map(cmd::IntoOutput::into_output)
+    }
+}
+
+#[derive(Debug, clap::Args)]
+pub struct Record {
+    #[clap(flatten)]
+    common: patch::Common,
+    #[clap(flatten)]
+    comment: patch::Comment,
+}
+
+#[derive(Debug, clap::Args)]
+pub struct Submit {
+    #[clap(flatten)]
+    common: patch::Common,
+    #[clap(flatten)]
+    comment: patch::Comment,
+    #[clap(flatten)]
+    remote: patch::Remote,
+}
+
+pub fn record(Record { common, comment }: Record) -> cmd::Result<patches::Record> {
+    patch::create(patch::Kind::Comment {
+        common,
+        remote: None,
+        comment,
+    })
+}
+
+pub fn submit(
+    Submit {
+        common,
+        comment,
+        remote,
+    }: Submit,
+) -> cmd::Result<patches::Record> {
+    patch::create(patch::Kind::Comment {
+        common,
+        remote: Some(remote),
+        comment,
+    })
+}
diff --git a/src/cmd/topic/ls.rs b/src/cmd/topic/ls.rs
new file mode 100644
index 0000000..430cc6e
--- /dev/null
+++ b/src/cmd/topic/ls.rs
@@ -0,0 +1,32 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use crate::{
+    git,
+    patches::{
+        self,
+        Topic,
+    },
+};
+
+use super::Common;
+use crate::cmd;
+
+#[derive(Debug, clap::Args)]
+pub struct Ls {
+    #[clap(flatten)]
+    common: Common,
+}
+
+#[derive(serde::Serialize)]
+pub struct Output {
+    topic: Topic,
+    subject: String,
+}
+
+pub fn ls(args: Ls) -> cmd::Result<Vec<cmd::Result<Output>>> {
+    let repo = git::repo::open(&args.common.git_dir)?;
+    Ok(patches::iter::unbundled::topics_with_subject(&repo)
+        .map(|i| i.map(|(topic, subject)| Output { topic, subject }))
+        .collect())
+}
diff --git a/src/cmd/topic/show.rs b/src/cmd/topic/show.rs
new file mode 100644
index 0000000..1d19720
--- /dev/null
+++ b/src/cmd/topic/show.rs
@@ -0,0 +1,34 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use super::Common;
+use crate::{
+    cmd,
+    git,
+    patches::{
+        self,
+        iter::Note,
+        Topic,
+    },
+};
+
+#[derive(Debug, clap::Args)]
+pub struct Show {
+    #[clap(flatten)]
+    common: Common,
+    /// Traverse the topic in reverse order, ie. oldest first
+    #[clap(long, value_parser)]
+    reverse: bool,
+    #[clap(value_parser)]
+    topic: Topic,
+}
+
+pub fn show(args: Show) -> cmd::Result<Vec<cmd::Result<Note>>> {
+    let repo = git::repo::open(&args.common.git_dir)?;
+    let iter = patches::iter::topic(&repo, &args.topic);
+    if args.reverse {
+        Ok(iter.rev().collect())
+    } else {
+        Ok(iter.collect())
+    }
+}
diff --git a/src/cmd/topic/unbundle.rs b/src/cmd/topic/unbundle.rs
new file mode 100644
index 0000000..3aab54b
--- /dev/null
+++ b/src/cmd/topic/unbundle.rs
@@ -0,0 +1,174 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    collections::{
+        BTreeMap,
+        BTreeSet,
+    },
+    path::PathBuf,
+};
+
+use anyhow::anyhow;
+use clap::ValueHint;
+
+use super::Common;
+use crate::{
+    cmd::{
+        self,
+        ui::{
+            debug,
+            info,
+            warn,
+        },
+        Aborted,
+    },
+    git::{
+        self,
+        if_not_found_none,
+        refs,
+        Refname,
+    },
+    metadata::{
+        self,
+        git::FromGit,
+    },
+    patches::{
+        self,
+        iter::dropped,
+        Bundle,
+        Record,
+        Topic,
+        REF_IT_BUNDLES,
+        REF_IT_PATCHES,
+        TOPIC_MERGES,
+        TOPIC_SNAPSHOTS,
+    },
+    paths,
+};
+
+// TODO:
+//
+// - don't require patch bundle to be present on-disk when snapshots would do
+
+#[derive(Debug, clap::Args)]
+pub struct Unbundle {
+    #[clap(flatten)]
+    common: Common,
+    /// The directory where to write the bundle to
+    ///
+    /// Unless this is an absolute path, it is treated as relative to $GIT_DIR.
+    #[clap(
+        long,
+        value_parser,
+        value_name = "DIR",
+        default_value_os_t = paths::bundles().to_owned(),
+        value_hint = ValueHint::DirPath,
+    )]
+    bundle_dir: PathBuf,
+    /// The topic to unbundle
+    #[clap(value_parser)]
+    topic: Topic,
+    /// The drop history to find the topic in
+    #[clap(value_parser)]
+    drop: Option<String>,
+}
+
+#[derive(serde::Serialize)]
+pub struct Output {
+    updated: BTreeMap<Refname, git::serde::oid::Oid>,
+}
+
+pub fn unbundle(args: Unbundle) -> cmd::Result<Output> {
+    let repo = git::repo::open(&args.common.git_dir)?;
+    let bundle_dir = if args.bundle_dir.is_relative() {
+        repo.path().join(args.bundle_dir)
+    } else {
+        args.bundle_dir
+    };
+    let drop = match args.drop {
+        Some(rev) => if_not_found_none(repo.resolve_reference_from_short_name(&rev))?
+            .ok_or_else(|| anyhow!("no ref matching {rev} found"))?
+            .name()
+            .ok_or_else(|| anyhow!("invalid drop"))?
+            .to_owned(),
+        None => REF_IT_PATCHES.to_owned(),
+    };
+
+    let filter = [&args.topic, &TOPIC_MERGES, &TOPIC_SNAPSHOTS];
+    let mut on_topic: Vec<Record> = Vec::new();
+    let mut checkpoints: Vec<Record> = Vec::new();
+    for row in dropped::topics(&repo, &drop) {
+        let (t, id) = row?;
+
+        if filter.into_iter().any(|f| f == &t) {
+            let commit = repo.find_commit(id)?;
+            let record = Record::from_commit(&repo, &commit)?;
+            if t == args.topic {
+                on_topic.push(record);
+                continue;
+            }
+
+            // Skip checkpoint which came after the most recent record on the topic
+            if !on_topic.is_empty() {
+                checkpoints.push(record);
+            }
+        }
+    }
+
+    let odb = repo.odb()?;
+
+    info!("Indexing checkpoints...");
+    for rec in checkpoints.into_iter().rev() {
+        Bundle::from_stored(&bundle_dir, rec.bundle_info().as_expect())?
+            .packdata()?
+            .index(&odb)?
+    }
+
+    let mut missing = BTreeSet::new();
+    for oid in on_topic
+        .iter()
+        .flat_map(|rec| &rec.bundle_info().prerequisites)
+    {
+        let oid = git2::Oid::try_from(oid)?;
+        if !odb.exists(oid) {
+            missing.insert(oid);
+        }
+    }
+
+    if !missing.is_empty() {
+        warn!("Unable to satisfy all prerequisites");
+        info!("The following prerequisite commits are missing:\n");
+        for oid in missing {
+            info!("{oid}");
+        }
+        info!("\nYou may try to unbundle the entire drop history");
+        cmd::abort!();
+    }
+
+    info!("Unbundling topic records...");
+    let mut tx = refs::Transaction::new(&repo)?;
+    let topic_ref = tx.lock_ref(args.topic.as_refname())?;
+    let mut up = BTreeMap::new();
+    for rec in on_topic.into_iter().rev() {
+        let hash = rec.bundle_hash();
+        let bundle = Bundle::from_stored(&bundle_dir, rec.bundle_info().as_expect())?;
+        if bundle.is_encrypted() {
+            warn!("Skipping encrypted bundle {hash}");
+            continue;
+        }
+        bundle.packdata()?.index(&odb)?;
+        debug!("{hash}: unbundle");
+        let updated = patches::unbundle(&odb, &mut tx, REF_IT_BUNDLES, &rec)?;
+        for (name, oid) in updated {
+            up.insert(name, oid.into());
+        }
+        debug!("{hash}: merge notes");
+        let submitter = metadata::Identity::from_content_hash(&repo, &rec.meta.signature.signer)?
+            .verified(metadata::git::find_parent(&repo))?;
+        patches::merge_notes(&repo, &submitter, &topic_ref, &rec)?;
+    }
+    tx.commit()?;
+
+    Ok(Output { updated: up })
+}
diff --git a/src/cmd/ui.rs b/src/cmd/ui.rs
new file mode 100644
index 0000000..c1ad214
--- /dev/null
+++ b/src/cmd/ui.rs
@@ -0,0 +1,131 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    borrow::Cow,
+    env,
+    ffi::OsStr,
+    io,
+    process::{
+        self,
+        Command,
+        Stdio,
+    },
+};
+
+use anyhow::ensure;
+use console::Term;
+use zeroize::Zeroizing;
+
+use crate::{
+    cmd::{
+        self,
+        Aborted,
+    },
+    patches::notes,
+};
+
+mod editor;
+mod output;
+pub use output::{
+    debug,
+    error,
+    info,
+    warn,
+    Output,
+};
+
+pub fn edit_commit_message(
+    repo: &git2::Repository,
+    branch: &str,
+    old: &git2::Tree,
+    new: &git2::Tree,
+) -> cmd::Result<String> {
+    let diff = repo.diff_tree_to_tree(
+        Some(old),
+        Some(new),
+        Some(
+            git2::DiffOptions::new()
+                .patience(true)
+                .minimal(true)
+                .context_lines(5),
+        ),
+    )?;
+    abort_if_empty(
+        "commit message",
+        editor::Commit::new(repo.path())?.edit(branch, diff),
+    )
+}
+
+pub fn edit_cover_letter(repo: &git2::Repository) -> cmd::Result<notes::Simple> {
+    abort_if_empty(
+        "cover letter",
+        editor::CoverLetter::new(repo.path())?.edit(),
+    )
+}
+
+pub fn edit_comment(
+    repo: &git2::Repository,
+    re: Option<&notes::Simple>,
+) -> cmd::Result<notes::Simple> {
+    abort_if_empty("comment", editor::Comment::new(repo.path())?.edit(re))
+}
+
+pub fn edit_metadata<T>(template: T) -> cmd::Result<T>
+where
+    T: serde::Serialize + serde::de::DeserializeOwned,
+{
+    abort_if_empty("metadata", editor::Metadata::new()?.edit(template))
+}
+
+fn abort_if_empty<T>(ctx: &str, edit: io::Result<Option<T>>) -> cmd::Result<T> {
+    edit?.map(Ok).unwrap_or_else(|| {
+        info!("Aborting due to empty {ctx}");
+        cmd::abort!()
+    })
+}
+
+pub fn askpass(prompt: &str) -> cmd::Result<Zeroizing<Vec<u8>>> {
+    const DEFAULT_ASKPASS: &str = "ssh-askpass";
+
+    fn ssh_askpass() -> Cow<'static, OsStr> {
+        env::var_os("SSH_ASKPASS")
+            .map(Into::into)
+            .unwrap_or_else(|| OsStr::new(DEFAULT_ASKPASS).into())
+    }
+
+    let ssh = env::var_os("SSH_ASKPASS_REQUIRE").and_then(|require| {
+        if require == "force" {
+            Some(ssh_askpass())
+        } else if require == "prefer" {
+            env::var_os("DISPLAY").map(|_| ssh_askpass())
+        } else {
+            None
+        }
+    });
+
+    match ssh {
+        Some(cmd) => {
+            let process::Output { status, stdout, .. } = Command::new(&cmd)
+                .arg(prompt)
+                .stderr(Stdio::inherit())
+                .output()?;
+            ensure!(
+                status.success(),
+                "{} failed with {:?}",
+                cmd.to_string_lossy(),
+                status.code()
+            );
+            Ok(Zeroizing::new(stdout))
+        },
+        None => {
+            let tty = Term::stderr();
+            if tty.is_term() {
+                tty.write_line(prompt)?;
+            }
+            tty.read_secure_line()
+                .map(|s| Zeroizing::new(s.into_bytes()))
+                .map_err(Into::into)
+        },
+    }
+}
diff --git a/src/cmd/ui/editor.rs b/src/cmd/ui/editor.rs
new file mode 100644
index 0000000..a2a7a5e
--- /dev/null
+++ b/src/cmd/ui/editor.rs
@@ -0,0 +1,228 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    env,
+    ffi::OsString,
+    io::{
+        self,
+        BufRead as _,
+        Write as _,
+    },
+    path::{
+        Path,
+        PathBuf,
+    },
+    process::Command,
+};
+
+use tempfile::TempPath;
+
+use crate::{
+    fs::LockedFile,
+    patches::notes,
+};
+
+const SCISSORS: &str = "# ------------------------ >8 ------------------------";
+
+pub struct Commit(Editmsg);
+
+impl Commit {
+    pub fn new<P: AsRef<Path>>(git_dir: P) -> io::Result<Self> {
+        Editmsg::new(git_dir.as_ref().join("COMMIT_EDITMSG")).map(Self)
+    }
+
+    pub fn edit(self, branch: &str, diff: git2::Diff) -> io::Result<Option<String>> {
+        let branch = branch.strip_prefix("refs/heads/").unwrap_or(branch);
+        self.0.edit(|buf| {
+            write!(
+                buf,
+                "
+# Please enter the commit message for your changes. Lines starting
+# with '#' will be ignored, and an empty message aborts the commit.
+#
+# On branch {branch}
+#
+{SCISSORS}
+# Do not modify or remove the line above.
+# Everything below it will be ignored.
+#
+# Changes to be committed:
+"
+            )?;
+            diff.print(git2::DiffFormat::Patch, |_delta, _hunk, line| {
+                use git2::DiffLineType::{
+                    Addition,
+                    Context,
+                    Deletion,
+                };
+                let ok = if matches!(line.origin_value(), Context | Addition | Deletion) {
+                    write!(buf, "{}", line.origin()).is_ok()
+                } else {
+                    true
+                };
+                ok && buf.write_all(line.content()).is_ok()
+            })
+            .map_err(|e| io::Error::new(io::ErrorKind::Other, e))?;
+            Ok(())
+        })
+    }
+}
+
+pub struct CoverLetter(Editmsg);
+
+impl CoverLetter {
+    pub fn new<P: AsRef<Path>>(git_dir: P) -> io::Result<Self> {
+        Editmsg::new(git_dir.as_ref().join("NOTES_EDITMSG")).map(Self)
+    }
+
+    // TODO: render patch series a la git log
+    pub fn edit(self) -> io::Result<Option<notes::Simple>> {
+        let txt = self.0.edit(|buf| {
+            writeln!(
+                buf,
+                "
+# Please describe your patch as you would in a cover letter or PR.
+# Lines starting with '#' will be ignored, and an empty message
+# aborts the patch creation.
+#
+{SCISSORS}
+# Do not modify or remove the line above.
+# Everything below it will be ignored.
+#
+# Changes to be committed:
+
+TODO (sorry)
+"
+            )?;
+
+            Ok(())
+        })?;
+
+        Ok(txt.map(notes::Simple::new))
+    }
+}
+
+pub struct Comment(Editmsg);
+
+impl Comment {
+    pub fn new<P: AsRef<Path>>(git_dir: P) -> io::Result<Self> {
+        Editmsg::new(git_dir.as_ref().join("NOTES_EDITMSG")).map(Self)
+    }
+
+    pub fn edit(self, re: Option<&notes::Simple>) -> io::Result<Option<notes::Simple>> {
+        let txt = self.0.edit(|buf| {
+            write!(
+                buf,
+                "
+# Enter your comment above. Lines starting with '#' will be ignored,
+# and an empty message aborts the comment creation.
+"
+            )?;
+
+            if let Some(prev) = re {
+                write!(
+                    buf,
+                    "#
+{SCISSORS}
+# Do not modify or remove the line above.
+# Everything below it will be ignored.
+#
+# Replying to:
+"
+                )?;
+
+                serde_json::to_writer_pretty(buf, prev)?;
+            }
+
+            Ok(())
+        })?;
+
+        Ok(txt.map(notes::Simple::new))
+    }
+}
+
+pub struct Metadata {
+    _tmp: TempPath,
+    msg: Editmsg,
+}
+
+impl Metadata {
+    pub fn new() -> io::Result<Self> {
+        let _tmp = tempfile::Builder::new()
+            .suffix(".json")
+            .tempfile()?
+            .into_temp_path();
+        let msg = Editmsg::new(&_tmp)?;
+
+        Ok(Self { _tmp, msg })
+    }
+
+    // TODO: explainers, edit errors
+    pub fn edit<T>(self, template: T) -> io::Result<Option<T>>
+    where
+        T: serde::Serialize + serde::de::DeserializeOwned,
+    {
+        let txt = self.msg.edit(|buf| {
+            serde_json::to_writer_pretty(buf, &template)?;
+
+            Ok(())
+        })?;
+
+        Ok(txt.as_deref().map(serde_json::from_str).transpose()?)
+    }
+}
+
+struct Editmsg {
+    file: LockedFile,
+}
+
+impl Editmsg {
+    fn new<P: Into<PathBuf>>(path: P) -> io::Result<Self> {
+        LockedFile::in_place(path, true, 0o644).map(|file| Self { file })
+    }
+
+    fn edit<F>(mut self, pre_fill: F) -> io::Result<Option<String>>
+    where
+        F: FnOnce(&mut LockedFile) -> io::Result<()>,
+    {
+        pre_fill(&mut self.file)?;
+        Command::new(editor())
+            .arg(self.file.edit_path())
+            .spawn()?
+            .wait()?;
+        self.file.reopen()?;
+        let mut msg = String::new();
+        for line in io::BufReader::new(self.file).lines() {
+            let line = line?;
+            if line == SCISSORS {
+                break;
+            }
+            if line.starts_with('#') {
+                continue;
+            }
+
+            msg.push_str(&line);
+            msg.push('\n');
+        }
+        let len = msg.trim_end().len();
+        msg.truncate(len);
+
+        Ok(if msg.is_empty() { None } else { Some(msg) })
+    }
+}
+
+fn editor() -> OsString {
+    #[cfg(windows)]
+    const DEFAULT_EDITOR: &str = "notepad.exe";
+    #[cfg(not(windows))]
+    const DEFAULT_EDITOR: &str = "vi";
+
+    if let Some(exe) = env::var_os("VISUAL") {
+        return exe;
+    }
+    if let Some(exe) = env::var_os("EDITOR") {
+        return exe;
+    }
+    DEFAULT_EDITOR.into()
+}
diff --git a/src/cmd/ui/output.rs b/src/cmd/ui/output.rs
new file mode 100644
index 0000000..f1ad598
--- /dev/null
+++ b/src/cmd/ui/output.rs
@@ -0,0 +1,44 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+pub use log::{
+    debug,
+    error,
+    info,
+    warn,
+};
+
+pub struct Output;
+
+impl log::Log for Output {
+    fn enabled(&self, metadata: &log::Metadata) -> bool {
+        metadata.level() <= log::max_level()
+    }
+
+    fn log(&self, record: &log::Record) {
+        let meta = record.metadata();
+        if !self.enabled(meta) {
+            return;
+        }
+        let level = meta.level();
+        let style = {
+            let s = console::Style::new().for_stderr();
+            if level < log::Level::Info
+                && console::user_attended_stderr()
+                && console::colors_enabled_stderr()
+            {
+                match level {
+                    log::Level::Error => s.red(),
+                    log::Level::Warn => s.yellow(),
+                    log::Level::Info | log::Level::Debug | log::Level::Trace => unreachable!(),
+                }
+            } else {
+                s
+            }
+        };
+
+        eprintln!("{}", style.apply_to(record.args()));
+    }
+
+    fn flush(&self) {}
+}
diff --git a/src/cmd/util.rs b/src/cmd/util.rs
new file mode 100644
index 0000000..27654d8
--- /dev/null
+++ b/src/cmd/util.rs
@@ -0,0 +1,4 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+pub mod args;
diff --git a/src/cmd/util/args.rs b/src/cmd/util/args.rs
new file mode 100644
index 0000000..e372c82
--- /dev/null
+++ b/src/cmd/util/args.rs
@@ -0,0 +1,139 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use core::{
+    fmt,
+    slice,
+    str::FromStr,
+};
+use std::{
+    borrow::Borrow,
+    convert::Infallible,
+    env,
+    path::PathBuf,
+    vec,
+};
+
+pub use crate::git::Refname;
+use crate::{
+    cfg::paths,
+    git,
+};
+
+/// Search path akin to the `PATH` environment variable.
+#[derive(Clone, Debug)]
+pub struct SearchPath(Vec<PathBuf>);
+
+impl SearchPath {
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+}
+
+impl fmt::Display for SearchPath {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        std::env::join_paths(&self.0)
+            .unwrap()
+            .to_string_lossy()
+            .fmt(f)
+    }
+}
+
+impl FromStr for SearchPath {
+    type Err = Infallible;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(Self(env::split_paths(s).collect()))
+    }
+}
+
+impl IntoIterator for SearchPath {
+    type Item = PathBuf;
+    type IntoIter = vec::IntoIter<PathBuf>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.into_iter()
+    }
+}
+
+impl<'a> IntoIterator for &'a SearchPath {
+    type Item = &'a PathBuf;
+    type IntoIter = slice::Iter<'a, PathBuf>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.iter()
+    }
+}
+
+/// A [`SearchPath`] with a [`Default`] appropriate for `it` identity
+/// repositories.
+#[derive(Clone, Debug)]
+pub struct IdSearchPath(SearchPath);
+
+impl IdSearchPath {
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    /// Attempt to open each path element as a git repository
+    ///
+    /// The repositories will be opened as bare, even if they aren't. No error
+    /// is returned if a repo could not be opened (e.g. because it is not a git
+    /// repository).
+    pub fn open_git(&self) -> Vec<git2::Repository> {
+        let mut rs = Vec::with_capacity(self.len());
+        for path in self {
+            if let Ok(repo) = git::repo::open_bare(path) {
+                rs.push(repo);
+            }
+        }
+
+        rs
+    }
+}
+
+impl Default for IdSearchPath {
+    fn default() -> Self {
+        Self(SearchPath(vec![paths::ids()]))
+    }
+}
+
+impl fmt::Display for IdSearchPath {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+impl FromStr for IdSearchPath {
+    type Err = <SearchPath as FromStr>::Err;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        s.parse().map(Self)
+    }
+}
+
+impl IntoIterator for IdSearchPath {
+    type Item = <SearchPath as IntoIterator>::Item;
+    type IntoIter = <SearchPath as IntoIterator>::IntoIter;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.into_iter()
+    }
+}
+
+impl<'a> IntoIterator for &'a IdSearchPath {
+    type Item = <&'a SearchPath as IntoIterator>::Item;
+    type IntoIter = <&'a SearchPath as IntoIterator>::IntoIter;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.0.borrow().into_iter()
+    }
+}
diff --git a/src/error.rs b/src/error.rs
new file mode 100644
index 0000000..e202dfa
--- /dev/null
+++ b/src/error.rs
@@ -0,0 +1,12 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+pub type Error = anyhow::Error;
+pub type Result<T> = anyhow::Result<T>;
+
+#[derive(Debug, thiserror::Error)]
+#[error("{what} not found in {whence}")]
+pub struct NotFound<T, U> {
+    pub what: T,
+    pub whence: U,
+}
diff --git a/src/fs.rs b/src/fs.rs
new file mode 100644
index 0000000..436ec83
--- /dev/null
+++ b/src/fs.rs
@@ -0,0 +1,192 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    fs::{
+        remove_file,
+        rename,
+        File,
+    },
+    io::{
+        self,
+        Read,
+        Seek,
+        Write,
+    },
+    path::{
+        Path,
+        PathBuf,
+    },
+};
+
+/// A [`File`] which is protected by a git-style lock file
+///
+/// When a [`LockedFile`] is created, a lock file named after its path with
+/// suffix ".lock" is created with `O_EXCL`. That is, if the lock file already
+/// exists, the operation will fail.
+///
+/// Then, either the lock file (when using [`LockedFile::atomic`]) or the base
+/// file (when using [`LockedFile::in_place`] is opened for writing.
+/// [`LockedFile`] implements [`Write`], [`Read`], and [`Seek`].
+///
+/// When a [`LockedFile`] is dropped, the lock file is unlinked. **NOTE** that
+/// this may leave the lock file in place if the process exits forcefully.
+///
+/// When using [`LockedFile::atomic`], the modified lock file is renamed to the
+/// base file atomically. For this to happen, [`LockedFile::persist`] must be
+/// called explicitly.
+pub struct LockedFile {
+    /// Path to the lock file
+    lock: PathBuf,
+    /// Path to the file being edited
+    path: PathBuf,
+    /// File being edited
+    edit: File,
+    /// Commit mode
+    mode: Commit,
+}
+
+enum Commit {
+    Atomic,
+    InPlace,
+}
+
+impl Drop for LockedFile {
+    fn drop(&mut self) {
+        remove_file(&self.lock).ok();
+    }
+}
+
+impl LockedFile {
+    pub const DEFAULT_PERMISSIONS: u32 = 0o644;
+
+    pub fn atomic<P, M>(path: P, truncate: bool, mode: M) -> io::Result<Self>
+    where
+        P: Into<PathBuf>,
+        M: Into<Option<u32>>,
+    {
+        let path = path.into();
+        let perm = mode.into().unwrap_or(Self::DEFAULT_PERMISSIONS);
+        let lock = path.with_extension("lock");
+        let mut edit = File::options()
+            .read(true)
+            .write(true)
+            .create_new(true)
+            .permissions(perm)
+            .open(&lock)?;
+        if !truncate && path.exists() {
+            std::fs::copy(&path, &lock)?;
+            edit = File::options().read(true).append(true).open(&lock)?;
+        }
+        let mode = Commit::Atomic;
+
+        Ok(Self {
+            lock,
+            path,
+            edit,
+            mode,
+        })
+    }
+
+    pub fn in_place<P, M>(path: P, truncate: bool, mode: M) -> io::Result<Self>
+    where
+        P: Into<PathBuf>,
+        M: Into<Option<u32>>,
+    {
+        let path = path.into();
+        let perm = mode.into().unwrap_or(Self::DEFAULT_PERMISSIONS);
+        let lock = path.with_extension("lock");
+        let _ = File::options()
+            .read(true)
+            .write(true)
+            .create_new(true)
+            .permissions(perm)
+            .open(&lock)?;
+        let edit = File::options()
+            .read(true)
+            .write(true)
+            .truncate(truncate)
+            .create(true)
+            .permissions(perm)
+            .open(&path)?;
+        let mode = Commit::InPlace;
+
+        Ok(Self {
+            lock,
+            path,
+            edit,
+            mode,
+        })
+    }
+
+    /// Reopen the file handle
+    ///
+    /// This is sometimes necessary, eg. when launching an editor to let the
+    /// user modify the file, in which case the file descriptor of the
+    /// handle is invalidated.
+    pub fn reopen(&mut self) -> io::Result<()> {
+        self.edit = File::options()
+            .read(true)
+            .write(true)
+            .open(self.edit_path())?;
+        Ok(())
+    }
+
+    pub fn edit_path(&self) -> &Path {
+        match self.mode {
+            Commit::Atomic => &self.lock,
+            Commit::InPlace => &self.path,
+        }
+    }
+
+    #[allow(unused)]
+    pub fn target_path(&self) -> &Path {
+        &self.path
+    }
+
+    pub fn persist(self) -> io::Result<()> {
+        match self.mode {
+            Commit::Atomic => rename(&self.lock, &self.path),
+            Commit::InPlace => remove_file(&self.lock),
+        }
+    }
+}
+
+impl Read for LockedFile {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        self.edit.read(buf)
+    }
+}
+
+impl Write for LockedFile {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.edit.write(buf)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.edit.flush()
+    }
+}
+
+impl Seek for LockedFile {
+    fn seek(&mut self, pos: io::SeekFrom) -> io::Result<u64> {
+        self.edit.seek(pos)
+    }
+}
+
+pub(crate) trait PermissionsExt {
+    fn permissions(&mut self, mode: u32) -> &mut Self;
+}
+
+impl PermissionsExt for std::fs::OpenOptions {
+    #[cfg(unix)]
+    fn permissions(&mut self, mode: u32) -> &mut Self {
+        use std::os::unix::fs::OpenOptionsExt as _;
+        self.mode(mode)
+    }
+
+    #[cfg(not(unix))]
+    fn permissions(&mut self, mode: u32) -> &mut Self {
+        self
+    }
+}
diff --git a/src/git.rs b/src/git.rs
new file mode 100644
index 0000000..f837711
--- /dev/null
+++ b/src/git.rs
@@ -0,0 +1,111 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::process::{
+    self,
+    Command,
+};
+
+use anyhow::{
+    anyhow,
+    ensure,
+    Context,
+};
+use once_cell::sync::Lazy;
+use sha2::{
+    Digest,
+    Sha256,
+};
+
+mod commit;
+pub use commit::{
+    commit_signed,
+    verify_commit_signature,
+};
+
+pub mod config;
+
+pub mod refs;
+pub use refs::{
+    ReferenceNames,
+    Refname,
+};
+pub mod repo;
+pub use repo::add_alternates;
+pub mod serde;
+
+pub static EMPTY_TREE: Lazy<git2::Oid> =
+    Lazy::new(|| git2::Oid::from_str("4b825dc642cb6eb9a060e54bf8d69288fbee4904").unwrap());
+
+pub type Result<T> = core::result::Result<T, git2::Error>;
+
+pub fn empty_tree(repo: &git2::Repository) -> Result<git2::Tree> {
+    repo.find_tree(*EMPTY_TREE)
+}
+
+pub fn if_not_found_none<T>(r: Result<T>) -> Result<Option<T>> {
+    if_not_found_then(r.map(Some), || Ok(None))
+}
+
+pub fn if_not_found_then<F, T>(r: Result<T>, f: F) -> Result<T>
+where
+    F: FnOnce() -> Result<T>,
+{
+    r.or_else(|e| match e.code() {
+        git2::ErrorCode::NotFound => f(),
+        _ => Err(e),
+    })
+}
+
+pub fn blob_hash(data: &[u8]) -> Result<git2::Oid> {
+    // very minimally faster than going through libgit2. not sure yet if that's
+    // worth the dependency.
+    #[cfg(feature = "sha1dc")]
+    {
+        use sha1collisiondetection::Sha1CD;
+
+        let mut hasher = Sha1CD::default();
+        hasher.update("blob ");
+        hasher.update(data.len().to_string().as_bytes());
+        hasher.update(b"\0");
+        hasher.update(data);
+        let hash = hasher.finalize_cd().expect("sha1 collision detected");
+        git2::Oid::from_bytes(&hash)
+    }
+    #[cfg(not(feature = "sha1dc"))]
+    git2::Oid::hash_object(git2::ObjectType::Blob, data)
+}
+
+pub fn blob_hash_sha2(data: &[u8]) -> [u8; 32] {
+    let mut hasher = Sha256::new();
+    hasher.update("blob ");
+    hasher.update(data.len().to_string().as_bytes());
+    hasher.update(b"\0");
+    hasher.update(data);
+    hasher.finalize().into()
+}
+
+/// Look up `key` from config and run the value as a command
+pub fn config_command(cfg: &git2::Config, key: &str) -> crate::Result<Option<String>> {
+    if_not_found_none(cfg.get_string(key))?
+        .map(|cmd| {
+            let process::Output { status, stdout, .. } = {
+                let invalid = || anyhow!("'{cmd}' is not a valid command");
+                let lex = shlex::split(&cmd).ok_or_else(invalid)?;
+                let (bin, args) = lex.split_first().ok_or_else(invalid)?;
+                Command::new(bin)
+                    .args(args)
+                    .stderr(process::Stdio::inherit())
+                    .output()?
+            };
+            ensure!(status.success(), "'{cmd}' failed");
+            const NL: u8 = b'\n';
+            let line1 = stdout
+                .into_iter()
+                .take_while(|b| b != &NL)
+                .collect::<Vec<_>>();
+            ensure!(!line1.is_empty(), "no output from '{cmd}'");
+            String::from_utf8(line1).with_context(|| format!("invalid output from '{cmd}'"))
+        })
+        .transpose()
+}
diff --git a/src/git/commit.rs b/src/git/commit.rs
new file mode 100644
index 0000000..cb4a516
--- /dev/null
+++ b/src/git/commit.rs
@@ -0,0 +1,46 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use crate::ssh;
+
+const SSHSIG_NAMESPACE: &str = "git";
+
+pub fn commit_signed<'a, S>(
+    signer: &mut S,
+    repo: &'a git2::Repository,
+    msg: impl AsRef<str>,
+    tree: &git2::Tree<'a>,
+    parents: &[&git2::Commit<'a>],
+) -> crate::Result<git2::Oid>
+where
+    S: crate::keys::Signer + ?Sized,
+{
+    let aut = repo.signature()?;
+    let buf = repo.commit_create_buffer(&aut, &aut, msg.as_ref(), tree, parents)?;
+    let sig = {
+        let hash = ssh::HashAlg::Sha512;
+        let data = ssh::SshSig::signed_data(SSHSIG_NAMESPACE, hash, &buf)?;
+        let sig = signer.sign(&data)?;
+        ssh::SshSig::new(signer.ident().key_data(), SSHSIG_NAMESPACE, hash, sig)?
+            .to_pem(ssh::LineEnding::LF)?
+    };
+    let oid = repo.commit_signed(
+        buf.as_str().expect("commit buffer to be utf8"),
+        sig.as_str(),
+        None,
+    )?;
+
+    Ok(oid)
+}
+
+pub fn verify_commit_signature(
+    repo: &git2::Repository,
+    oid: &git2::Oid,
+) -> crate::Result<ssh::PublicKey> {
+    let (sig, data) = repo.extract_signature(oid, None)?;
+    let sig = ssh::SshSig::from_pem(&*sig)?;
+    let pk = ssh::PublicKey::from(sig.public_key().clone());
+    pk.verify(SSHSIG_NAMESPACE, &data, &sig)?;
+
+    Ok(pk)
+}
diff --git a/src/git/config.rs b/src/git/config.rs
new file mode 100644
index 0000000..bc8dfcc
--- /dev/null
+++ b/src/git/config.rs
@@ -0,0 +1,31 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::ops::Deref;
+
+/// A read-only snapshot of a [`git2::Config`]
+pub struct Snapshot(git2::Config);
+
+impl Deref for Snapshot {
+    type Target = git2::Config;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl TryFrom<git2::Config> for Snapshot {
+    type Error = git2::Error;
+
+    fn try_from(mut cfg: git2::Config) -> Result<Self, Self::Error> {
+        cfg.snapshot().map(Self)
+    }
+}
+
+impl TryFrom<&mut git2::Config> for Snapshot {
+    type Error = git2::Error;
+
+    fn try_from(cfg: &mut git2::Config) -> Result<Self, Self::Error> {
+        cfg.snapshot().map(Self)
+    }
+}
diff --git a/src/git/refs.rs b/src/git/refs.rs
new file mode 100644
index 0000000..5960434
--- /dev/null
+++ b/src/git/refs.rs
@@ -0,0 +1,327 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use core::{
+    fmt,
+    ops::Deref,
+    str::FromStr,
+};
+use std::{
+    borrow::Cow,
+    cell::Cell,
+    collections::HashMap,
+    path::Path,
+    rc::Rc,
+};
+
+pub const MAX_FILENAME: usize = 255;
+
+#[derive(Clone, Copy)]
+pub struct Options {
+    pub allow_onelevel: bool,
+    pub allow_pattern: bool,
+}
+
+pub mod error {
+    use thiserror::Error;
+
+    #[derive(Debug, Error)]
+    pub enum RefFormat {
+        #[error("empty input")]
+        Empty,
+        #[error("name too long")]
+        NameTooLong,
+        #[error("invalid character {0:?}")]
+        InvalidChar(char),
+        #[error("invalid character sequence {0:?}")]
+        InvalidSeq(&'static str),
+        #[error("must contain at least one '/'")]
+        OneLevel,
+        #[error("must contain at most one '*'")]
+        Pattern,
+    }
+}
+
+pub fn check_ref_format(opts: Options, s: &str) -> Result<(), error::RefFormat> {
+    use error::RefFormat::*;
+
+    match s {
+        "" => Err(Empty),
+        "@" => Err(InvalidChar('@')),
+        "." => Err(InvalidChar('.')),
+        _ => {
+            let mut globs = 0;
+            let mut parts = 0;
+
+            for x in s.split('/') {
+                if x.is_empty() {
+                    return Err(InvalidSeq("//"));
+                }
+                if x.len() > MAX_FILENAME {
+                    return Err(NameTooLong);
+                }
+
+                parts += 1;
+
+                if x.ends_with(".lock") {
+                    return Err(InvalidSeq(".lock"));
+                }
+
+                let last_char = x.len() - 1;
+                for (i, y) in x.chars().zip(x.chars().cycle().skip(1)).enumerate() {
+                    match y {
+                        ('.', '.') => return Err(InvalidSeq("..")),
+                        ('@', '{') => return Err(InvalidSeq("@{")),
+                        ('*', _) => globs += 1,
+                        (z, _) => match z {
+                            '\0' | '\\' | '~' | '^' | ':' | '?' | '[' | ' ' => {
+                                return Err(InvalidChar(z))
+                            },
+                            '.' if i == 0 || i == last_char => return Err(InvalidChar('.')),
+                            _ if z.is_ascii_control() => return Err(InvalidChar(z)),
+
+                            _ => continue,
+                        },
+                    }
+                }
+            }
+
+            if parts < 2 && !opts.allow_onelevel {
+                Err(OneLevel)
+            } else if globs > 1 && opts.allow_pattern {
+                Err(Pattern)
+            } else if globs > 0 && !opts.allow_pattern {
+                Err(InvalidChar('*'))
+            } else {
+                Ok(())
+            }
+        },
+    }
+}
+
+/// A valid git refname.
+///
+/// If the input starts with 'refs/`, it is taken verbatim (after validation),
+/// otherwise `refs/heads/' is prepended (ie. the input is considered a branch
+/// name).
+#[derive(
+    Clone, Debug, Eq, Hash, Ord, PartialEq, PartialOrd, ::serde::Serialize, ::serde::Deserialize,
+)]
+#[serde(try_from = "String")]
+pub struct Refname(String);
+
+impl Refname {
+    pub fn main() -> Self {
+        Self("refs/heads/main".into())
+    }
+
+    pub fn master() -> Self {
+        Self("refs/heads/master".into())
+    }
+}
+
+impl fmt::Display for Refname {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self)
+    }
+}
+
+impl Deref for Refname {
+    type Target = str;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl AsRef<str> for Refname {
+    fn as_ref(&self) -> &str {
+        self
+    }
+}
+
+impl AsRef<Path> for Refname {
+    fn as_ref(&self) -> &Path {
+        Path::new(self.0.as_str())
+    }
+}
+
+impl From<Refname> for String {
+    fn from(r: Refname) -> Self {
+        r.0
+    }
+}
+
+impl FromStr for Refname {
+    type Err = error::RefFormat;
+
+    fn from_str(s: &str) -> core::result::Result<Self, Self::Err> {
+        Self::try_from(s.to_owned())
+    }
+}
+
+impl TryFrom<String> for Refname {
+    type Error = error::RefFormat;
+
+    fn try_from(value: String) -> core::result::Result<Self, Self::Error> {
+        const OPTIONS: Options = Options {
+            allow_onelevel: true,
+            allow_pattern: false,
+        };
+
+        check_ref_format(OPTIONS, &value)?;
+        let name = if value.starts_with("refs/") {
+            value
+        } else {
+            format!("refs/heads/{}", value)
+        };
+
+        Ok(Self(name))
+    }
+}
+
+/// Iterator over reference names
+///
+/// [`git2::ReferenceNames`] is advertised as more efficient if only the
+/// reference names are needed, and not a full [`git2::Reference`]. However,
+/// that type has overly restrictive lifetime constraints (because,
+/// inexplicably, it does **not** consume [`git2::References`] even though
+/// the documentation claims so).
+///
+/// We can work around this by transforming the reference `&str` into some other
+/// type which is not subject to its lifetime.
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+pub struct ReferenceNames<'a, F> {
+    inner: git2::References<'a>,
+    trans: F,
+}
+
+impl<'a, F> ReferenceNames<'a, F> {
+    pub fn new(refs: git2::References<'a>, trans: F) -> Self {
+        Self { inner: refs, trans }
+    }
+}
+
+impl<'a, F, E, T> Iterator for ReferenceNames<'a, F>
+where
+    F: FnMut(&str) -> core::result::Result<T, E>,
+    E: From<git2::Error>,
+{
+    type Item = core::result::Result<T, E>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.inner
+            .names()
+            .next()
+            .map(|r| r.map_err(E::from).and_then(|name| (self.trans)(name)))
+    }
+}
+
+pub struct Transaction<'a> {
+    tx: git2::Transaction<'a>,
+    locked: HashMap<Refname, Rc<Cell<Op>>>,
+}
+
+impl<'a> Transaction<'a> {
+    pub fn new(repo: &'a git2::Repository) -> super::Result<Self> {
+        let tx = repo.transaction()?;
+        Ok(Self {
+            tx,
+            locked: HashMap::new(),
+        })
+    }
+
+    pub fn lock_ref(&mut self, name: Refname) -> super::Result<LockedRef> {
+        use std::collections::hash_map::Entry;
+
+        let lref = match self.locked.entry(name) {
+            Entry::Vacant(v) => {
+                let name = v.key().clone();
+                self.tx.lock_ref(&name)?;
+                let op = Rc::new(Cell::new(Op::default()));
+                v.insert(Rc::clone(&op));
+                LockedRef { name, op }
+            },
+            Entry::Occupied(v) => LockedRef {
+                name: v.key().clone(),
+                op: Rc::clone(v.get()),
+            },
+        };
+
+        Ok(lref)
+    }
+
+    pub fn commit(mut self) -> super::Result<()> {
+        for (name, op) in self.locked {
+            match op.take() {
+                Op::None => continue,
+                Op::DirTarget { target, reflog } => {
+                    self.tx.set_target(&name, target, None, &reflog)?
+                },
+                Op::SymTarget { target, reflog } => {
+                    self.tx.set_symbolic_target(&name, &target, None, &reflog)?
+                },
+                Op::Remove => self.tx.remove(&name)?,
+            }
+        }
+        self.tx.commit()
+    }
+}
+
+#[derive(Debug, Default)]
+enum Op {
+    #[default]
+    None,
+    DirTarget {
+        target: git2::Oid,
+        reflog: Cow<'static, str>,
+    },
+    SymTarget {
+        target: Refname,
+        reflog: Cow<'static, str>,
+    },
+    #[allow(unused)]
+    Remove,
+}
+
+pub struct LockedRef {
+    name: Refname,
+    op: Rc<Cell<Op>>,
+}
+
+impl LockedRef {
+    pub fn name(&self) -> &Refname {
+        &self.name
+    }
+
+    pub fn set_target<S: Into<Cow<'static, str>>>(&self, target: git2::Oid, reflog: S) {
+        self.op.set(Op::DirTarget {
+            target,
+            reflog: reflog.into(),
+        })
+    }
+
+    pub fn set_symbolic_target<S: Into<Cow<'static, str>>>(&self, target: Refname, reflog: S) {
+        self.op.set(Op::SymTarget {
+            target,
+            reflog: reflog.into(),
+        })
+    }
+
+    #[allow(unused)]
+    pub fn remove(&self) {
+        self.op.set(Op::Remove)
+    }
+}
+
+impl fmt::Display for LockedRef {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self.name())
+    }
+}
+
+impl From<LockedRef> for Refname {
+    fn from(LockedRef { name, .. }: LockedRef) -> Self {
+        name
+    }
+}
diff --git a/src/git/repo.rs b/src/git/repo.rs
new file mode 100644
index 0000000..3fb8a16
--- /dev/null
+++ b/src/git/repo.rs
@@ -0,0 +1,93 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    collections::HashSet,
+    ffi::OsString,
+    io::{
+        BufReader,
+        Seek,
+        Write,
+    },
+    iter,
+    path::Path,
+    result::Result as StdResult,
+};
+
+use super::{
+    if_not_found_then,
+    Result,
+};
+use crate::{
+    fs::LockedFile,
+    io::Lines,
+};
+
+pub fn open<P: AsRef<Path>>(path: P) -> Result<git2::Repository> {
+    git2::Repository::open_ext(
+        path,
+        git2::RepositoryOpenFlags::FROM_ENV,
+        iter::empty::<OsString>(),
+    )
+}
+
+pub fn open_bare<P: AsRef<Path>>(path: P) -> Result<git2::Repository> {
+    git2::Repository::open_ext(
+        path,
+        git2::RepositoryOpenFlags::FROM_ENV | git2::RepositoryOpenFlags::BARE,
+        iter::empty::<OsString>(),
+    )
+}
+
+pub fn open_or_init<P: AsRef<Path>>(path: P, opts: InitOpts) -> Result<git2::Repository> {
+    if_not_found_then(open(path.as_ref()), || init(path, opts))
+}
+
+pub struct InitOpts<'a> {
+    pub bare: bool,
+    pub description: &'a str,
+    pub initial_head: &'a str,
+}
+
+pub fn init<P: AsRef<Path>>(path: P, opts: InitOpts) -> Result<git2::Repository> {
+    git2::Repository::init_opts(
+        path,
+        git2::RepositoryInitOptions::new()
+            .no_reinit(true)
+            .mkdir(true)
+            .mkpath(true)
+            .bare(opts.bare)
+            .description(opts.description)
+            .initial_head(opts.initial_head),
+    )
+}
+
+pub fn add_alternates<'a, I>(repo: &git2::Repository, alt: I) -> crate::Result<()>
+where
+    I: IntoIterator<Item = &'a git2::Repository>,
+{
+    let (mut persistent, known) = {
+        let mut lock = LockedFile::atomic(
+            repo.path().join("objects").join("info").join("alternates"),
+            false,
+            LockedFile::DEFAULT_PERMISSIONS,
+        )?;
+        lock.seek(std::io::SeekFrom::Start(0))?;
+        let mut bufread = BufReader::new(lock);
+        let known = Lines::new(&mut bufread).collect::<StdResult<HashSet<String>, _>>()?;
+        (bufread.into_inner(), known)
+    };
+    {
+        let odb = repo.odb()?;
+        for alternate in alt {
+            let path = format!("{}", alternate.path().join("objects").display());
+            odb.add_disk_alternate(&path)?;
+            if !known.contains(&path) {
+                writeln!(&mut persistent, "{}", path)?
+            }
+        }
+    }
+    persistent.persist()?;
+
+    Ok(())
+}
diff --git a/src/git/serde.rs b/src/git/serde.rs
new file mode 100644
index 0000000..e20df47
--- /dev/null
+++ b/src/git/serde.rs
@@ -0,0 +1,61 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::str::FromStr;
+
+use serde::{
+    Deserialize,
+    Deserializer,
+    Serialize,
+    Serializer,
+};
+
+pub mod oid {
+    use super::*;
+
+    #[derive(serde::Serialize, serde::Deserialize)]
+    pub struct Oid(#[serde(with = "self")] pub git2::Oid);
+
+    impl From<git2::Oid> for Oid {
+        fn from(oid: git2::Oid) -> Self {
+            Self(oid)
+        }
+    }
+
+    pub fn serialize<S>(oid: &git2::Oid, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: Serializer,
+    {
+        serializer.serialize_str(&oid.to_string())
+    }
+
+    pub fn deserialize<'de, D>(deserializer: D) -> Result<git2::Oid, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        let hex: &str = Deserialize::deserialize(deserializer)?;
+        git2::Oid::from_str(hex).map_err(serde::de::Error::custom)
+    }
+
+    pub mod option {
+        use super::*;
+
+        pub fn serialize<S>(oid: &Option<git2::Oid>, serializer: S) -> Result<S::Ok, S::Error>
+        where
+            S: Serializer,
+        {
+            oid.as_ref().map(ToString::to_string).serialize(serializer)
+        }
+
+        #[allow(unused)]
+        pub fn deserialize<'de, D>(deserializer: D) -> Result<Option<git2::Oid>, D::Error>
+        where
+            D: Deserializer<'de>,
+        {
+            let hex: Option<&str> = Deserialize::deserialize(deserializer)?;
+            hex.map(FromStr::from_str)
+                .transpose()
+                .map_err(serde::de::Error::custom)
+        }
+    }
+}
diff --git a/src/http.rs b/src/http.rs
new file mode 100644
index 0000000..d52ef8f
--- /dev/null
+++ b/src/http.rs
@@ -0,0 +1,355 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    fs::File,
+    io::Cursor,
+    net::ToSocketAddrs,
+    path::{
+        Path,
+        PathBuf,
+    },
+    sync::{
+        Arc,
+        Mutex,
+    },
+};
+
+use log::{
+    debug,
+    error,
+};
+use once_cell::sync::Lazy;
+use sha2::{
+    Digest,
+    Sha256,
+};
+use threadpool::ThreadPool;
+use tiny_http::{
+    Header,
+    HeaderField,
+    Method,
+    Request,
+    Response,
+    ServerConfig,
+    StatusCode,
+};
+use url::Url;
+
+use crate::{
+    bundle,
+    git,
+    keys,
+    patches::{
+        self,
+        AcceptArgs,
+        AcceptOptions,
+    },
+    ssh::agent,
+};
+
+pub use tiny_http::SslConfig;
+
+pub struct Options {
+    /// Directory of the drop repo
+    pub git_dir: PathBuf,
+    /// Directory from where to serve bundles
+    ///
+    /// Unless absolute, treated as relative to GIT_DIR.
+    pub bundle_dir: PathBuf,
+    /// Ref prefix under which to store the refs contained in patch bundles
+    pub unbundle_prefix: String,
+    /// The refname of the drop history
+    pub drop_ref: String,
+    /// The refname anchoring the seen objects tree
+    pub seen_ref: String,
+    /// Size of the server's threadpool
+    ///
+    /// If `None`, the number of available CPUs is used.
+    pub threads: Option<usize>,
+    /// Certificate and key for `serve`ing over TLS.
+    ///
+    /// It is generally recommended to proxy behind a terminating web server and
+    /// set this to `None`.
+    pub tls: Option<SslConfig>,
+    /// IPFS API to publish received bundles to
+    pub ipfs_api: Option<Url>,
+}
+
+pub fn serve<A>(addr: A, opts: Options) -> !
+where
+    A: ToSocketAddrs,
+{
+    let executor = ThreadPool::new(opts.threads.unwrap_or_else(num_cpus::get));
+    let server = tiny_http::Server::new(ServerConfig {
+        addr,
+        ssl: opts.tls,
+    })
+    .unwrap();
+
+    let repo = git::repo::open(&opts.git_dir).unwrap();
+    let config = repo.config().unwrap();
+
+    let git_dir = repo.path().to_owned();
+    let bundle_dir = if opts.bundle_dir.is_relative() {
+        git_dir.join(opts.bundle_dir)
+    } else {
+        opts.bundle_dir
+    };
+
+    let signer = keys::Agent::from_gitconfig(&config).unwrap();
+
+    let handler = Arc::new(Handler {
+        repo: Mutex::new(repo),
+        signer: Mutex::new(signer),
+        bundle_dir,
+        unbundle_prefix: opts.unbundle_prefix,
+        drop_ref: opts.drop_ref,
+        seen_ref: opts.seen_ref,
+        ipfs_api: opts.ipfs_api,
+    });
+    for req in server.incoming_requests() {
+        let handler = Arc::clone(&handler);
+        executor.execute(move || handler.route(req))
+    }
+
+    panic!("server died unexpectedly");
+}
+
+static CONTENT_TYPE: Lazy<HeaderField> = Lazy::new(|| "Content-Type".parse().unwrap());
+
+static OCTET_STREAM: Lazy<Header> = Lazy::new(|| Header {
+    field: CONTENT_TYPE.clone(),
+    value: "application/octet-stream".parse().unwrap(),
+});
+static TEXT_PLAIN: Lazy<Header> = Lazy::new(|| Header {
+    field: CONTENT_TYPE.clone(),
+    value: "text/plain".parse().unwrap(),
+});
+static JSON: Lazy<Header> = Lazy::new(|| Header {
+    field: CONTENT_TYPE.clone(),
+    value: "application/json".parse().unwrap(),
+});
+static SERVER: Lazy<Header> = Lazy::new(|| Header {
+    field: "Server".parse().unwrap(),
+    value: format!("it/{}", env!("CARGO_PKG_VERSION", "unknown"))
+        .parse()
+        .unwrap(),
+});
+
+enum Resp {
+    Empty {
+        code: StatusCode,
+    },
+    Text {
+        code: StatusCode,
+        body: String,
+    },
+    File {
+        file: File,
+    },
+    Json {
+        code: StatusCode,
+        body: Box<dyn erased_serde::Serialize>,
+    },
+}
+
+impl Resp {
+    const OK: Self = Self::Empty {
+        code: StatusCode(200),
+    };
+    const NOT_FOUND: Self = Self::Empty {
+        code: StatusCode(404),
+    };
+    const METHOD_NOT_ALLOWED: Self = Self::Empty {
+        code: StatusCode(405),
+    };
+    const INTERNAL_SERVER_ERROR: Self = Self::Empty {
+        code: StatusCode(500),
+    };
+
+    fn respond_to(self, req: Request) {
+        let remote_addr = *req.remote_addr();
+        let response = Response::empty(500).with_header(SERVER.clone());
+        let res = match self {
+            Self::Empty { code } => req.respond(response.with_status_code(code)),
+            Self::Text { code, body } => {
+                let len = body.len();
+                req.respond(
+                    response
+                        .with_status_code(code)
+                        .with_header(TEXT_PLAIN.clone())
+                        .with_data(Cursor::new(body.into_bytes()), Some(len)),
+                )
+            },
+            Self::File { file } => {
+                let len = file.metadata().ok().and_then(|v| v.len().try_into().ok());
+                req.respond(
+                    response
+                        .with_status_code(200)
+                        .with_header(OCTET_STREAM.clone())
+                        .with_data(file, len),
+                )
+            },
+            Self::Json { code, body } => {
+                let json = serde_json::to_vec(&body).unwrap();
+                let len = json.len();
+                req.respond(
+                    response
+                        .with_status_code(code)
+                        .with_header(JSON.clone())
+                        .with_data(Cursor::new(json), Some(len)),
+                )
+            },
+        };
+
+        if let Err(e) = res {
+            error!("failed to send response to {remote_addr}: {e}");
+        }
+    }
+}
+
+impl From<StatusCode> for Resp {
+    fn from(code: StatusCode) -> Self {
+        Self::Empty { code }
+    }
+}
+
+struct Handler {
+    repo: Mutex<git2::Repository>,
+    signer: Mutex<keys::Agent<agent::UnixStream>>,
+    bundle_dir: PathBuf,
+    unbundle_prefix: String,
+    drop_ref: String,
+    seen_ref: String,
+    ipfs_api: Option<Url>,
+}
+
+impl Handler {
+    fn route(&self, mut req: Request) {
+        use Method::*;
+
+        debug!("{} {}", req.method(), req.url());
+        let resp = match req.method() {
+            Get => match &request_target(&req)[..] {
+                ["-", "status"] => Resp::OK,
+                ["bundles", hash] => self.get_bundle(hash),
+                _ => Resp::NOT_FOUND,
+            },
+
+            Post => match &request_target(&req)[..] {
+                ["patches"] => self.post_patch(&mut req),
+                _ => Resp::NOT_FOUND,
+            },
+
+            _ => Resp::METHOD_NOT_ALLOWED,
+        };
+
+        resp.respond_to(req)
+    }
+
+    fn get_bundle(&self, hash: &str) -> Resp {
+        fn base_path(root: &Path, s: &str) -> Result<PathBuf, Resp> {
+            bundle::Hash::is_valid(s)
+                .then(|| root.join(s))
+                .ok_or_else(|| Resp::Text {
+                    code: 400.into(),
+                    body: "invalid bundle hash".into(),
+                })
+        }
+
+        if let Some(hash) = hash.strip_suffix(bundle::list::DOT_FILE_EXTENSION) {
+            base_path(&self.bundle_dir, hash).map_or_else(
+                |x| x,
+                |base| {
+                    let path = base.with_extension(bundle::list::FILE_EXTENSION);
+                    if !path.exists() && base.with_extension(bundle::FILE_EXTENSION).exists() {
+                        default_bundle_list(hash)
+                    } else {
+                        serve_file(path)
+                    }
+                },
+            )
+        } else if let Some(hash) = hash.strip_suffix(bundle::DOT_FILE_EXTENSION) {
+            base_path(&self.bundle_dir, hash).map_or_else(
+                |x| x,
+                |mut path| {
+                    path.set_extension(bundle::FILE_EXTENSION);
+                    serve_file(path)
+                },
+            )
+        } else {
+            base_path(&self.bundle_dir, hash).map_or_else(
+                |x| x,
+                |mut base| {
+                    base.set_extension(bundle::FILE_EXTENSION);
+                    if !base.exists() {
+                        base.set_extension(bundle::list::FILE_EXTENSION);
+                    }
+                    serve_file(base)
+                },
+            )
+        }
+    }
+
+    fn post_patch(&self, req: &mut Request) -> Resp {
+        patches::Submission::from_http(&self.bundle_dir, req)
+            .and_then(|mut sub| {
+                let repo = self.repo.lock().unwrap();
+                let mut signer = self.signer.lock().unwrap();
+                sub.try_accept(AcceptArgs {
+                    unbundle_prefix: &self.unbundle_prefix,
+                    drop_ref: &self.drop_ref,
+                    seen_ref: &self.seen_ref,
+                    repo: &repo,
+                    signer: &mut *signer,
+                    ipfs_api: self.ipfs_api.as_ref(),
+                    options: AcceptOptions::default(),
+                })
+            })
+            .map(|record| Resp::Json {
+                code: 200.into(),
+                body: Box::new(record),
+            })
+            .unwrap_or_else(|e| Resp::Text {
+                code: 400.into(),
+                body: e.to_string(),
+            })
+    }
+}
+
+// We've been calling this "request URL", but acc. to RFC7230 it is the
+// "request-target".
+fn request_target(req: &Request) -> Vec<&str> {
+    req.url().split('/').filter(|s| !s.is_empty()).collect()
+}
+
+fn serve_file<P: AsRef<Path>>(path: P) -> Resp {
+    let path = path.as_ref();
+    if path.exists() {
+        File::open(path)
+            .map(|file| Resp::File { file })
+            .unwrap_or_else(|e| {
+                error!("failed to open file {}: {e}", path.display());
+                Resp::INTERNAL_SERVER_ERROR
+            })
+    } else {
+        Resp::NOT_FOUND
+    }
+}
+
+fn default_bundle_list(hash: &str) -> Resp {
+    let uri = bundle::Uri::Relative(format!("/bundle/{}.bundle", hash));
+    let id = hex::encode(Sha256::digest(uri.as_str()));
+
+    let body = bundle::List {
+        bundles: vec![bundle::Location::new(id, uri)],
+        ..bundle::List::any()
+    }
+    .to_str();
+
+    Resp::Text {
+        code: 200.into(),
+        body,
+    }
+}
diff --git a/src/io.rs b/src/io.rs
new file mode 100644
index 0000000..86f91c6
--- /dev/null
+++ b/src/io.rs
@@ -0,0 +1,146 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use sha2::{
+    digest::generic_array::GenericArray,
+    Digest,
+};
+
+/// Created by [`Lines::until_blank`], stops iteration at the first blank line.
+pub struct UntilBlank<B> {
+    inner: Lines<B>,
+}
+
+impl<B: std::io::BufRead> Iterator for UntilBlank<B> {
+    type Item = std::io::Result<String>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.inner.next().and_then(|res| match res {
+            Ok(line) => {
+                if line.is_empty() {
+                    None
+                } else {
+                    Some(Ok(line))
+                }
+            },
+            Err(e) => Some(Err(e)),
+        })
+    }
+}
+
+impl<B: std::io::Seek> std::io::Seek for UntilBlank<B> {
+    fn seek(&mut self, pos: std::io::SeekFrom) -> std::io::Result<u64> {
+        self.inner.seek(pos)
+    }
+}
+
+/// Like [`std::io::Lines`], but allows to retain ownership of the underlying
+/// [`std::io::BufRead`].
+pub struct Lines<B> {
+    buf: B,
+}
+
+impl<B: std::io::BufRead> Lines<B> {
+    pub fn new(buf: B) -> Self {
+        Self { buf }
+    }
+
+    pub fn until_blank(self) -> UntilBlank<B> {
+        UntilBlank { inner: self }
+    }
+}
+
+impl<B: std::io::BufRead> Iterator for Lines<B> {
+    type Item = std::io::Result<String>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let mut buf = String::new();
+        match self.buf.read_line(&mut buf) {
+            Ok(0) => None,
+            Ok(_) => {
+                if buf.ends_with('\n') {
+                    buf.pop();
+                    if buf.ends_with('\r') {
+                        buf.pop();
+                    }
+                }
+                Some(Ok(buf))
+            },
+            Err(e) => Some(Err(e)),
+        }
+    }
+}
+
+impl<B: std::io::Seek> std::io::Seek for Lines<B> {
+    fn seek(&mut self, pos: std::io::SeekFrom) -> std::io::Result<u64> {
+        self.buf.seek(pos)
+    }
+}
+
+/// A [`std::io::Write`] which also computes a hash digest from the bytes
+/// written to it.
+pub struct HashWriter<D, W> {
+    hasher: D,
+    writer: W,
+}
+
+impl<D, W> HashWriter<D, W> {
+    pub fn new(hasher: D, writer: W) -> Self {
+        Self { hasher, writer }
+    }
+}
+
+impl<D, W> HashWriter<D, W>
+where
+    D: Digest,
+{
+    pub fn hash(self) -> GenericArray<u8, D::OutputSize> {
+        self.hasher.finalize()
+    }
+}
+
+impl<D, W> std::io::Write for HashWriter<D, W>
+where
+    D: Digest,
+    W: std::io::Write,
+{
+    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+        self.hasher.update(buf);
+        self.writer.write(buf)
+    }
+
+    fn flush(&mut self) -> std::io::Result<()> {
+        self.writer.flush()
+    }
+}
+
+/// A [`std::io::Write`] which keeps track of the number of bytes written to it
+pub struct LenWriter<W> {
+    written: u64,
+    writer: W,
+}
+
+impl<W> LenWriter<W> {
+    pub fn new(writer: W) -> Self {
+        Self { written: 0, writer }
+    }
+
+    pub fn bytes_written(&self) -> u64 {
+        self.written
+    }
+}
+
+impl<W> std::io::Write for LenWriter<W>
+where
+    W: std::io::Write,
+{
+    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+        let n = self.writer.write(buf)?;
+        self.written += n as u64;
+        Ok(n)
+    }
+
+    fn flush(&mut self) -> std::io::Result<()> {
+        self.writer.flush()
+    }
+}
diff --git a/src/iter.rs b/src/iter.rs
new file mode 100644
index 0000000..1289c52
--- /dev/null
+++ b/src/iter.rs
@@ -0,0 +1,109 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::ops::ControlFlow;
+
+/// Iterator with a lazy fallible initialiser
+///
+/// It is a common pattern that instantiating an effectful iterator is fallible,
+/// while traversing it is fallible, too. This yields unwieldy signatures like:
+///
+/// ```no_run
+/// fn my_iterator() -> Result<impl Iterator<Item = Result<T, F>>, E>
+/// ```
+///
+/// Often, however, we can unify the error types (`E` and `F` above), which
+/// allows for the more pleasant pattern that constructing the iterator is
+/// infallible, but an initialiser error is returned upon the first call to
+/// `next()`. Ie.:
+///
+/// ```no_run
+/// fn my_iterator() -> impl Iterator<Item = Result<T, E>>
+/// ```
+#[must_use = "iterators are lazy and do nothing unless consumed"]
+pub struct Iter<E, F, I, G> {
+    init: Option<F>,
+    iter: Option<Result<I, E>>,
+    next: G,
+}
+
+impl<E, F, I, G> Iter<E, F, I, G> {
+    pub fn new(init: F, next: G) -> Self {
+        Self {
+            init: Some(init),
+            iter: None,
+            next,
+        }
+    }
+}
+
+impl<E, F, I, G, T, U> Iterator for Iter<E, F, I, G>
+where
+    F: FnOnce() -> Result<I, E>,
+    I: Iterator<Item = Result<T, E>>,
+    G: FnMut(Result<T, E>) -> Option<Result<U, E>>,
+{
+    type Item = Result<U, E>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.iter.take() {
+            None => {
+                let init = self.init.take()?;
+                self.iter = Some(init());
+                self.next()
+            },
+            Some(Err(e)) => Some(Err(e)),
+            Some(Ok(mut iter)) => {
+                let item = iter.next()?;
+                let next = (self.next)(item);
+                self.iter = Some(Ok(iter));
+                next
+            },
+        }
+    }
+}
+
+impl<E, F, I, G, T, U> DoubleEndedIterator for Iter<E, F, I, G>
+where
+    F: FnOnce() -> Result<I, E>,
+    I: Iterator<Item = Result<T, E>> + DoubleEndedIterator,
+    G: FnMut(Result<T, E>) -> Option<Result<U, E>>,
+{
+    fn next_back(&mut self) -> Option<Self::Item> {
+        match self.iter.take() {
+            None => {
+                let init = self.init.take()?;
+                self.iter = Some(init());
+                self.next_back()
+            },
+            Some(Err(e)) => Some(Err(e)),
+            Some(Ok(mut iter)) => {
+                let item = iter.next_back()?;
+                let next = (self.next)(item);
+                self.iter = Some(Ok(iter));
+                next
+            },
+        }
+    }
+}
+
+pub(crate) trait IteratorExt {
+    fn try_find_map<F, T, E>(&mut self, mut f: F) -> crate::Result<Option<T>>
+    where
+        Self: Iterator + Sized,
+        F: FnMut(Self::Item) -> Result<Option<T>, E>,
+        E: Into<crate::Error>,
+    {
+        let x = self.try_fold((), |(), i| match f(i) {
+            Err(e) => ControlFlow::Break(Err(e.into())),
+            Ok(v) if v.is_some() => ControlFlow::Break(Ok(v)),
+            Ok(_) => ControlFlow::Continue(()),
+        });
+        match x {
+            ControlFlow::Continue(()) => Ok(None),
+            ControlFlow::Break(v) => v,
+        }
+    }
+}
+
+impl<T: Iterator> IteratorExt for T {}
diff --git a/src/json.rs b/src/json.rs
new file mode 100644
index 0000000..52f6215
--- /dev/null
+++ b/src/json.rs
@@ -0,0 +1,49 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    fs::File,
+    io::BufReader,
+    path::Path,
+};
+
+use serde::{
+    de::DeserializeOwned,
+    Deserialize,
+    Serialize,
+};
+
+pub mod canonical;
+
+pub fn from_blob<'a, T>(blob: &'a git2::Blob) -> crate::Result<T>
+where
+    T: Deserialize<'a>,
+{
+    Ok(serde_json::from_slice(blob.content())?)
+}
+
+pub fn to_blob<T>(repo: &git2::Repository, data: &T) -> crate::Result<git2::Oid>
+where
+    T: Serialize,
+{
+    let mut writer = repo.blob_writer(None)?;
+    serde_json::to_writer_pretty(&mut writer, data)?;
+    Ok(writer.commit()?)
+}
+
+pub fn from_file<P, T>(path: P) -> crate::Result<T>
+where
+    P: AsRef<Path>,
+    T: DeserializeOwned,
+{
+    let file = File::open(path)?;
+    Ok(serde_json::from_reader(BufReader::new(file))?)
+}
+
+pub fn load<P, T>(path: P) -> crate::Result<T>
+where
+    P: AsRef<Path>,
+    T: DeserializeOwned,
+{
+    from_file(path)
+}
diff --git a/src/json/canonical.rs b/src/json/canonical.rs
new file mode 100644
index 0000000..6de9517
--- /dev/null
+++ b/src/json/canonical.rs
@@ -0,0 +1,166 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    collections::BTreeMap,
+    io::Write,
+};
+
+use unicode_normalization::{
+    is_nfc_quick,
+    IsNormalized,
+    UnicodeNormalization as _,
+};
+
+use crate::metadata;
+
+pub mod error {
+    use std::io;
+
+    use thiserror::Error;
+
+    #[derive(Debug, Error)]
+    pub enum Canonicalise {
+        #[error(transparent)]
+        Cjson(#[from] Float),
+
+        #[error(transparent)]
+        Json(#[from] serde_json::Error),
+
+        #[error(transparent)]
+        Io(#[from] io::Error),
+    }
+
+    #[derive(Debug, Error)]
+    #[error("cannot canonicalise floating-point number")]
+    pub struct Float;
+}
+
+pub(crate) enum Value {
+    Null,
+    Bool(bool),
+    Number(Number),
+    String(String),
+    Array(Vec<Value>),
+    Object(BTreeMap<String, Value>),
+}
+
+impl TryFrom<&serde_json::Value> for Value {
+    type Error = error::Float;
+
+    fn try_from(js: &serde_json::Value) -> Result<Self, Self::Error> {
+        match js {
+            serde_json::Value::Null => Ok(Self::Null),
+            serde_json::Value::Bool(b) => Ok(Self::Bool(*b)),
+            serde_json::Value::Number(n) => n
+                .as_i64()
+                .map(Number::I64)
+                .or_else(|| n.as_u64().map(Number::U64))
+                .map(Self::Number)
+                .ok_or(error::Float),
+            serde_json::Value::String(s) => Ok(Self::String(to_nfc(s))),
+            serde_json::Value::Array(v) => {
+                let mut out = Vec::with_capacity(v.len());
+                for w in v.iter().map(TryFrom::try_from) {
+                    out.push(w?);
+                }
+                Ok(Self::Array(out))
+            },
+            serde_json::Value::Object(m) => {
+                let mut out = BTreeMap::new();
+                for (k, v) in m {
+                    out.insert(to_nfc(k), Self::try_from(v)?);
+                }
+                Ok(Self::Object(out))
+            },
+        }
+    }
+}
+
+impl TryFrom<&metadata::Custom> for Value {
+    type Error = error::Float;
+
+    fn try_from(js: &metadata::Custom) -> Result<Self, Self::Error> {
+        let mut out = BTreeMap::new();
+        for (k, v) in js {
+            out.insert(to_nfc(k), Self::try_from(v)?);
+        }
+        Ok(Self::Object(out))
+    }
+}
+
+impl serde::Serialize for Value {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        match self {
+            Value::Null => serializer.serialize_unit(),
+            Value::Bool(b) => serializer.serialize_bool(*b),
+            Value::Number(n) => n.serialize(serializer),
+            Value::String(s) => serializer.serialize_str(s),
+            Value::Array(v) => v.serialize(serializer),
+            Value::Object(m) => {
+                use serde::ser::SerializeMap;
+
+                let mut map = serializer.serialize_map(Some(m.len()))?;
+                for (k, v) in m {
+                    map.serialize_entry(k, v)?;
+                }
+                map.end()
+            },
+        }
+    }
+}
+
+pub(crate) enum Number {
+    I64(i64),
+    U64(u64),
+}
+
+impl serde::Serialize for Number {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        match self {
+            Number::I64(n) => serializer.serialize_i64(*n),
+            Number::U64(n) => serializer.serialize_u64(*n),
+        }
+    }
+}
+
+fn to_nfc(s: &String) -> String {
+    match is_nfc_quick(s.chars()) {
+        IsNormalized::Yes => s.clone(),
+        IsNormalized::No | IsNormalized::Maybe => s.nfc().collect(),
+    }
+}
+
+pub fn to_writer<W, T>(out: W, v: T) -> Result<(), error::Canonicalise>
+where
+    W: Write,
+    T: serde::Serialize,
+{
+    let js = serde_json::to_value(v)?;
+    let cj = Value::try_from(&js)?;
+    serde_json::to_writer(out, &cj).map_err(|e| {
+        if e.is_io() {
+            error::Canonicalise::Io(e.into())
+        } else {
+            error::Canonicalise::Json(e)
+        }
+    })?;
+
+    Ok(())
+}
+
+pub fn to_vec<T>(v: T) -> Result<Vec<u8>, error::Canonicalise>
+where
+    T: serde::Serialize,
+{
+    let mut buf = Vec::new();
+    to_writer(&mut buf, v)?;
+
+    Ok(buf)
+}
diff --git a/src/keys.rs b/src/keys.rs
new file mode 100644
index 0000000..c6be894
--- /dev/null
+++ b/src/keys.rs
@@ -0,0 +1,206 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use core::fmt;
+use std::{
+    borrow::Cow,
+    io,
+    ops::{
+        Deref,
+        DerefMut,
+    },
+    str::FromStr,
+};
+
+use anyhow::anyhow;
+use signature::SignerMut;
+
+use crate::{
+    cfg,
+    metadata,
+    ssh::{
+        self,
+        agent,
+    },
+};
+
+pub type Signature = ssh::Signature;
+
+pub trait Signer {
+    fn ident(&self) -> VerificationKey;
+    fn sign(&mut self, msg: &[u8]) -> Result<ssh::Signature, signature::Error>;
+}
+
+impl<T> Signer for Box<T>
+where
+    T: Signer + ?Sized,
+{
+    fn ident(&self) -> VerificationKey {
+        self.deref().ident()
+    }
+
+    fn sign(&mut self, msg: &[u8]) -> Result<ssh::Signature, signature::Error> {
+        self.deref_mut().sign(msg)
+    }
+}
+
+impl Signer for ssh::PrivateKey {
+    fn ident(&self) -> VerificationKey {
+        self.public_key().into()
+    }
+
+    fn sign(&mut self, msg: &[u8]) -> Result<ssh::Signature, signature::Error> {
+        self.try_sign(msg)
+    }
+}
+
+pub struct Agent<T> {
+    client: agent::Client<T>,
+    ident: ssh::PublicKey,
+}
+
+impl Agent<agent::UnixStream> {
+    pub fn from_gitconfig(cfg: &git2::Config) -> crate::Result<Self> {
+        let client = agent::Client::from_env()?;
+        let ident = VerificationKey::from_gitconfig(cfg)?.0.into_owned();
+
+        Ok(Self { client, ident })
+    }
+
+    pub fn boxed(self) -> Box<dyn Signer> {
+        Box::new(self)
+    }
+
+    pub fn as_dyn(&mut self) -> &mut dyn Signer {
+        self
+    }
+}
+
+impl<T> Agent<T> {
+    pub fn new(client: agent::Client<T>, key: VerificationKey<'_>) -> Self {
+        let ident = key.0.into_owned();
+        Self { client, ident }
+    }
+
+    pub fn verification_key(&self) -> VerificationKey {
+        VerificationKey::from(&self.ident)
+    }
+}
+
+impl<T> Signer for Agent<T>
+where
+    T: io::Read + io::Write,
+{
+    fn ident(&self) -> VerificationKey {
+        self.verification_key()
+    }
+
+    fn sign(&mut self, msg: &[u8]) -> Result<ssh::Signature, signature::Error> {
+        self.client
+            .sign(&self.ident, msg)
+            .map_err(signature::Error::from_source)
+    }
+}
+
+impl<T> Signer for &mut Agent<T>
+where
+    T: io::Read + io::Write,
+{
+    fn ident(&self) -> VerificationKey {
+        self.verification_key()
+    }
+
+    fn sign(&mut self, msg: &[u8]) -> Result<ssh::Signature, signature::Error> {
+        self.client
+            .sign(&self.ident, msg)
+            .map_err(signature::Error::from_source)
+    }
+}
+
+#[derive(Clone)]
+pub struct VerificationKey<'a>(Cow<'a, ssh::PublicKey>);
+
+impl<'a> VerificationKey<'a> {
+    pub fn from_openssh(key: &str) -> Result<Self, ssh::Error> {
+        ssh::PublicKey::from_openssh(key).map(Cow::Owned).map(Self)
+    }
+
+    pub fn to_openssh(&self) -> Result<String, ssh::Error> {
+        self.0.to_openssh()
+    }
+
+    pub fn from_gitconfig(cfg: &git2::Config) -> crate::Result<Self> {
+        let key = cfg::git::signing_key(cfg)?
+            .ok_or_else(|| anyhow!("unable to determine signing key from git config"))?
+            .public()
+            .to_owned();
+        Ok(Self(Cow::Owned(key)))
+    }
+
+    pub fn algorithm(&self) -> ssh::Algorithm {
+        self.0.algorithm()
+    }
+
+    pub fn strip_comment(&mut self) {
+        self.0.to_mut().set_comment("")
+    }
+
+    pub fn without_comment(mut self) -> Self {
+        self.strip_comment();
+        self
+    }
+
+    pub fn sha256(&self) -> [u8; 32] {
+        self.0.fingerprint(ssh::HashAlg::Sha256).sha256().unwrap()
+    }
+
+    pub fn to_owned<'b>(&self) -> VerificationKey<'b> {
+        VerificationKey(Cow::Owned(self.0.clone().into_owned()))
+    }
+
+    pub fn keyid(&self) -> metadata::KeyId {
+        metadata::KeyId::from(self)
+    }
+
+    pub(crate) fn key_data(&self) -> ssh::public::KeyData {
+        self.as_ref().into()
+    }
+}
+
+impl AsRef<ssh::PublicKey> for VerificationKey<'_> {
+    fn as_ref(&self) -> &ssh::PublicKey {
+        &self.0
+    }
+}
+
+impl fmt::Display for VerificationKey<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str(&self.0.to_string())
+    }
+}
+
+impl From<ssh::PublicKey> for VerificationKey<'_> {
+    fn from(key: ssh::PublicKey) -> Self {
+        Self(Cow::Owned(key))
+    }
+}
+
+impl<'a> From<&'a ssh::PublicKey> for VerificationKey<'a> {
+    fn from(key: &'a ssh::PublicKey) -> Self {
+        Self(Cow::Borrowed(key))
+    }
+}
+
+impl FromStr for VerificationKey<'_> {
+    type Err = ssh::Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Self::from_openssh(s)
+    }
+}
+
+impl signature::Verifier<ssh::Signature> for VerificationKey<'_> {
+    fn verify(&self, msg: &[u8], signature: &ssh::Signature) -> Result<(), signature::Error> {
+        signature::Verifier::verify(&*self.0, msg, signature)
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..789f99f
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,33 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+mod bundle;
+mod cfg;
+mod fs;
+mod git;
+mod http;
+mod io;
+mod iter;
+mod json;
+mod keys;
+mod metadata;
+mod patches;
+mod serde;
+mod ssh;
+mod str;
+
+pub const SPEC_VERSION: metadata::SpecVersion = metadata::SpecVersion::current();
+
+pub mod cmd;
+pub use cmd::{
+    ui::Output,
+    Cmd,
+};
+
+pub mod error;
+pub use error::{
+    Error,
+    Result,
+};
+
+pub use cfg::paths;
diff --git a/src/metadata.rs b/src/metadata.rs
new file mode 100644
index 0000000..9caee96
--- /dev/null
+++ b/src/metadata.rs
@@ -0,0 +1,749 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use core::{
+    convert::TryFrom,
+    fmt,
+    ops::Deref,
+    str::FromStr,
+};
+use std::{
+    borrow::Cow,
+    collections::BTreeMap,
+    io,
+    marker::PhantomData,
+    ops::DerefMut,
+};
+
+use serde::ser::SerializeSeq;
+use sha2::{
+    Digest,
+    Sha512,
+};
+use time::{
+    Duration,
+    OffsetDateTime,
+    UtcOffset,
+};
+use versions::SemVer;
+
+use crate::{
+    git::blob_hash_sha2,
+    json::canonical,
+    keys::{
+        Signer,
+        VerificationKey,
+    },
+    ssh,
+};
+
+pub mod drop;
+pub use drop::Drop;
+
+pub mod error;
+pub mod git;
+
+mod mirrors;
+pub use mirrors::{
+    Alternates,
+    Mirrors,
+};
+
+pub mod identity;
+pub use identity::{
+    Identity,
+    IdentityId,
+};
+
+#[derive(Clone, Eq, Ord, PartialEq, PartialOrd)]
+pub struct SpecVersion(SemVer);
+
+impl SpecVersion {
+    pub const fn current() -> Self {
+        Self::new(0, 1, 0)
+    }
+
+    const fn new(major: u32, minor: u32, patch: u32) -> Self {
+        Self(SemVer {
+            major,
+            minor,
+            patch,
+            pre_rel: None,
+            meta: None,
+        })
+    }
+
+    /// This spec version is compatible if its major version is greater than or
+    /// equal to `other`'s
+    pub fn is_compatible(&self, other: &Self) -> bool {
+        self.0.major >= other.major()
+    }
+
+    pub fn major(&self) -> u32 {
+        self.0.major
+    }
+
+    pub fn minor(&self) -> u32 {
+        self.0.minor
+    }
+
+    pub fn patch(&self) -> u32 {
+        self.0.patch
+    }
+}
+
+impl Default for SpecVersion {
+    fn default() -> Self {
+        Self::current()
+    }
+}
+
+impl fmt::Display for SpecVersion {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+impl FromStr for SpecVersion {
+    type Err = <SemVer as FromStr>::Err;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        SemVer::from_str(s).map(Self)
+    }
+}
+
+impl<'a> TryFrom<&'a str> for SpecVersion {
+    type Error = <SemVer as TryFrom<&'a str>>::Error;
+
+    fn try_from(value: &str) -> Result<Self, Self::Error> {
+        SemVer::try_from(value).map(Self)
+    }
+}
+
+impl AsRef<SemVer> for SpecVersion {
+    fn as_ref(&self) -> &SemVer {
+        &self.0
+    }
+}
+
+impl serde::Serialize for SpecVersion {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        serializer.serialize_str(&self.to_string())
+    }
+}
+
+impl<'de> serde::Deserialize<'de> for SpecVersion {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let s: &str = serde::Deserialize::deserialize(deserializer)?;
+        Self::try_from(s).map_err(|_| serde::de::Error::custom("invalid version string"))
+    }
+}
+
+pub type Custom = serde_json::Map<String, serde_json::Value>;
+
+#[derive(
+    Clone, Copy, Eq, Ord, PartialEq, PartialOrd, Hash, serde::Serialize, serde::Deserialize,
+)]
+pub struct KeyId(#[serde(with = "hex::serde")] [u8; 32]);
+
+impl KeyId {
+    pub fn as_bytes(&self) -> &[u8] {
+        self.as_ref()
+    }
+}
+
+impl AsRef<[u8]> for KeyId {
+    fn as_ref(&self) -> &[u8] {
+        &self.0
+    }
+}
+
+impl From<&Key<'_>> for KeyId {
+    fn from(key: &Key<'_>) -> Self {
+        Self::from(&key.0)
+    }
+}
+
+impl From<Key<'_>> for KeyId {
+    fn from(key: Key<'_>) -> Self {
+        Self::from(key.0)
+    }
+}
+
+impl From<&VerificationKey<'_>> for KeyId {
+    fn from(key: &VerificationKey<'_>) -> Self {
+        Self(key.sha256())
+    }
+}
+
+impl From<VerificationKey<'_>> for KeyId {
+    fn from(key: VerificationKey<'_>) -> Self {
+        Self(key.sha256())
+    }
+}
+
+impl fmt::Display for KeyId {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str(&hex::encode(self.0))
+    }
+}
+
+impl fmt::Debug for KeyId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("KeyId").field(&hex::encode(self.0)).finish()
+    }
+}
+
+#[derive(Clone, Eq, PartialEq, serde::Serialize, serde::Deserialize)]
+pub struct ContentHash {
+    #[serde(with = "hex::serde")]
+    pub sha1: [u8; 20],
+    #[serde(with = "hex::serde")]
+    pub sha2: [u8; 32],
+}
+
+impl ContentHash {
+    pub fn as_oid(&self) -> git2::Oid {
+        self.into()
+    }
+}
+
+impl From<&git2::Blob<'_>> for ContentHash {
+    fn from(blob: &git2::Blob) -> Self {
+        let sha1 = blob
+            .id()
+            .as_bytes()
+            .try_into()
+            .expect("libgit2 to support only sha1 oids");
+        let sha2 = blob_hash_sha2(blob.content());
+
+        Self { sha1, sha2 }
+    }
+}
+
+impl From<&ContentHash> for git2::Oid {
+    fn from(ContentHash { sha1, .. }: &ContentHash) -> Self {
+        Self::from_bytes(sha1).expect("20 bytes are a valid git2::Oid")
+    }
+}
+
+impl PartialEq<git2::Oid> for ContentHash {
+    fn eq(&self, other: &git2::Oid) -> bool {
+        self.sha1.as_slice() == other.as_bytes()
+    }
+}
+
+impl fmt::Debug for ContentHash {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.debug_struct("ContentHash")
+            .field("sha1", &hex::encode(self.sha1))
+            .field("sha2", &hex::encode(self.sha2))
+            .finish()
+    }
+}
+
+impl fmt::Display for ContentHash {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str(&hex::encode(self.sha1))
+    }
+}
+
+#[derive(
+    Clone, Copy, Debug, Eq, Ord, PartialEq, PartialOrd, serde::Serialize, serde::Deserialize,
+)]
+pub struct DateTime(#[serde(with = "time::serde::rfc3339")] OffsetDateTime);
+
+impl DateTime {
+    pub fn now() -> Self {
+        Self(time::OffsetDateTime::now_utc())
+    }
+
+    pub const fn checked_add(self, duration: Duration) -> Option<Self> {
+        // `map` is not const yet
+        match self.0.checked_add(duration) {
+            None => None,
+            Some(x) => Some(Self(x)),
+        }
+    }
+}
+
+impl FromStr for DateTime {
+    type Err = time::error::Parse;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        OffsetDateTime::parse(s, &time::format_description::well_known::Rfc3339)
+            .map(|dt| dt.to_offset(UtcOffset::UTC))
+            .map(Self)
+    }
+}
+
+impl Deref for DateTime {
+    type Target = time::OffsetDateTime;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+#[derive(serde::Serialize, serde::Deserialize)]
+#[serde(tag = "_type")]
+pub enum Metadata<'a> {
+    #[serde(rename = "eagain.io/it/identity")]
+    Identity(Cow<'a, Identity>),
+    #[serde(rename = "eagain.io/it/drop")]
+    Drop(Cow<'a, Drop>),
+    #[serde(rename = "eagain.io/it/mirrors")]
+    Mirrors(Cow<'a, Mirrors>),
+    #[serde(rename = "eagain.io/it/alternates")]
+    Alternates(Cow<'a, Alternates>),
+}
+
+impl<'a> Metadata<'a> {
+    pub fn identity<T>(s: T) -> Self
+    where
+        T: Into<Cow<'a, Identity>>,
+    {
+        Self::Identity(s.into())
+    }
+
+    pub fn drop<T>(d: T) -> Self
+    where
+        T: Into<Cow<'a, Drop>>,
+    {
+        Self::Drop(d.into())
+    }
+
+    pub fn mirrors<T>(a: T) -> Self
+    where
+        T: Into<Cow<'a, Mirrors>>,
+    {
+        Self::Mirrors(a.into())
+    }
+
+    pub fn alternates<T>(a: T) -> Self
+    where
+        T: Into<Cow<'a, Alternates>>,
+    {
+        Self::Alternates(a.into())
+    }
+
+    pub fn sign<'b, I, S>(self, keys: I) -> crate::Result<Signed<Self>>
+    where
+        I: IntoIterator<Item = &'b mut S>,
+        S: Signer + ?Sized + 'b,
+    {
+        let payload = Sha512::digest(canonical::to_vec(&self)?);
+        let signatures = keys
+            .into_iter()
+            .map(|signer| {
+                let keyid = KeyId::from(signer.ident());
+                let sig = signer.sign(&payload)?;
+                Ok::<_, crate::Error>((keyid, Signature::from(sig)))
+            })
+            .collect::<Result<_, _>>()?;
+
+        Ok(Signed {
+            signed: self,
+            signatures,
+        })
+    }
+}
+
+impl From<Identity> for Metadata<'static> {
+    fn from(s: Identity) -> Self {
+        Self::identity(s)
+    }
+}
+
+impl<'a> From<&'a Identity> for Metadata<'a> {
+    fn from(s: &'a Identity) -> Self {
+        Self::identity(s)
+    }
+}
+
+impl From<Drop> for Metadata<'static> {
+    fn from(d: Drop) -> Self {
+        Self::drop(d)
+    }
+}
+
+impl<'a> From<&'a Drop> for Metadata<'a> {
+    fn from(d: &'a Drop) -> Self {
+        Self::drop(d)
+    }
+}
+
+impl From<Mirrors> for Metadata<'static> {
+    fn from(m: Mirrors) -> Self {
+        Self::mirrors(m)
+    }
+}
+
+impl<'a> From<&'a Mirrors> for Metadata<'a> {
+    fn from(m: &'a Mirrors) -> Self {
+        Self::mirrors(m)
+    }
+}
+
+impl From<Alternates> for Metadata<'static> {
+    fn from(a: Alternates) -> Self {
+        Self::alternates(a)
+    }
+}
+
+impl<'a> From<&'a Alternates> for Metadata<'a> {
+    fn from(a: &'a Alternates) -> Self {
+        Self::alternates(a)
+    }
+}
+
+impl<'a> TryFrom<Metadata<'a>> for Cow<'a, Identity> {
+    type Error = Metadata<'a>;
+
+    fn try_from(value: Metadata<'a>) -> Result<Self, Self::Error> {
+        match value {
+            Metadata::Identity(inner) => Ok(inner),
+            _ => Err(value),
+        }
+    }
+}
+
+impl<'a> TryFrom<Metadata<'a>> for Cow<'a, Drop> {
+    type Error = Metadata<'a>;
+
+    fn try_from(value: Metadata<'a>) -> Result<Self, Self::Error> {
+        match value {
+            Metadata::Drop(inner) => Ok(inner),
+            _ => Err(value),
+        }
+    }
+}
+
+impl<'a> TryFrom<Metadata<'a>> for Cow<'a, Mirrors> {
+    type Error = Metadata<'a>;
+
+    fn try_from(value: Metadata<'a>) -> Result<Self, Self::Error> {
+        match value {
+            Metadata::Mirrors(inner) => Ok(inner),
+            _ => Err(value),
+        }
+    }
+}
+
+impl<'a> TryFrom<Metadata<'a>> for Cow<'a, Alternates> {
+    type Error = Metadata<'a>;
+
+    fn try_from(value: Metadata<'a>) -> Result<Self, Self::Error> {
+        match value {
+            Metadata::Alternates(inner) => Ok(inner),
+            _ => Err(value),
+        }
+    }
+}
+
+#[derive(Clone, serde::Serialize, serde::Deserialize)]
+pub struct Signed<T> {
+    pub signed: T,
+    pub signatures: BTreeMap<KeyId, Signature>,
+}
+
+impl<T> Signed<T> {
+    pub fn fmap<U, F>(self, f: F) -> Signed<U>
+    where
+        F: FnOnce(T) -> U,
+    {
+        Signed {
+            signed: f(self.signed),
+            signatures: self.signatures,
+        }
+    }
+}
+
+impl<T, E> Signed<Result<T, E>> {
+    pub fn transpose(self) -> Result<Signed<T>, E> {
+        let Self { signed, signatures } = self;
+        signed.map(|signed| Signed { signed, signatures })
+    }
+}
+
+impl<T: HasPrev> Signed<T> {
+    pub fn ancestors<F>(&self, find_prev: F) -> impl Iterator<Item = io::Result<Self>>
+    where
+        F: FnMut(&ContentHash) -> io::Result<Self>,
+    {
+        Ancestors {
+            prev: self.signed.prev().cloned(),
+            find_prev,
+            _marker: PhantomData,
+        }
+    }
+
+    pub fn has_ancestor<F>(&self, ancestor: &ContentHash, find_prev: F) -> io::Result<bool>
+    where
+        F: FnMut(&ContentHash) -> io::Result<Self>,
+    {
+        match self.signed.prev() {
+            None => Ok(false),
+            Some(parent) if parent == ancestor => Ok(true),
+            Some(_) => {
+                for prev in self.ancestors(find_prev) {
+                    match prev?.signed.prev() {
+                        None => return Ok(false),
+                        Some(parent) if parent == ancestor => return Ok(true),
+                        _ => continue,
+                    }
+                }
+
+                Ok(false)
+            },
+        }
+    }
+}
+
+impl Signed<Drop> {
+    pub fn verified<'a, F, G>(
+        self,
+        find_prev: F,
+        find_signer: G,
+    ) -> Result<drop::Verified, error::Verification>
+    where
+        F: FnMut(&ContentHash) -> io::Result<Self>,
+        G: FnMut(&IdentityId) -> io::Result<KeySet<'a>>,
+    {
+        self.signed
+            .verified(&self.signatures, find_prev, find_signer)
+    }
+}
+
+impl Signed<Identity> {
+    pub fn verified<F>(self, find_prev: F) -> Result<identity::Verified, error::Verification>
+    where
+        F: FnMut(&ContentHash) -> io::Result<Self>,
+    {
+        self.signed.verified(&self.signatures, find_prev)
+    }
+
+    pub fn verify<F>(&self, find_prev: F) -> Result<IdentityId, error::Verification>
+    where
+        F: FnMut(&ContentHash) -> io::Result<Self>,
+    {
+        self.signed.verify(&self.signatures, find_prev)
+    }
+}
+
+impl<T> AsRef<T> for Signed<T> {
+    fn as_ref(&self) -> &T {
+        &self.signed
+    }
+}
+
+struct Ancestors<T, F> {
+    prev: Option<ContentHash>,
+    find_prev: F,
+    _marker: PhantomData<T>,
+}
+
+impl<T, F> Iterator for Ancestors<T, F>
+where
+    T: HasPrev,
+    F: FnMut(&ContentHash) -> io::Result<Signed<T>>,
+{
+    type Item = io::Result<Signed<T>>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let prev = self.prev.take()?;
+        (self.find_prev)(&prev)
+            .map(|parent| {
+                self.prev = parent.signed.prev().cloned();
+                Some(parent)
+            })
+            .transpose()
+    }
+}
+
+pub trait HasPrev {
+    fn prev(&self) -> Option<&ContentHash>;
+}
+
+impl HasPrev for Identity {
+    fn prev(&self) -> Option<&ContentHash> {
+        self.prev.as_ref()
+    }
+}
+
+impl HasPrev for Drop {
+    fn prev(&self) -> Option<&ContentHash> {
+        self.prev.as_ref()
+    }
+}
+
+#[derive(Clone)]
+pub struct Key<'a>(VerificationKey<'a>);
+
+impl Key<'_> {
+    pub fn id(&self) -> KeyId {
+        self.into()
+    }
+}
+
+impl fmt::Debug for Key<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_tuple("Key").field(&self.0.to_string()).finish()
+    }
+}
+
+impl<'a> From<VerificationKey<'a>> for Key<'a> {
+    fn from(vk: VerificationKey<'a>) -> Self {
+        Self(vk.without_comment())
+    }
+}
+
+impl signature::Verifier<Signature> for Key<'_> {
+    fn verify(&self, msg: &[u8], signature: &Signature) -> Result<(), signature::Error> {
+        let ssh = ssh::Signature::new(self.0.algorithm(), signature.as_ref())?;
+        self.0.verify(msg, &ssh)
+    }
+}
+
+impl serde::Serialize for Key<'_> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        serializer.serialize_str(&self.0.to_openssh().map_err(serde::ser::Error::custom)?)
+    }
+}
+
+impl<'de> serde::Deserialize<'de> for Key<'_> {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let s: &str = serde::Deserialize::deserialize(deserializer)?;
+        VerificationKey::from_openssh(s)
+            .map(Self)
+            .map_err(serde::de::Error::custom)
+    }
+}
+
+impl FromStr for Key<'_> {
+    type Err = ssh_key::Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        VerificationKey::from_openssh(s).map(Self)
+    }
+}
+
+#[derive(Clone, Default)]
+pub struct KeySet<'a>(BTreeMap<KeyId, Key<'a>>);
+
+impl<'a> Deref for KeySet<'a> {
+    type Target = BTreeMap<KeyId, Key<'a>>;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<'a> DerefMut for KeySet<'a> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        &mut self.0
+    }
+}
+
+impl<'a> FromIterator<Key<'a>> for KeySet<'a> {
+    fn from_iter<T: IntoIterator<Item = Key<'a>>>(iter: T) -> Self {
+        let mut kv = BTreeMap::new();
+        for key in iter {
+            kv.insert(KeyId::from(&key), key);
+        }
+        Self(kv)
+    }
+}
+
+impl serde::Serialize for KeySet<'_> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        let mut seq = serializer.serialize_seq(Some(self.0.len()))?;
+        for key in self.0.values() {
+            seq.serialize_element(key)?;
+        }
+        seq.end()
+    }
+}
+
+impl<'de> serde::Deserialize<'de> for KeySet<'static> {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        struct Visitor;
+
+        impl<'de> serde::de::Visitor<'de> for Visitor {
+            type Value = KeySet<'static>;
+
+            fn expecting(&self, f: &mut fmt::Formatter) -> fmt::Result {
+                f.write_str("a sequence of keys")
+            }
+
+            fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+            where
+                A: serde::de::SeqAccess<'de>,
+            {
+                let mut kv = BTreeMap::new();
+                while let Some(key) = seq.next_element()? {
+                    kv.insert(KeyId::from(&key), key);
+                }
+
+                Ok(KeySet(kv))
+            }
+        }
+
+        deserializer.deserialize_seq(Visitor)
+    }
+}
+
+#[derive(Clone, Debug, serde::Serialize, serde::Deserialize)]
+pub struct Signature(#[serde(with = "hex::serde")] Vec<u8>);
+
+impl From<ssh::Signature> for Signature {
+    fn from(sig: ssh::Signature) -> Self {
+        Self(sig.as_bytes().to_vec())
+    }
+}
+
+impl AsRef<[u8]> for Signature {
+    fn as_ref(&self) -> &[u8] {
+        self.0.as_ref()
+    }
+}
+
+impl signature::Signature for Signature {
+    fn from_bytes(bytes: &[u8]) -> Result<Self, signature::Error> {
+        Ok(Self(bytes.to_vec()))
+    }
+}
+
+pub struct Verified<T>(T);
+
+impl<T> Verified<T> {
+    pub fn into_inner(self) -> T {
+        self.0
+    }
+}
+
+impl<T> Deref for Verified<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
diff --git a/src/metadata/drop.rs b/src/metadata/drop.rs
new file mode 100644
index 0000000..d231712
--- /dev/null
+++ b/src/metadata/drop.rs
@@ -0,0 +1,274 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    borrow::Cow,
+    collections::{
+        BTreeMap,
+        BTreeSet,
+        HashMap,
+    },
+    io,
+    num::NonZeroUsize,
+};
+
+use log::warn;
+use sha2::{
+    Digest,
+    Sha512,
+};
+use signature::Verifier;
+
+use super::{
+    error,
+    Alternates,
+    ContentHash,
+    Custom,
+    DateTime,
+    IdentityId,
+    KeyId,
+    KeySet,
+    Metadata,
+    Mirrors,
+    Signature,
+    Signed,
+    SpecVersion,
+};
+use crate::{
+    git::Refname,
+    json::canonical,
+    str::Varchar,
+};
+
+#[derive(Clone, serde::Serialize, serde::Deserialize)]
+pub struct Roles {
+    pub root: Role,
+    pub snapshot: Role,
+    pub mirrors: Role,
+    pub branches: HashMap<Refname, Annotated>,
+}
+
+impl Roles {
+    pub(crate) fn ids(&self) -> BTreeSet<IdentityId> {
+        let Self {
+            root: Role { ids: root, .. },
+            snapshot: Role { ids: snapshot, .. },
+            mirrors: Role { ids: mirrors, .. },
+            branches,
+        } = self;
+
+        let mut ids = BTreeSet::new();
+        ids.extend(root);
+        ids.extend(snapshot);
+        ids.extend(mirrors);
+        ids.extend(branches.values().flat_map(|a| &a.role.ids));
+        ids
+    }
+}
+
+#[derive(Clone, serde::Serialize, serde::Deserialize)]
+pub struct Role {
+    pub ids: BTreeSet<IdentityId>,
+    pub threshold: NonZeroUsize,
+}
+
+pub type Description = Varchar<String, 128>;
+
+#[derive(Clone, serde::Serialize, serde::Deserialize)]
+pub struct Annotated {
+    #[serde(flatten)]
+    pub role: Role,
+    pub description: Description,
+}
+
+pub type Verified = super::Verified<Drop>;
+
+#[derive(Clone, serde::Serialize, serde::Deserialize)]
+pub struct Drop {
+    pub spec_version: SpecVersion,
+    #[serde(default = "Description::new")]
+    pub description: Description,
+    pub prev: Option<ContentHash>,
+    pub roles: Roles,
+    #[serde(default)]
+    pub custom: Custom,
+}
+
+impl Drop {
+    pub fn verified<'a, F, G>(
+        self,
+        signatures: &BTreeMap<KeyId, Signature>,
+        find_prev: F,
+        find_signer: G,
+    ) -> Result<Verified, error::Verification>
+    where
+        F: FnMut(&ContentHash) -> io::Result<Signed<Self>>,
+        G: FnMut(&IdentityId) -> io::Result<KeySet<'a>>,
+    {
+        self.verify(signatures, find_prev, find_signer)?;
+        Ok(super::Verified(self))
+    }
+
+    pub fn verify<'a, F, G>(
+        &self,
+        signatures: &BTreeMap<KeyId, Signature>,
+        mut find_prev: F,
+        mut find_signer: G,
+    ) -> Result<(), error::Verification>
+    where
+        F: FnMut(&ContentHash) -> io::Result<Signed<Self>>,
+        G: FnMut(&IdentityId) -> io::Result<KeySet<'a>>,
+    {
+        use error::Verification::*;
+
+        if !crate::SPEC_VERSION.is_compatible(&self.spec_version) {
+            return Err(IncompatibleSpecVersion);
+        }
+
+        let canonical = self.canonicalise()?;
+        let payload = Sha512::digest(&canonical);
+        verify::AuthorisedSigners::from_ids(&self.roles.root.ids, &mut find_signer)?
+            .verify_signatures(&payload, self.roles.root.threshold, signatures)?;
+        if let Some(prev) = self.prev.as_ref().map(&mut find_prev).transpose()? {
+            verify::AuthorisedSigners::from_ids(&prev.signed.roles.root.ids, &mut find_signer)?
+                .verify_signatures(&payload, prev.signed.roles.root.threshold, signatures)?;
+            return prev.signed.verify(&prev.signatures, find_prev, find_signer);
+        }
+
+        Ok(())
+    }
+
+    pub fn verify_mirrors<'a, F>(
+        &self,
+        mirrors: &Signed<Mirrors>,
+        find_signer: F,
+    ) -> Result<(), error::Verification>
+    where
+        F: FnMut(&IdentityId) -> io::Result<KeySet<'a>>,
+    {
+        use error::Verification::*;
+
+        if let Some(deadline) = &mirrors.signed.expires {
+            if deadline < &DateTime::now() {
+                return Err(Expired);
+            }
+        }
+        if !crate::SPEC_VERSION.is_compatible(&mirrors.signed.spec_version) {
+            return Err(IncompatibleSpecVersion);
+        }
+
+        let payload = Sha512::digest(mirrors.signed.canonicalise()?);
+        verify::AuthorisedSigners::from_ids(&self.roles.mirrors.ids, find_signer)?
+            .verify_signatures(&payload, self.roles.mirrors.threshold, &mirrors.signatures)
+    }
+
+    pub fn verify_alternates<'a, F>(
+        &self,
+        alt: &Signed<Alternates>,
+        find_signer: F,
+    ) -> Result<(), error::Verification>
+    where
+        F: FnMut(&IdentityId) -> io::Result<KeySet<'a>>,
+    {
+        use error::Verification::*;
+
+        if let Some(deadline) = &alt.signed.expires {
+            if deadline < &DateTime::now() {
+                return Err(Expired);
+            }
+        }
+        if !crate::SPEC_VERSION.is_compatible(&alt.signed.spec_version) {
+            return Err(IncompatibleSpecVersion);
+        }
+
+        let payload = Sha512::digest(alt.signed.canonicalise()?);
+        verify::AuthorisedSigners::from_ids(&self.roles.mirrors.ids, find_signer)?
+            .verify_signatures(&payload, self.roles.mirrors.threshold, &alt.signatures)
+    }
+
+    pub fn canonicalise(&self) -> Result<Vec<u8>, canonical::error::Canonicalise> {
+        canonical::to_vec(Metadata::drop(self))
+    }
+}
+
+impl From<Drop> for Cow<'static, Drop> {
+    fn from(d: Drop) -> Self {
+        Self::Owned(d)
+    }
+}
+
+impl<'a> From<&'a Drop> for Cow<'a, Drop> {
+    fn from(d: &'a Drop) -> Self {
+        Self::Borrowed(d)
+    }
+}
+
+mod verify {
+    use super::*;
+
+    pub struct AuthorisedSigners<'a, 'b>(BTreeMap<&'a IdentityId, KeySet<'b>>);
+
+    impl<'a, 'b> AuthorisedSigners<'a, 'b> {
+        pub fn from_ids<F>(
+            ids: &'a BTreeSet<IdentityId>,
+            mut find_signer: F,
+        ) -> Result<AuthorisedSigners<'a, 'b>, error::Verification>
+        where
+            F: FnMut(&IdentityId) -> io::Result<KeySet<'b>>,
+        {
+            let mut signers = BTreeMap::new();
+            for id in ids {
+                signers.insert(id, find_signer(id)?);
+            }
+            signers
+                .values()
+                .try_fold(BTreeSet::new(), |mut all_keys, keys| {
+                    for key in keys.keys() {
+                        if !all_keys.insert(key) {
+                            return Err(error::Verification::DuplicateKey(*key));
+                        }
+                    }
+
+                    Ok(all_keys)
+                })?;
+
+            Ok(Self(signers))
+        }
+
+        pub fn verify_signatures<'c, S>(
+            &mut self,
+            payload: &[u8],
+            threshold: NonZeroUsize,
+            signatures: S,
+        ) -> Result<(), error::Verification>
+        where
+            S: IntoIterator<Item = (&'c KeyId, &'c Signature)>,
+        {
+            use error::Verification::SignatureThreshold;
+
+            let mut need_signatures = threshold.get();
+            for (key_id, signature) in signatures {
+                if let Some(sig_id) = self.0.iter().find_map(|(id, keys)| {
+                    #[allow(clippy::unnecessary_lazy_evaluations)]
+                    keys.contains_key(key_id).then(|| *id)
+                }) {
+                    let key = self.0.remove(sig_id).unwrap().remove(key_id).unwrap();
+                    if key.verify(payload, signature).is_ok() {
+                        need_signatures -= 1;
+                    } else {
+                        warn!("Bad signature by {key_id}");
+                    }
+
+                    if need_signatures == 0 {
+                        break;
+                    }
+                }
+            }
+            if need_signatures > 0 {
+                return Err(SignatureThreshold);
+            }
+
+            Ok(())
+        }
+    }
+}
diff --git a/src/metadata/error.rs b/src/metadata/error.rs
new file mode 100644
index 0000000..66173f9
--- /dev/null
+++ b/src/metadata/error.rs
@@ -0,0 +1,40 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::io;
+
+use thiserror::Error;
+
+use super::KeyId;
+use crate::json::canonical::error::Canonicalise;
+
+#[derive(Debug, Error)]
+pub enum SigId {
+    #[error("payload not at root revision")]
+    NotRoot,
+
+    #[error("invalid payload: canonicalisation failed")]
+    Canonical(#[from] Canonicalise),
+}
+
+#[derive(Debug, Error)]
+#[non_exhaustive]
+pub enum Verification {
+    #[error("incompatible spec version")]
+    IncompatibleSpecVersion,
+
+    #[error("canonicalisation failed")]
+    Canonicalise(#[from] Canonicalise),
+
+    #[error("required signature threshold not met")]
+    SignatureThreshold,
+
+    #[error("metadata past its expiry date")]
+    Expired,
+
+    #[error("duplicate key: key {0} appears in more than one identity")]
+    DuplicateKey(KeyId),
+
+    #[error(transparent)]
+    Io(#[from] io::Error),
+}
diff --git a/src/metadata/git.rs b/src/metadata/git.rs
new file mode 100644
index 0000000..1dde3da
--- /dev/null
+++ b/src/metadata/git.rs
@@ -0,0 +1,232 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    borrow::Cow,
+    io,
+};
+
+use anyhow::anyhow;
+
+use super::{
+    drop,
+    identity,
+    Alternates,
+    ContentHash,
+    Drop,
+    Identity,
+    IdentityId,
+    KeySet,
+    Metadata,
+    Mirrors,
+    Signed,
+};
+use crate::{
+    cmd,
+    git::if_not_found_none,
+    json,
+};
+
+pub const META_FILE_ALTERNATES: &str = "alternates.json";
+pub const META_FILE_DROP: &str = "drop.json";
+pub const META_FILE_ID: &str = "id.json";
+pub const META_FILE_MIRRORS: &str = "mirrors.json";
+
+pub mod error {
+    use thiserror::Error;
+
+    #[derive(Debug, Error)]
+    #[error("unexpected metadata type")]
+    pub struct TypeMismatch;
+
+    #[derive(Debug, Error)]
+    #[error("{file} not found in tree")]
+    pub struct FileNotFound {
+        pub file: &'static str,
+    }
+}
+
+pub struct GitMeta<T> {
+    pub hash: ContentHash,
+    pub signed: Signed<T>,
+}
+
+pub type GitIdentity = GitMeta<Identity>;
+pub type GitDrop = GitMeta<Drop>;
+pub type GitMirrors = GitMeta<Mirrors>;
+pub type GitAlternates = GitMeta<Alternates>;
+
+impl GitMeta<Drop> {
+    pub fn verified<'a, F, G>(
+        self,
+        find_prev: F,
+        find_signer: G,
+    ) -> Result<drop::Verified, super::error::Verification>
+    where
+        F: FnMut(&ContentHash) -> io::Result<Signed<Drop>>,
+        G: FnMut(&IdentityId) -> io::Result<KeySet<'a>>,
+    {
+        self.signed.verified(find_prev, find_signer)
+    }
+}
+
+impl GitMeta<Identity> {
+    pub fn verified<F>(self, find_prev: F) -> Result<identity::Verified, super::error::Verification>
+    where
+        F: FnMut(&ContentHash) -> io::Result<Signed<Identity>>,
+    {
+        self.signed.verified(find_prev)
+    }
+}
+
+pub struct FromSearchPath<'a, T> {
+    /// The repository (from the search path) the object was found in
+    pub repo: &'a git2::Repository,
+    pub meta: GitMeta<T>,
+}
+
+pub trait FromGit: Sized + Clone
+where
+    for<'a> Cow<'a, Self>: TryFrom<Metadata<'a>>,
+{
+    const METADATA_JSON: &'static str;
+
+    fn from_blob(blob: &git2::Blob) -> crate::Result<GitMeta<Self>> {
+        let hash = ContentHash::from(blob);
+        let signed = json::from_blob::<Signed<Metadata>>(blob)?
+            .fmap(Cow::<Self>::try_from)
+            .transpose()
+            .map_err(|_| error::TypeMismatch)?
+            .fmap(Cow::into_owned);
+
+        Ok(GitMeta { hash, signed })
+    }
+
+    fn from_tip<R: AsRef<str>>(
+        repo: &git2::Repository,
+        refname: R,
+    ) -> crate::Result<GitMeta<Self>> {
+        Self::from_reference(repo, &repo.find_reference(refname.as_ref())?)
+    }
+
+    fn from_reference(
+        repo: &git2::Repository,
+        reference: &git2::Reference,
+    ) -> crate::Result<GitMeta<Self>> {
+        Self::from_commit(repo, &reference.peel_to_commit()?)
+    }
+
+    fn from_commit(repo: &git2::Repository, commit: &git2::Commit) -> crate::Result<GitMeta<Self>> {
+        Self::from_tree(repo, &commit.tree()?)
+    }
+
+    fn from_tree(repo: &git2::Repository, tree: &git2::Tree) -> crate::Result<GitMeta<Self>> {
+        let entry = tree
+            .get_name(Self::METADATA_JSON)
+            .ok_or(error::FileNotFound {
+                file: Self::METADATA_JSON,
+            })?;
+        let blob = entry.to_object(repo)?.peel_to_blob()?;
+
+        Self::from_blob(&blob)
+    }
+
+    fn from_content_hash(
+        repo: &git2::Repository,
+        hash: &ContentHash,
+    ) -> crate::Result<GitMeta<Self>> {
+        let blob = repo.find_blob(hash.into())?;
+        Self::from_blob(&blob)
+    }
+
+    fn from_search_path<R: AsRef<str>>(
+        search_path: &[git2::Repository],
+        refname: R,
+    ) -> crate::Result<FromSearchPath<Self>> {
+        let (repo, reference) = find_ref_in_path(search_path, refname.as_ref())?
+            .ok_or_else(|| anyhow!("{} not found in search path", refname.as_ref()))?;
+        Self::from_reference(repo, &reference).map(|meta| FromSearchPath { repo, meta })
+    }
+}
+
+impl FromGit for Identity {
+    const METADATA_JSON: &'static str = META_FILE_ID;
+}
+
+impl FromGit for Drop {
+    const METADATA_JSON: &'static str = META_FILE_DROP;
+}
+
+impl FromGit for Mirrors {
+    const METADATA_JSON: &'static str = META_FILE_MIRRORS;
+}
+
+impl FromGit for Alternates {
+    const METADATA_JSON: &'static str = META_FILE_ALTERNATES;
+}
+
+pub fn find_parent<T>(
+    repo: &git2::Repository,
+) -> impl Fn(&ContentHash) -> io::Result<Signed<T>> + '_
+where
+    T: FromGit,
+    for<'a> Cow<'a, T>: TryFrom<Metadata<'a>>,
+{
+    |hash| {
+        T::from_content_hash(repo, hash)
+            .map_err(as_io)
+            .map(|meta| meta.signed)
+    }
+}
+
+pub fn find_parent_in_tree<'a, T>(
+    repo: &'a git2::Repository,
+    tree: &'a git2::Tree<'a>,
+) -> impl Fn(&ContentHash) -> io::Result<Signed<T>> + 'a
+where
+    T: FromGit,
+    for<'b> Cow<'b, T>: TryFrom<Metadata<'b>>,
+{
+    fn go<T>(
+        repo: &git2::Repository,
+        tree: &git2::Tree,
+        hash: &ContentHash,
+    ) -> crate::Result<Signed<T>>
+    where
+        T: FromGit,
+        for<'b> Cow<'b, T>: TryFrom<Metadata<'b>>,
+    {
+        let oid = git2::Oid::from(hash);
+        let blob = tree
+            .get_id(oid)
+            .ok_or_else(|| anyhow!("parent {} not found in tree {}", oid, tree.id()))?
+            .to_object(repo)?
+            .into_blob()
+            .map_err(|_| anyhow!("parent {} is not a file", oid))?;
+
+        T::from_blob(&blob).map(|meta| meta.signed)
+    }
+
+    move |hash| go(repo, tree, hash).map_err(as_io)
+}
+
+pub fn find_ref_in_path<'a>(
+    search_path: &'a [git2::Repository],
+    name: &str,
+) -> cmd::Result<Option<(&'a git2::Repository, git2::Reference<'a>)>> {
+    for repo in search_path {
+        let have_ref = if_not_found_none(repo.resolve_reference_from_short_name(name))?;
+        if let Some(r) = have_ref {
+            return Ok(Some((repo, r)));
+        }
+    }
+
+    Ok(None)
+}
+
+fn as_io<E>(e: E) -> io::Error
+where
+    E: Into<Box<dyn std::error::Error + Send + Sync>>,
+{
+    io::Error::new(io::ErrorKind::Other, e)
+}
diff --git a/src/metadata/identity.rs b/src/metadata/identity.rs
new file mode 100644
index 0000000..8071e84
--- /dev/null
+++ b/src/metadata/identity.rs
@@ -0,0 +1,366 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    borrow::Cow,
+    collections::{
+        BTreeMap,
+        BTreeSet,
+    },
+    fmt,
+    io,
+    marker::PhantomData,
+    num::NonZeroUsize,
+    path::PathBuf,
+    str::FromStr,
+};
+
+use anyhow::{
+    anyhow,
+    ensure,
+};
+use hex::FromHex;
+use log::warn;
+use sha2::{
+    Digest,
+    Sha256,
+    Sha512,
+};
+use signature::Verifier;
+use url::Url;
+
+use super::{
+    error,
+    git::{
+        find_parent_in_tree,
+        FromGit,
+        META_FILE_ID,
+    },
+    Ancestors,
+    ContentHash,
+    Custom,
+    DateTime,
+    Key,
+    KeyId,
+    KeySet,
+    Metadata,
+    Signature,
+    Signed,
+    SpecVersion,
+};
+use crate::{
+    json::{
+        self,
+        canonical,
+    },
+    metadata::git::find_parent,
+};
+
+#[derive(
+    Clone, Copy, Eq, Ord, PartialEq, PartialOrd, Hash, serde::Serialize, serde::Deserialize,
+)]
+pub struct IdentityId(#[serde(with = "hex::serde")] [u8; 32]);
+
+impl TryFrom<&Identity> for IdentityId {
+    type Error = error::SigId;
+
+    fn try_from(id: &Identity) -> Result<Self, Self::Error> {
+        if id.prev.is_some() {
+            return Err(error::SigId::NotRoot);
+        }
+        let digest = Sha256::digest(id.canonicalise()?);
+        Ok(Self(digest.into()))
+    }
+}
+
+impl fmt::Display for IdentityId {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.write_str(&hex::encode(self.0))
+    }
+}
+
+impl fmt::Debug for IdentityId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&self.to_string())
+    }
+}
+
+impl FromStr for IdentityId {
+    type Err = hex::FromHexError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        FromHex::from_hex(s).map(Self)
+    }
+}
+
+impl TryFrom<String> for IdentityId {
+    type Error = hex::FromHexError;
+
+    fn try_from(value: String) -> Result<Self, Self::Error> {
+        FromHex::from_hex(value).map(Self)
+    }
+}
+
+pub struct Verified {
+    id: IdentityId,
+    cur: Identity,
+}
+
+impl Verified {
+    pub fn id(&self) -> &IdentityId {
+        &self.id
+    }
+
+    pub fn identity(&self) -> &Identity {
+        &self.cur
+    }
+
+    pub fn into_parts(self) -> (IdentityId, Identity) {
+        (self.id, self.cur)
+    }
+
+    /// `true` if signature is valid over message for any of the signer's
+    /// _current_ set of keys
+    pub fn did_sign<T: AsRef<[u8]>>(&self, msg: T, sig: &Signature) -> bool {
+        self.cur
+            .keys
+            .values()
+            .any(|key| key.verify(msg.as_ref(), sig).is_ok())
+    }
+}
+
+impl AsRef<Identity> for Verified {
+    fn as_ref(&self) -> &Identity {
+        self.identity()
+    }
+}
+
+#[derive(Clone, serde::Serialize, serde::Deserialize)]
+pub struct Identity {
+    pub spec_version: SpecVersion,
+    pub prev: Option<ContentHash>,
+    pub keys: KeySet<'static>,
+    pub threshold: NonZeroUsize,
+    pub mirrors: BTreeSet<Url>,
+    pub expires: Option<DateTime>,
+    #[serde(default)]
+    pub custom: Custom,
+}
+
+impl Identity {
+    pub fn verified<F>(
+        self,
+        signatures: &BTreeMap<KeyId, Signature>,
+        find_prev: F,
+    ) -> Result<Verified, error::Verification>
+    where
+        F: FnMut(&ContentHash) -> io::Result<Signed<Self>>,
+    {
+        let id = self.verify(signatures, find_prev)?;
+        Ok(Verified { id, cur: self })
+    }
+
+    pub fn verify<F>(
+        &self,
+        signatures: &BTreeMap<KeyId, Signature>,
+        find_prev: F,
+    ) -> Result<IdentityId, error::Verification>
+    where
+        F: FnMut(&ContentHash) -> io::Result<Signed<Self>>,
+    {
+        use error::Verification::Expired;
+
+        if let Some(deadline) = &self.expires {
+            if deadline < &DateTime::now() {
+                return Err(Expired);
+            }
+        }
+        self.verify_tail(Cow::Borrowed(signatures), find_prev)
+    }
+
+    fn verify_tail<F>(
+        &self,
+        signatures: Cow<BTreeMap<KeyId, Signature>>,
+        mut find_prev: F,
+    ) -> Result<IdentityId, error::Verification>
+    where
+        F: FnMut(&ContentHash) -> io::Result<Signed<Self>>,
+    {
+        use error::Verification::IncompatibleSpecVersion;
+
+        if !crate::SPEC_VERSION.is_compatible(&self.spec_version) {
+            return Err(IncompatibleSpecVersion);
+        }
+
+        let canonical = self.canonicalise()?;
+        let signed = Sha512::digest(&canonical);
+        verify_signatures(&signed, self.threshold, signatures.iter(), &self.keys)?;
+        if let Some(prev) = self.prev.as_ref().map(&mut find_prev).transpose()? {
+            verify_signatures(
+                &signed,
+                prev.signed.threshold,
+                signatures.iter(),
+                &prev.signed.keys,
+            )?;
+            return prev
+                .signed
+                .verify_tail(Cow::Owned(prev.signatures), find_prev);
+        }
+
+        Ok(IdentityId(Sha256::digest(canonical).into()))
+    }
+
+    pub fn canonicalise(&self) -> Result<Vec<u8>, canonical::error::Canonicalise> {
+        canonical::to_vec(Metadata::identity(self))
+    }
+
+    pub fn ancestors<F>(&self, find_prev: F) -> impl Iterator<Item = io::Result<Signed<Self>>>
+    where
+        F: FnMut(&ContentHash) -> io::Result<Signed<Self>>,
+    {
+        Ancestors {
+            prev: self.prev.clone(),
+            find_prev,
+            _marker: PhantomData,
+        }
+    }
+
+    pub fn has_ancestor<F>(&self, ancestor: &ContentHash, find_prev: F) -> io::Result<bool>
+    where
+        F: FnMut(&ContentHash) -> io::Result<Signed<Self>>,
+    {
+        match &self.prev {
+            None => Ok(false),
+            Some(parent) if parent == ancestor => Ok(true),
+            Some(_) => {
+                for prev in self.ancestors(find_prev) {
+                    match &prev?.signed.prev {
+                        None => return Ok(false),
+                        Some(parent) if parent == ancestor => return Ok(true),
+                        _ => continue,
+                    }
+                }
+
+                Ok(false)
+            },
+        }
+    }
+}
+
+impl From<Identity> for Cow<'static, Identity> {
+    fn from(s: Identity) -> Self {
+        Self::Owned(s)
+    }
+}
+
+impl<'a> From<&'a Identity> for Cow<'a, Identity> {
+    fn from(s: &'a Identity) -> Self {
+        Self::Borrowed(s)
+    }
+}
+
+fn verify_signatures<'a, S>(
+    payload: &[u8],
+    threshold: NonZeroUsize,
+    signatures: S,
+    keys: &BTreeMap<KeyId, Key>,
+) -> Result<(), error::Verification>
+where
+    S: IntoIterator<Item = (&'a KeyId, &'a Signature)>,
+{
+    use error::Verification::SignatureThreshold;
+
+    let mut need_signatures = threshold.get();
+    for (key_id, signature) in signatures {
+        if let Some(key) = keys.get(key_id) {
+            if key.verify(payload, signature).is_ok() {
+                need_signatures -= 1;
+            } else {
+                warn!("Bad signature by {key_id}");
+            }
+
+            if need_signatures == 0 {
+                break;
+            }
+        }
+    }
+    if need_signatures > 0 {
+        return Err(SignatureThreshold);
+    }
+
+    Ok(())
+}
+
+const FOLDED_HISTORY: &str = ".history";
+
+pub fn fold_to_tree<'a>(
+    repo: &'a git2::Repository,
+    tree: &mut git2::TreeBuilder<'a>,
+    Signed { signed, signatures }: Signed<Identity>,
+) -> crate::Result<()> {
+    use git2::FileMode::{
+        Blob,
+        Tree,
+    };
+
+    let meta = Signed {
+        signed: Metadata::from(&signed),
+        signatures,
+    };
+    tree.insert(META_FILE_ID, json::to_blob(repo, &meta)?, Blob.into())?;
+
+    let mut history = {
+        let existing = tree
+            .get(FOLDED_HISTORY)?
+            .map(|t| t.to_object(repo))
+            .transpose()?;
+        repo.treebuilder(existing.as_ref().and_then(git2::Object::as_tree))?
+    };
+    let mut parents = Vec::new();
+    for parent in signed.ancestors(find_parent(repo)) {
+        let meta = parent?.fmap(Metadata::from);
+        let blob = json::to_blob(repo, &meta)?;
+        parents.push(blob);
+    }
+    for (n, oid) in parents.into_iter().rev().enumerate() {
+        history.insert(&format!("{n}.json"), oid, Blob.into())?;
+    }
+    tree.insert(FOLDED_HISTORY, history.write()?, Tree.into())?;
+
+    Ok(())
+}
+
+pub fn find_in_tree(
+    repo: &git2::Repository,
+    root: &git2::Tree,
+    id: &IdentityId,
+) -> crate::Result<Verified> {
+    let (id_path, hist_path) = {
+        let base = PathBuf::from(id.to_string());
+        (base.join(META_FILE_ID), base.join(FOLDED_HISTORY))
+    };
+
+    let blob = root
+        .get_path(&id_path)?
+        .to_object(repo)?
+        .into_blob()
+        .map_err(|_| anyhow!("{} is not a file", id_path.display()))?;
+    let meta = Identity::from_blob(&blob)?.signed;
+    let hist = root
+        .get_path(&hist_path)?
+        .to_object(repo)?
+        .into_tree()
+        .map_err(|_| anyhow!("{} is not a directory", hist_path.display()))?;
+
+    let verified = meta
+        .signed
+        .verified(&meta.signatures, find_parent_in_tree(repo, &hist))?;
+    ensure!(
+        verified.id() == id,
+        "ids don't match after verification: expected {} found {}",
+        id,
+        verified.id()
+    );
+
+    Ok(verified)
+}
diff --git a/src/metadata/mirrors.rs b/src/metadata/mirrors.rs
new file mode 100644
index 0000000..9124dd3
--- /dev/null
+++ b/src/metadata/mirrors.rs
@@ -0,0 +1,95 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    borrow::Cow,
+    collections::BTreeSet,
+};
+
+use url::Url;
+
+use super::{
+    Custom,
+    DateTime,
+    Metadata,
+    SpecVersion,
+};
+use crate::{
+    json::canonical,
+    str::Varchar,
+};
+
+#[derive(Clone, serde::Serialize, serde::Deserialize)]
+pub struct Mirror {
+    pub url: Url,
+    #[serde(default)]
+    pub kind: Kind,
+    #[serde(default)]
+    pub custom: Custom,
+}
+
+#[derive(Clone, Default, serde::Serialize, serde::Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum Kind {
+    /// Can fetch bundles
+    Bundled,
+    /// Can fetch packs via git-protocol
+    #[default]
+    Packed,
+    /// Not serving bundles at all
+    Sparse,
+    /// Unknown kind
+    Unknown(Varchar<String, 16>),
+}
+
+#[derive(Clone, Default, serde::Serialize, serde::Deserialize)]
+pub struct Mirrors {
+    pub spec_version: SpecVersion,
+    pub mirrors: Vec<Mirror>,
+    pub expires: Option<DateTime>,
+}
+
+impl Mirrors {
+    pub fn canonicalise(&self) -> Result<Vec<u8>, canonical::error::Canonicalise> {
+        canonical::to_vec(Metadata::mirrors(self))
+    }
+}
+
+impl From<Mirrors> for Cow<'static, Mirrors> {
+    fn from(m: Mirrors) -> Self {
+        Self::Owned(m)
+    }
+}
+
+impl<'a> From<&'a Mirrors> for Cow<'a, Mirrors> {
+    fn from(m: &'a Mirrors) -> Self {
+        Self::Borrowed(m)
+    }
+}
+
+#[derive(Clone, Default, serde::Serialize, serde::Deserialize)]
+pub struct Alternates {
+    pub spec_version: SpecVersion,
+    pub alternates: BTreeSet<Url>,
+    #[serde(default)]
+    pub custom: Custom,
+    pub expires: Option<DateTime>,
+}
+
+impl Alternates {
+    pub fn canonicalise(&self) -> Result<Vec<u8>, canonical::error::Canonicalise> {
+        canonical::to_vec(Metadata::alternates(self))
+    }
+}
+
+impl From<Alternates> for Cow<'static, Alternates> {
+    fn from(a: Alternates) -> Self {
+        Self::Owned(a)
+    }
+}
+
+impl<'a> From<&'a Alternates> for Cow<'a, Alternates> {
+    fn from(a: &'a Alternates) -> Self {
+        Self::Borrowed(a)
+    }
+}
diff --git a/src/patches.rs b/src/patches.rs
new file mode 100644
index 0000000..8623e4e
--- /dev/null
+++ b/src/patches.rs
@@ -0,0 +1,212 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use core::{
+    fmt,
+    ops::Deref,
+};
+use std::{
+    io::BufRead,
+    str::FromStr,
+};
+
+use anyhow::{
+    anyhow,
+    bail,
+};
+
+use hex::FromHex;
+use once_cell::sync::Lazy;
+use sha2::{
+    digest::{
+        generic_array::GenericArray,
+        typenum::U32,
+    },
+    Digest,
+    Sha256,
+};
+
+use crate::{
+    git::Refname,
+    iter::IteratorExt,
+};
+
+mod traits;
+pub use traits::{
+    to_blob,
+    to_tree,
+    Seen,
+};
+use traits::{
+    write_sharded,
+    Blob,
+};
+
+mod bundle;
+pub use bundle::Bundle;
+
+mod error;
+pub use error::FromTree;
+
+pub mod iter;
+pub mod notes;
+
+pub mod record;
+pub use record::{
+    Record,
+    Signature,
+};
+
+mod state;
+pub use state::{
+    merge_notes,
+    unbundle,
+    unbundled_ref,
+    DropHead,
+};
+
+mod submit;
+pub use submit::{
+    AcceptArgs,
+    AcceptOptions,
+    Submission,
+    ALLOWED_REFS,
+    GLOB_HEADS,
+    GLOB_IT_BUNDLES,
+    GLOB_IT_IDS,
+    GLOB_IT_TOPICS,
+    GLOB_NOTES,
+    GLOB_TAGS,
+};
+
+pub const MAX_LEN_BUNDLE: usize = 5_000_000;
+
+pub const HTTP_HEADER_SIGNATURE: &str = "X-it-Signature";
+
+pub const REF_HEADS_PATCHES: &str = "refs/heads/patches";
+
+pub const REF_IT_BRANCHES: &str = "refs/it/branches";
+pub const REF_IT_BUNDLES: &str = "refs/it/bundles";
+pub const REF_IT_PATCHES: &str = "refs/it/patches";
+pub const REF_IT_SEEN: &str = "refs/it/seen";
+pub const REF_IT_TOPICS: &str = "refs/it/topics";
+
+pub const BLOB_HEADS: &str = "heads";
+pub const BLOB_META: &str = "record.json";
+
+pub static TOPIC_MERGES: Lazy<Topic> = Lazy::new(|| Topic::hashed("merges"));
+pub static TOPIC_SNAPSHOTS: Lazy<Topic> = Lazy::new(|| Topic::hashed("snapshots"));
+
+#[derive(Clone, Eq, PartialEq, serde::Serialize, serde::Deserialize)]
+pub struct Topic(#[serde(with = "hex::serde")] [u8; 32]);
+
+impl Topic {
+    const TRAILER_PREFIX: &str = "Re:";
+
+    pub fn hashed<T: AsRef<[u8]>>(v: T) -> Self {
+        Self(Sha256::digest(v).into())
+    }
+
+    pub fn from_commit(commit: &git2::Commit) -> crate::Result<Option<Self>> {
+        commit
+            .message_raw_bytes()
+            .lines()
+            .try_find_map(|line| -> crate::Result<Option<Topic>> {
+                let val = line?
+                    .strip_prefix(Self::TRAILER_PREFIX)
+                    .map(|v| Self::from_hex(v.trim()))
+                    .transpose()?;
+                Ok(val)
+            })
+    }
+
+    pub fn as_trailer(&self) -> String {
+        format!("{} {}", Self::TRAILER_PREFIX, self)
+    }
+
+    pub fn from_refname(name: &str) -> crate::Result<Self> {
+        let last = name
+            .split('/')
+            .next_back()
+            .ok_or_else(|| anyhow!("invalid topic ref {name}"))?;
+        Ok(Self::from_hex(last)?)
+    }
+
+    pub fn as_refname(&self) -> Refname {
+        let name = format!("{}/{}", REF_IT_TOPICS, self);
+        Refname::try_from(name).unwrap()
+    }
+}
+
+impl FromHex for Topic {
+    type Error = hex::FromHexError;
+
+    fn from_hex<T: AsRef<[u8]>>(hex: T) -> Result<Self, Self::Error> {
+        <[u8; 32]>::from_hex(hex).map(Self)
+    }
+}
+
+impl FromStr for Topic {
+    type Err = <Self as FromHex>::Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Self::from_hex(s)
+    }
+}
+
+impl fmt::Display for Topic {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&hex::encode(self.0))
+    }
+}
+
+impl fmt::Debug for Topic {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&hex::encode(self.0))
+    }
+}
+
+impl From<GenericArray<u8, U32>> for Topic {
+    fn from(a: GenericArray<u8, U32>) -> Self {
+        Self(a.into())
+    }
+}
+
+/// Maps a [`Refname`] to the [`REF_IT_BRANCHES`] namespace
+///
+/// The [`Refname`] must be a branch, ie. start with 'refs/heads/'.
+pub struct TrackingBranch(String);
+
+impl TrackingBranch {
+    pub fn master() -> Self {
+        Self([REF_IT_BRANCHES, "master"].join("/"))
+    }
+
+    pub fn main() -> Self {
+        Self([REF_IT_BRANCHES, "main"].join("/"))
+    }
+
+    pub fn into_refname(self) -> Refname {
+        Refname::try_from(self.0).unwrap()
+    }
+}
+
+impl Deref for TrackingBranch {
+    type Target = str;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl TryFrom<&Refname> for TrackingBranch {
+    type Error = crate::Error;
+
+    fn try_from(r: &Refname) -> Result<Self, Self::Error> {
+        match r.strip_prefix("refs/heads/") {
+            None => bail!("not a branch: {r}"),
+            Some("patches") => bail!("reserved name: {r}"),
+            Some(suf) => Ok(Self([REF_IT_BRANCHES, suf].join("/"))),
+        }
+    }
+}
diff --git a/src/patches/bundle.rs b/src/patches/bundle.rs
new file mode 100644
index 0000000..296b24a
--- /dev/null
+++ b/src/patches/bundle.rs
@@ -0,0 +1,344 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    fs::File,
+    io::{
+        self,
+        Read,
+        Seek,
+        SeekFrom,
+    },
+    iter,
+    path::{
+        Path,
+        PathBuf,
+    },
+};
+
+use anyhow::{
+    bail,
+    ensure,
+    Context,
+};
+use multipart::client::lazy::Multipart;
+use sha2::{
+    Digest,
+    Sha256,
+};
+use tempfile::NamedTempFile;
+use url::Url;
+
+use super::record::{
+    self,
+    Encryption,
+};
+use crate::{
+    bundle,
+    io::HashWriter,
+    keys::Signature,
+    Result,
+};
+
+pub struct Bundle {
+    pub(super) header: bundle::Header,
+    pub(super) path: PathBuf,
+    pub(super) info: bundle::Info,
+    pub(super) encryption: Option<Encryption>,
+    pack_start: u64,
+}
+
+impl Bundle {
+    pub fn create<P>(bundle_dir: P, repo: &git2::Repository, header: bundle::Header) -> Result<Self>
+    where
+        P: AsRef<Path>,
+    {
+        let bundle_dir = bundle_dir.as_ref();
+        std::fs::create_dir_all(bundle_dir)?;
+
+        let mut tmp = NamedTempFile::new_in(bundle_dir)?;
+        let info = bundle::create(&mut tmp, repo, &header)?;
+        let path = bundle_dir
+            .join(info.hash.to_string())
+            .with_extension(bundle::FILE_EXTENSION);
+        tmp.persist(&path)?;
+        let mut buf = Vec::new();
+        header.to_writer(&mut buf)?;
+        let pack_start = buf.len() as u64;
+
+        Ok(Self {
+            header,
+            path,
+            info,
+            encryption: None,
+            pack_start,
+        })
+    }
+
+    pub fn from_fetched(bundle: bundle::Fetched) -> Result<Self> {
+        let (path, info) = bundle.into_inner();
+        let (header, mut pack) = split(&path)?;
+        let pack_start = pack.offset;
+        let encryption = pack.encryption()?;
+
+        Ok(Self {
+            header,
+            path,
+            info,
+            encryption,
+            pack_start,
+        })
+    }
+
+    // TODO: defer computing the checksum until needed
+    pub fn from_stored<P>(bundle_dir: P, expect: bundle::Expect) -> Result<Self>
+    where
+        P: AsRef<Path>,
+    {
+        let path = bundle_dir
+            .as_ref()
+            .join(expect.hash.to_string())
+            .with_extension(bundle::FILE_EXTENSION);
+
+        let (header, mut pack) = split(&path)?;
+        let pack_start = pack.offset;
+        let encryption = pack.encryption()?;
+        drop(pack);
+        let mut file = File::open(&path)?;
+        let mut sha2 = Sha256::new();
+
+        let len = io::copy(&mut file, &mut sha2)?;
+        let hash = header.hash();
+        ensure!(expect.hash == &hash, "header hash mismatch");
+        let checksum = sha2.finalize().into();
+        if let Some(expect) = expect.checksum {
+            ensure!(expect == checksum, "claimed and actual hash differ");
+        }
+
+        let info = bundle::Info {
+            len,
+            hash,
+            checksum,
+            uris: vec![],
+        };
+
+        Ok(Self {
+            header,
+            path,
+            info,
+            encryption,
+            pack_start,
+        })
+    }
+
+    pub fn copy<R, P>(mut from: R, to: P) -> Result<Self>
+    where
+        R: Read,
+        P: AsRef<Path>,
+    {
+        std::fs::create_dir_all(&to)?;
+        let mut tmp = NamedTempFile::new_in(&to)?;
+        let mut out = HashWriter::new(Sha256::new(), &mut tmp);
+
+        let len = io::copy(&mut from, &mut out)?;
+        let checksum = out.hash().into();
+
+        let (header, mut pack) = split(tmp.path())?;
+        let hash = header.hash();
+        let pack_start = pack.offset;
+        let encryption = pack.encryption()?;
+
+        let info = bundle::Info {
+            len,
+            hash,
+            checksum,
+            uris: vec![],
+        };
+
+        let path = to
+            .as_ref()
+            .join(hash.to_string())
+            .with_extension(bundle::FILE_EXTENSION);
+        tmp.persist(&path)?;
+
+        Ok(Self {
+            header,
+            path,
+            info,
+            encryption,
+            pack_start,
+        })
+    }
+
+    pub fn encryption(&self) -> Option<Encryption> {
+        self.encryption
+    }
+
+    pub fn is_encrypted(&self) -> bool {
+        self.encryption.is_some()
+    }
+
+    pub fn reader(&self) -> Result<impl io::Read> {
+        Ok(File::open(&self.path)?)
+    }
+
+    pub fn header(&self) -> &bundle::Header {
+        &self.header
+    }
+
+    pub fn info(&self) -> &bundle::Info {
+        &self.info
+    }
+
+    pub fn packdata(&self) -> Result<Packdata> {
+        let bundle = File::open(&self.path)?;
+        Ok(Packdata {
+            offset: self.pack_start,
+            bundle,
+        })
+    }
+
+    pub fn default_location(&self) -> bundle::Location {
+        let uri = bundle::Uri::Relative(format!("/bundles/{}.bundle", self.info.hash));
+        let id = hex::encode(Sha256::digest(uri.as_str()));
+
+        bundle::Location {
+            id,
+            uri,
+            filter: None,
+            creation_token: None,
+            location: None,
+        }
+    }
+
+    pub fn bundle_list_path(&self) -> PathBuf {
+        self.path.with_extension(bundle::list::FILE_EXTENSION)
+    }
+
+    pub fn write_bundle_list<I>(&self, extra: I) -> Result<()>
+    where
+        I: IntoIterator<Item = bundle::Location>,
+    {
+        let mut blist = bundle::List::any();
+        blist.extend(
+            iter::once(self.default_location())
+                .chain(self.info.uris.iter().map(|url| {
+                    let uri = bundle::Uri::Absolute(url.clone());
+                    let id = hex::encode(Sha256::digest(uri.as_str()));
+
+                    bundle::Location {
+                        id,
+                        uri,
+                        filter: None,
+                        creation_token: None,
+                        location: None,
+                    }
+                }))
+                .chain(extra),
+        );
+
+        let mut cfg = git2::Config::open(&self.bundle_list_path())?;
+        blist.to_config(&mut cfg)?;
+
+        Ok(())
+    }
+
+    pub fn sign<S>(&self, signer: &mut S) -> Result<Signature>
+    where
+        S: crate::keys::Signer,
+    {
+        Ok(signer.sign(record::Heads::from(&self.header).as_slice())?)
+    }
+
+    pub fn ipfs_add(&mut self, via: &Url) -> Result<Url> {
+        let name = format!("{}.{}", self.info.hash, bundle::FILE_EXTENSION);
+        let mut api = via.join("api/v0/add")?;
+        api.query_pairs_mut()
+            // FIXME: we may want this, but `rust-chunked-transfer` (used by
+            // `ureq`) doesn't know about trailers
+            // .append_pair("to-files", &name)
+            .append_pair("quiet", "true");
+        let mpart = Multipart::new()
+            .add_file(name, self.path.as_path())
+            .prepare()?;
+
+        #[derive(serde::Deserialize)]
+        struct Response {
+            #[serde(rename = "Hash")]
+            cid: String,
+        }
+
+        let Response { cid } = ureq::post(api.as_str())
+            .set(
+                "Content-Length",
+                &mpart
+                    .content_len()
+                    .expect("zero-size bundle file?")
+                    .to_string(),
+            )
+            .set(
+                "Content-Type",
+                &format!("multipart/form-data; boundary={}", mpart.boundary()),
+            )
+            .send(mpart)
+            .context("posting to IPFS API")?
+            .into_json()
+            .context("parsing IPFS API response")?;
+
+        let url = Url::parse(&format!("ipfs://{cid}"))?;
+        self.info.uris.push(url.clone());
+
+        Ok(url)
+    }
+}
+
+impl From<Bundle> for bundle::Info {
+    fn from(Bundle { info, .. }: Bundle) -> Self {
+        info
+    }
+}
+
+fn split(bundle: &Path) -> Result<(bundle::Header, Packdata)> {
+    let mut bundle = File::open(bundle)?;
+    let header = bundle::Header::from_reader(&mut bundle)?;
+    let offset = bundle.stream_position()?;
+    let pack = Packdata { offset, bundle };
+    Ok((header, pack))
+}
+
+pub struct Packdata {
+    offset: u64,
+    bundle: File,
+}
+
+impl Packdata {
+    pub fn index(&mut self, odb: &git2::Odb) -> Result<()> {
+        self.bundle.seek(SeekFrom::Start(self.offset))?;
+
+        let mut pw = odb.packwriter()?;
+        io::copy(&mut self.bundle, &mut pw)?;
+        pw.commit()?;
+
+        Ok(())
+    }
+
+    pub fn encryption(&mut self) -> Result<Option<Encryption>> {
+        const PACK: &[u8] = b"PACK";
+        const AGE: &[u8] = b"age-encryption.org/v1";
+        const GPG: &[u8] = b"-----BEGIN PGP MESSAGE-----";
+
+        self.bundle.seek(SeekFrom::Start(self.offset))?;
+
+        let mut buf = [0; 32];
+        self.bundle.read_exact(&mut buf)?;
+        if buf.starts_with(PACK) {
+            Ok(None)
+        } else if buf.starts_with(AGE) {
+            Ok(Some(Encryption::Age))
+        } else if buf.starts_with(GPG) {
+            Ok(Some(Encryption::Gpg))
+        } else {
+            bail!("packdata does not appear to be in a known format")
+        }
+    }
+}
diff --git a/src/patches/error.rs b/src/patches/error.rs
new file mode 100644
index 0000000..a02ed94
--- /dev/null
+++ b/src/patches/error.rs
@@ -0,0 +1,29 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+#[non_exhaustive]
+pub enum FromTree {
+    #[error("'{name}' not found in tree")]
+    NotFound { name: &'static str },
+
+    #[error("expected '{name}' to be a blob, but found {kind:?}")]
+    TypeMismatch {
+        name: &'static str,
+        kind: Option<git2::ObjectType>,
+    },
+
+    #[error("max blob size {max} exceeded: {found}")]
+    BlobSize { max: usize, found: usize },
+
+    #[error("type conversion from byte slice to T failed")]
+    TypeConversion(#[source] crate::Error),
+
+    #[error("invalid signature")]
+    InvalidSignature(#[from] signature::Error),
+
+    #[error(transparent)]
+    Git(#[from] git2::Error),
+}
diff --git a/src/patches/iter.rs b/src/patches/iter.rs
new file mode 100644
index 0000000..6023247
--- /dev/null
+++ b/src/patches/iter.rs
@@ -0,0 +1,395 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    collections::BTreeSet,
+    rc::Rc,
+    str::FromStr,
+};
+
+use anyhow::anyhow;
+use time::{
+    OffsetDateTime,
+    UtcOffset,
+};
+
+use super::{
+    notes,
+    record::{
+        Heads,
+        Record,
+    },
+    Topic,
+    GLOB_IT_TOPICS,
+    TOPIC_MERGES,
+};
+use crate::{
+    git::{
+        self,
+        Refname,
+        EMPTY_TREE,
+    },
+    iter,
+    patches::REF_IT_BUNDLES,
+    Result,
+};
+
+pub mod dropped {
+    use super::*;
+    use crate::{
+        error,
+        patches::TOPIC_SNAPSHOTS,
+    };
+
+    pub fn topics<'a>(
+        repo: &'a git2::Repository,
+        drop_ref: &'a str,
+    ) -> impl Iterator<Item = Result<(Topic, git2::Oid)>> + 'a {
+        let topic = move |oid| -> Result<Option<(Topic, git2::Oid)>> {
+            let commit = repo.find_commit(oid)?;
+            Ok(Topic::from_commit(&commit)?.map(|topic| (topic, oid)))
+        };
+        let init = || {
+            let mut walk = repo.revwalk()?;
+            walk.push_ref(drop_ref)?;
+            Ok(walk.map(|i| i.map_err(Into::into)))
+        };
+
+        iter::Iter::new(init, Some).filter_map(move |oid| oid.and_then(topic).transpose())
+    }
+
+    pub fn topic<'a>(
+        repo: &'a git2::Repository,
+        drop_ref: &'a str,
+        topic: &'a Topic,
+    ) -> impl Iterator<Item = Result<git2::Oid>> + 'a {
+        topics(repo, drop_ref).filter_map(move |i| {
+            i.map(|(top, oid)| (&top == topic).then_some(oid))
+                .transpose()
+        })
+    }
+
+    #[allow(unused)]
+    pub fn merges<'a>(
+        repo: &'a git2::Repository,
+        drop_ref: &'a str,
+    ) -> impl Iterator<Item = Result<git2::Oid>> + 'a {
+        topic(repo, drop_ref, &TOPIC_MERGES)
+    }
+
+    #[allow(unused)]
+    pub fn snapshots<'a>(
+        repo: &'a git2::Repository,
+        drop_ref: &'a str,
+    ) -> impl Iterator<Item = Result<git2::Oid>> + 'a {
+        topic(repo, drop_ref, &TOPIC_SNAPSHOTS)
+    }
+
+    pub fn records<'a>(
+        repo: &'a git2::Repository,
+        drop_ref: &'a str,
+    ) -> impl Iterator<Item = Result<Record>> + 'a {
+        _records(repo, drop_ref, false)
+    }
+
+    pub fn records_rev<'a>(
+        repo: &'a git2::Repository,
+        drop_ref: &'a str,
+    ) -> impl Iterator<Item = Result<Record>> + 'a {
+        _records(repo, drop_ref, true)
+    }
+
+    fn _records<'a>(
+        repo: &'a git2::Repository,
+        drop_ref: &'a str,
+        rev: bool,
+    ) -> impl Iterator<Item = Result<Record>> + 'a {
+        let record = move |oid| -> Result<Option<Record>> {
+            let commit = repo.find_commit(oid)?;
+            match Record::from_commit(repo, &commit) {
+                Ok(r) => Ok(Some(r)),
+                Err(e) => match e.downcast_ref::<error::NotFound<&str, String>>() {
+                    Some(error::NotFound { what: "topic", .. }) => Ok(None),
+                    _ => Err(e),
+                },
+            }
+        };
+        let init = move || {
+            let mut walk = repo.revwalk()?;
+            walk.push_ref(drop_ref)?;
+            if rev {
+                walk.set_sorting(git2::Sort::REVERSE)?;
+            }
+            Ok(walk.map(|i| i.map_err(Into::into)))
+        };
+
+        iter::Iter::new(init, Some).filter_map(move |oid| oid.and_then(record).transpose())
+    }
+}
+
+pub mod unbundled {
+    use super::*;
+
+    #[allow(unused)]
+    pub fn topics(repo: &git2::Repository) -> impl Iterator<Item = Result<Topic>> + '_ {
+        iter::Iter::new(
+            move || {
+                let refs = repo.references_glob(GLOB_IT_TOPICS.glob())?;
+                Ok(git::ReferenceNames::new(refs, Topic::from_refname))
+            },
+            Some,
+        )
+    }
+
+    pub fn topics_with_subject(
+        repo: &git2::Repository,
+    ) -> impl Iterator<Item = Result<(Topic, String)>> + '_ {
+        let topic_and_subject = move |refname: &str| -> Result<(Topic, String)> {
+            let topic = Topic::from_refname(refname)?;
+            let subject = find_subject(repo, refname)?;
+            Ok((topic, subject))
+        };
+        iter::Iter::new(
+            move || {
+                let refs = repo.references_glob(GLOB_IT_TOPICS.glob())?;
+                Ok(git::ReferenceNames::new(refs, topic_and_subject))
+            },
+            Some,
+        )
+    }
+
+    // TODO: cache this somewhere
+    fn find_subject(repo: &git2::Repository, topic_ref: &str) -> Result<String> {
+        let mut walk = repo.revwalk()?;
+        walk.push_ref(topic_ref)?;
+        walk.simplify_first_parent()?;
+        walk.set_sorting(git2::Sort::TOPOLOGICAL | git2::Sort::REVERSE)?;
+        match walk.next() {
+            None => Ok(String::default()),
+            Some(oid) => {
+                let tree = repo.find_commit(oid?)?.tree()?;
+                let note = notes::Note::from_tree(repo, &tree)?;
+                let subj = match note {
+                    notes::Note::Simple(n) => n
+                        .checkpoint_kind()
+                        .map(|k| {
+                            match k {
+                                notes::CheckpointKind::Merge => "Merges",
+                                notes::CheckpointKind::Snapshot => "Snapshots",
+                            }
+                            .to_owned()
+                        })
+                        .unwrap_or_else(|| n.subject().unwrap_or_default().to_owned()),
+                    _ => String::default(),
+                };
+
+                Ok(subj)
+            },
+        }
+    }
+}
+
+#[derive(Eq, PartialEq, serde::Serialize)]
+pub struct Subject {
+    pub name: String,
+    pub email: String,
+}
+
+impl TryFrom<git2::Signature<'_>> for Subject {
+    type Error = std::str::Utf8Error;
+
+    fn try_from(git: git2::Signature<'_>) -> std::result::Result<Self, Self::Error> {
+        let utf8 = |bs| std::str::from_utf8(bs).map(ToOwned::to_owned);
+
+        let name = utf8(git.name_bytes())?;
+        let email = utf8(git.email_bytes())?;
+
+        Ok(Self { name, email })
+    }
+}
+
+#[derive(serde::Serialize)]
+#[serde(rename_all = "kebab-case")]
+pub struct NoteHeader {
+    #[serde(with = "git::serde::oid")]
+    pub id: git2::Oid,
+    pub author: Subject,
+    /// `Some` iff different from `author`
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub committer: Option<Subject>,
+    /// Committer time
+    #[serde(with = "time::serde::rfc3339")]
+    pub time: OffsetDateTime,
+    pub patch: Rc<PatchInfo>,
+    #[serde(
+        with = "git::serde::oid::option",
+        skip_serializing_if = "Option::is_none"
+    )]
+    pub in_reply_to: Option<git2::Oid>,
+}
+
+#[derive(serde::Serialize)]
+pub struct PatchInfo {
+    pub id: Heads,
+    pub tips: BTreeSet<Refname>,
+}
+
+#[derive(serde::Serialize)]
+pub struct Note {
+    pub header: NoteHeader,
+    pub message: notes::Note,
+}
+
+pub fn topic<'a>(
+    repo: &'a git2::Repository,
+    topic: &'a Topic,
+) -> impl Iterator<Item = Result<Note>> + DoubleEndedIterator + 'a {
+    let init = move || {
+        let topic_ref = topic.as_refname();
+        let mut walk = repo.revwalk()?;
+        walk.push_ref(&topic_ref)?;
+        walk.set_sorting(git2::Sort::TOPOLOGICAL)?;
+
+        fn patch_id(c: &git2::Commit) -> Result<Option<Heads>> {
+            let parse = || Heads::try_from(c);
+            let is_merge = c.tree_id() == *EMPTY_TREE;
+            is_merge.then(parse).transpose()
+        }
+
+        fn patch_info(repo: &git2::Repository, id: Heads) -> Result<PatchInfo> {
+            let prefix = format!("{}/{}", REF_IT_BUNDLES, id);
+            let glob = format!("{prefix}/**");
+            let mut iter = repo.references_glob(&glob)?;
+            let tips = iter
+                .names()
+                .filter_map(|i| match i {
+                    Err(e) => Some(Err(e.into())),
+                    Ok(name)
+                        if name
+                            .strip_prefix(&prefix)
+                            .expect("glob yields prefix")
+                            .starts_with("/it/") =>
+                    {
+                        None
+                    },
+                    Ok(name) => Refname::from_str(name)
+                        .map_err(Into::into)
+                        .map(Some)
+                        .transpose(),
+                })
+                .collect::<Result<_>>()?;
+
+            Ok(PatchInfo { id, tips })
+        }
+
+        let mut patches: Vec<Rc<PatchInfo>> = Vec::new();
+        let mut commits: Vec<(git2::Tree<'a>, NoteHeader)> = Vec::new();
+
+        if let Some(tip) = walk.next() {
+            // ensure tip is a merge
+            {
+                let tip = repo.find_commit(tip?)?;
+                let id = patch_id(&tip)?.ok_or_else(|| {
+                    anyhow!("invalid topic '{topic_ref}': tip must be a merge commit")
+                })?;
+                let patch = patch_info(repo, id)?;
+                patches.push(Rc::new(patch));
+            }
+
+            for id in walk {
+                let commit = repo.find_commit(id?)?;
+                match patch_id(&commit)? {
+                    Some(id) => {
+                        let patch = patch_info(repo, id)?;
+                        patches.push(Rc::new(patch))
+                    },
+                    None => {
+                        let id = commit.id();
+                        let (author, committer) = {
+                            let a = commit.author();
+                            let c = commit.committer();
+
+                            if a.name_bytes() != c.name_bytes()
+                                && a.email_bytes() != c.email_bytes()
+                            {
+                                let author = Subject::try_from(a)?;
+                                let committer = Subject::try_from(c).map(Some)?;
+
+                                (author, committer)
+                            } else {
+                                (Subject::try_from(a)?, None)
+                            }
+                        };
+                        let time = {
+                            let t = commit.time();
+                            let ofs = UtcOffset::from_whole_seconds(t.offset_minutes() * 60)?;
+                            OffsetDateTime::from_unix_timestamp(t.seconds())?.replace_offset(ofs)
+                        };
+                        let tree = commit.tree()?;
+                        let patch = Rc::clone(&patches[patches.len() - 1]);
+                        let in_reply_to = commit.parent_ids().next();
+
+                        let header = NoteHeader {
+                            id,
+                            author,
+                            committer,
+                            time,
+                            patch,
+                            in_reply_to,
+                        };
+
+                        commits.push((tree, header));
+                    },
+                }
+            }
+        }
+
+        Ok(commits.into_iter().map(move |(tree, header)| {
+            notes::Note::from_tree(repo, &tree).map(|message| Note { header, message })
+        }))
+    };
+
+    iter::Iter::new(init, Some)
+}
+
+pub mod topic {
+    use crate::git::if_not_found_none;
+
+    use super::*;
+
+    pub(crate) fn default_reply_to(
+        repo: &git2::Repository,
+        topic: &Topic,
+    ) -> Result<Option<git2::Oid>> {
+        let topic_ref = topic.as_refname();
+        if if_not_found_none(repo.refname_to_id(&topic_ref))?.is_none() {
+            return Ok(None);
+        }
+
+        let mut walk = repo.revwalk()?;
+        walk.set_sorting(git2::Sort::TOPOLOGICAL | git2::Sort::REVERSE)?;
+        walk.push_ref(&topic_ref)?;
+
+        let first = walk
+            .next()
+            .expect("topic can't be empty, because {topic_ref} exists")?;
+        let mut last = first;
+        let mut seen = BTreeSet::<git2::Oid>::new();
+        for id in walk {
+            let id = id?;
+            let commit = repo.find_commit(id)?;
+            if commit.tree_id() != *EMPTY_TREE {
+                let first_parent = commit
+                    .parent_ids()
+                    .next()
+                    .expect("commit {id} must have a parent");
+                if first_parent == first || !seen.contains(&first_parent) {
+                    last = id;
+                }
+                seen.insert(id);
+            }
+        }
+
+        Ok(Some(last))
+    }
+}
diff --git a/src/patches/notes.rs b/src/patches/notes.rs
new file mode 100644
index 0000000..b85ca64
--- /dev/null
+++ b/src/patches/notes.rs
@@ -0,0 +1,181 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    cmp,
+    collections::BTreeMap,
+    convert::Infallible,
+    io,
+    ops::Range,
+};
+
+use super::{
+    error,
+    traits::{
+        Blob,
+        BlobData,
+        TreeData,
+    },
+};
+use crate::{
+    bundle::ObjectId,
+    git::Refname,
+};
+
+#[derive(serde::Serialize)]
+#[serde(untagged)]
+pub enum Note {
+    Simple(Simple),
+    Automerge(Automerge),
+}
+
+impl Note {
+    pub fn from_tree<'a>(repo: &'a git2::Repository, tree: &git2::Tree<'a>) -> crate::Result<Self> {
+        Blob::<Simple>::from_tree(repo, tree)
+            .map(|Blob { content, .. }| Self::Simple(content))
+            .or_else(|e| match e {
+                error::FromTree::NotFound { .. } => {
+                    let Blob { content, .. } = Blob::<Automerge>::from_tree(repo, tree)?;
+                    Ok(Self::Automerge(content))
+                },
+                x => Err(x.into()),
+            })
+    }
+}
+
+#[derive(serde::Serialize)]
+pub struct Automerge(Vec<u8>);
+
+impl BlobData for Automerge {
+    type Error = Infallible;
+
+    const MAX_BYTES: usize = 1_000_000;
+
+    fn from_blob(data: &[u8]) -> Result<Self, Self::Error> {
+        Ok(Self(data.to_vec()))
+    }
+
+    fn write_blob<W: io::Write>(&self, mut writer: W) -> io::Result<()> {
+        writer.write_all(&self.0)
+    }
+}
+
+impl TreeData for Automerge {
+    const BLOB_NAME: &'static str = "c";
+}
+
+#[derive(Clone, Debug, serde::Serialize, serde::Deserialize)]
+#[serde(untagged)]
+pub enum Simple {
+    Known(Predef),
+    Unknown(serde_json::Map<String, serde_json::Value>),
+}
+
+impl Simple {
+    pub fn new(message: String) -> Self {
+        Self::basic(message)
+    }
+
+    pub fn basic(message: String) -> Self {
+        Self::Known(Predef::Basic { message })
+    }
+
+    pub fn checkpoint(
+        kind: CheckpointKind,
+        refs: BTreeMap<Refname, ObjectId>,
+        message: Option<String>,
+    ) -> Self {
+        Self::Known(Predef::Checkpoint {
+            kind,
+            refs,
+            message,
+        })
+    }
+
+    pub fn from_commit(repo: &git2::Repository, commit: &git2::Commit) -> crate::Result<Self> {
+        let tree = commit.tree()?;
+        let blob = Blob::from_tree(repo, &tree)?;
+
+        Ok(blob.content)
+    }
+
+    pub fn subject(&self) -> Option<&str> {
+        match self {
+            Self::Known(k) => k.subject(),
+            _ => None,
+        }
+    }
+
+    pub fn is_checkpoint(&self) -> bool {
+        matches!(self, Self::Known(Predef::Checkpoint { .. }))
+    }
+
+    pub fn checkpoint_kind(&self) -> Option<&CheckpointKind> {
+        match self {
+            Self::Known(Predef::Checkpoint { kind, .. }) => Some(kind),
+            _ => None,
+        }
+    }
+}
+
+impl BlobData for Simple {
+    type Error = serde_json::Error;
+
+    const MAX_BYTES: usize = 1_000_000;
+
+    fn from_blob(data: &[u8]) -> Result<Self, Self::Error> {
+        serde_json::from_slice(data)
+    }
+
+    fn write_blob<W: io::Write>(&self, writer: W) -> io::Result<()> {
+        serde_json::to_writer_pretty(writer, self).map_err(Into::into)
+    }
+}
+
+impl TreeData for Simple {
+    const BLOB_NAME: &'static str = "m";
+}
+
+#[derive(Clone, Debug, serde::Serialize, serde::Deserialize)]
+#[serde(tag = "_type")]
+pub enum Predef {
+    #[serde(rename = "eagain.io/it/notes/basic")]
+    Basic { message: String },
+    #[serde(rename = "eagain.io/it/notes/code-comment")]
+    CodeComment { loc: SourceLoc, message: String },
+    #[serde(rename = "eagain.io/it/notes/checkpoint")]
+    Checkpoint {
+        kind: CheckpointKind,
+        refs: BTreeMap<Refname, ObjectId>,
+        #[serde(skip_serializing_if = "Option::is_none")]
+        message: Option<String>,
+    },
+}
+
+impl Predef {
+    pub fn subject(&self) -> Option<&str> {
+        let msg = match self {
+            Self::Basic { message } | Self::CodeComment { message, .. } => Some(message),
+            Self::Checkpoint { message, .. } => message.as_ref(),
+        }?;
+        let line = msg.lines().next()?;
+        let subj = &line[..cmp::min(72, line.len())];
+
+        (!subj.is_empty()).then_some(subj)
+    }
+}
+
+#[derive(Clone, Debug, serde::Serialize, serde::Deserialize)]
+pub struct SourceLoc {
+    #[serde(with = "crate::git::serde::oid")]
+    pub file: git2::Oid,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub line: Option<Range<usize>>,
+}
+
+#[derive(Clone, Copy, Debug, serde::Serialize, serde::Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum CheckpointKind {
+    Merge,
+    Snapshot,
+}
diff --git a/src/patches/record.rs b/src/patches/record.rs
new file mode 100644
index 0000000..6a95973
--- /dev/null
+++ b/src/patches/record.rs
@@ -0,0 +1,472 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use core::ops::Deref;
+use std::{
+    collections::{
+        BTreeMap,
+        BTreeSet,
+    },
+    fmt,
+    io::{
+        self,
+        BufRead,
+    },
+    path::{
+        Path,
+        PathBuf,
+    },
+    str::FromStr,
+};
+
+use anyhow::{
+    anyhow,
+    bail,
+    ensure,
+    Context,
+};
+
+use hex::{
+    FromHex,
+    ToHex,
+};
+
+use sha2::{
+    Digest,
+    Sha256,
+};
+use signature::{
+    Signature as _,
+    Verifier,
+};
+
+use super::{
+    traits::{
+        to_tree,
+        BlobData,
+        Foldable,
+        TreeData,
+    },
+    write_sharded,
+    Blob,
+    Bundle,
+    Topic,
+    BLOB_HEADS,
+    BLOB_META,
+    HTTP_HEADER_SIGNATURE,
+    TOPIC_MERGES,
+    TOPIC_SNAPSHOTS,
+};
+use crate::{
+    bundle,
+    error::NotFound,
+    git::{
+        self,
+        Refname,
+    },
+    iter::IteratorExt,
+    metadata::{
+        self,
+        identity,
+        ContentHash,
+    },
+};
+
+#[derive(Clone, Copy, Eq, PartialEq, serde::Serialize, serde::Deserialize)]
+pub struct Heads(#[serde(with = "hex::serde")] [u8; 32]);
+
+impl Heads {
+    const TRAILER_PREFIX: &str = "Patch:";
+
+    pub fn from_commit(commit: &git2::Commit) -> crate::Result<Option<Self>> {
+        commit.message_raw_bytes().lines().try_find_map(|line| {
+            line?
+                .strip_prefix(Self::TRAILER_PREFIX)
+                .map(|s| Self::from_str(s.trim()).map_err(crate::Error::from))
+                .transpose()
+        })
+    }
+
+    pub fn as_trailer(&self) -> String {
+        format!("{} {}", Self::TRAILER_PREFIX, self)
+    }
+}
+
+impl Deref for Heads {
+    type Target = [u8; 32];
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl AsRef<[u8]> for Heads {
+    fn as_ref(&self) -> &[u8] {
+        &self.0
+    }
+}
+
+impl From<&bundle::Header> for Heads {
+    fn from(h: &bundle::Header) -> Self {
+        let tips = h.references.values().collect::<BTreeSet<_>>();
+        let mut hasher = Sha256::new();
+        for sha in tips {
+            hasher.update(sha.as_bytes());
+        }
+        Self(hasher.finalize().into())
+    }
+}
+
+impl TryFrom<&git2::Commit<'_>> for Heads {
+    type Error = crate::Error;
+
+    fn try_from(commit: &git2::Commit) -> Result<Self, Self::Error> {
+        Self::from_commit(commit)?.ok_or_else(|| {
+            anyhow!(NotFound {
+                what: "patch trailer",
+                whence: format!("commit {}", commit.id()),
+            })
+        })
+    }
+}
+
+impl FromStr for Heads {
+    type Err = hex::FromHexError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Self::from_hex(s)
+    }
+}
+
+impl FromHex for Heads {
+    type Error = hex::FromHexError;
+
+    fn from_hex<T: AsRef<[u8]>>(hex: T) -> Result<Self, Self::Error> {
+        <[u8; 32]>::from_hex(hex).map(Self)
+    }
+}
+
+impl fmt::Display for Heads {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&hex::encode(self.0))
+    }
+}
+
+impl fmt::Debug for Heads {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(&hex::encode(self.0))
+    }
+}
+
+impl BlobData for Heads {
+    type Error = <[u8; 32] as FromHex>::Error;
+
+    const MAX_BYTES: usize = 64;
+
+    fn from_blob(data: &[u8]) -> Result<Self, Self::Error> {
+        Self::from_hex(data)
+    }
+
+    fn write_blob<W: io::Write>(&self, mut writer: W) -> io::Result<()> {
+        writer.write_all(self.encode_hex::<String>().as_bytes())
+    }
+}
+
+impl TreeData for Heads {
+    const BLOB_NAME: &'static str = BLOB_HEADS;
+}
+
+impl Foldable for Heads {
+    fn folded_name(&self) -> String {
+        self.encode_hex()
+    }
+}
+
+#[derive(Clone, Debug, serde::Serialize, serde::Deserialize)]
+pub struct Signature {
+    pub signer: metadata::ContentHash,
+    pub signature: metadata::Signature,
+}
+
+impl From<Signature> for tiny_http::Header {
+    fn from(s: Signature) -> Self {
+        let value = format!(
+            "s1={}; s2={}; sd={}",
+            hex::encode(s.signer.sha1),
+            hex::encode(s.signer.sha2),
+            hex::encode(s.signature.as_ref())
+        );
+
+        Self::from_bytes(HTTP_HEADER_SIGNATURE.as_bytes(), value).unwrap()
+    }
+}
+
+impl TryFrom<&tiny_http::Header> for Signature {
+    type Error = crate::Error;
+
+    fn try_from(hdr: &tiny_http::Header) -> Result<Self, Self::Error> {
+        ensure!(
+            hdr.field.equiv(HTTP_HEADER_SIGNATURE),
+            "not a {HTTP_HEADER_SIGNATURE} header"
+        );
+
+        let mut sha1: Option<[u8; 20]> = None;
+        let mut sha2: Option<[u8; 32]> = None;
+        let mut signature = None;
+        for part in hdr.value.as_str().split(';') {
+            match part.trim().split_at(2) {
+                ("s1", val) => {
+                    let bytes = <[u8; 20]>::from_hex(val)?;
+                    sha1 = Some(bytes);
+                },
+                ("s2", val) => {
+                    let bytes = <[u8; 32]>::from_hex(val)?;
+                    sha2 = Some(bytes);
+                },
+                ("sd", val) => {
+                    let bytes = hex::decode(val)?;
+                    signature = Some(metadata::Signature::from_bytes(&bytes)?);
+                },
+
+                _ => continue,
+            }
+        }
+
+        let sha1 = sha1.ok_or_else(|| anyhow!("missing sha1 identity content hash"))?;
+        let sha2 = sha2.ok_or_else(|| anyhow!("missing sha2 identity content hash"))?;
+        let signature = signature.ok_or_else(|| anyhow!("missing signature bytes"))?;
+
+        Ok(Self {
+            signer: metadata::ContentHash { sha1, sha2 },
+            signature,
+        })
+    }
+}
+
+#[derive(Debug, serde::Serialize, serde::Deserialize)]
+pub struct Meta {
+    pub bundle: BundleInfo,
+    pub signature: Signature,
+}
+
+impl BlobData for Meta {
+    type Error = serde_json::Error;
+
+    const MAX_BYTES: usize = 100_000;
+
+    fn from_blob(data: &[u8]) -> Result<Self, Self::Error> {
+        serde_json::from_slice(data)
+    }
+
+    fn write_blob<W: io::Write>(&self, writer: W) -> io::Result<()> {
+        serde_json::to_writer_pretty(writer, self).map_err(Into::into)
+    }
+}
+
+impl TreeData for Meta {
+    const BLOB_NAME: &'static str = BLOB_META;
+}
+
+#[derive(Clone, Copy, Debug, Eq, PartialEq, serde::Serialize, serde::Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum Encryption {
+    Age,
+    Gpg,
+}
+
+impl Encryption {
+    pub fn as_str(&self) -> &str {
+        match self {
+            Self::Age => "age",
+            Self::Gpg => "gpg",
+        }
+    }
+}
+
+impl FromStr for Encryption {
+    type Err = serde_json::Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        serde_json::from_str(s)
+    }
+}
+
+#[derive(Debug, serde::Serialize, serde::Deserialize)]
+pub struct BundleInfo {
+    #[serde(flatten)]
+    pub info: bundle::Info,
+    pub prerequisites: BTreeSet<bundle::ObjectId>,
+    pub references: BTreeMap<Refname, bundle::ObjectId>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub encryption: Option<Encryption>,
+}
+
+impl BundleInfo {
+    pub fn as_expect(&self) -> bundle::Expect {
+        bundle::Expect::from(&self.info)
+    }
+}
+
+impl From<&Bundle> for BundleInfo {
+    fn from(bundle: &Bundle) -> Self {
+        let (prerequisites, references) = {
+            let h = bundle.header();
+            (h.prerequisites.clone(), h.references.clone())
+        };
+        Self {
+            info: bundle.info().clone(),
+            prerequisites,
+            references,
+            encryption: bundle.encryption(),
+        }
+    }
+}
+
+/// Log record of a patch submission
+#[derive(Debug, serde::Serialize, serde::Deserialize)]
+pub struct Record {
+    pub topic: Topic,
+    pub heads: Heads,
+    pub meta: Meta,
+}
+
+impl Record {
+    pub fn from_commit<'a>(
+        repo: &'a git2::Repository,
+        commit: &git2::Commit<'a>,
+    ) -> crate::Result<Self> {
+        let topic = Topic::from_commit(commit)?.ok_or_else(|| crate::error::NotFound {
+            what: "topic",
+            whence: format!("message of commit {}", commit.id()),
+        })?;
+
+        let tree = commit.tree()?;
+
+        let mut heads: Option<Heads> = None;
+        let mut meta: Option<Meta> = None;
+
+        for entry in &tree {
+            match entry.name() {
+                Some(BLOB_HEADS) => {
+                    heads = Some(Blob::<Heads>::from_entry(repo, entry)?.content);
+                },
+                Some(BLOB_META) => {
+                    meta = Some(Blob::<Meta>::from_entry(repo, entry)?.content);
+                },
+
+                None | Some(_) => continue,
+            }
+        }
+
+        let whence = || format!("tree {}", tree.id());
+        let heads = heads.ok_or_else(|| crate::error::NotFound {
+            what: BLOB_HEADS,
+            whence: whence(),
+        })?;
+        let meta = meta.ok_or_else(|| crate::error::NotFound {
+            what: BLOB_META,
+            whence: whence(),
+        })?;
+
+        Ok(Self { topic, heads, meta })
+    }
+
+    pub fn commit<S>(
+        &self,
+        signer: &mut S,
+        repo: &git2::Repository,
+        ids: &git2::Tree,
+        parent: Option<&git2::Commit>,
+        seen: Option<&mut git2::TreeBuilder>,
+    ) -> crate::Result<git2::Oid>
+    where
+        S: crate::keys::Signer,
+    {
+        let tree = {
+            let mut tb = repo.treebuilder(parent.map(|p| p.tree()).transpose()?.as_ref())?;
+            tb.insert("ids", ids.id(), git2::FileMode::Tree.into())?;
+            to_tree(repo, &mut tb, &self.heads)?;
+            to_tree(repo, &mut tb, &self.meta)?;
+            repo.find_tree(tb.write()?)?
+        };
+        let oid = git::commit_signed(
+            signer,
+            repo,
+            self.topic.as_trailer(),
+            &tree,
+            &parent.into_iter().collect::<Vec<_>>(),
+        )?;
+
+        if let Some(seen) = seen {
+            write_sharded(
+                repo,
+                seen,
+                &self.heads,
+                tree.get_name(Heads::BLOB_NAME)
+                    .expect("heads blob written above")
+                    .id(),
+            )?;
+        }
+
+        Ok(oid)
+    }
+
+    pub fn signed_part(&self) -> [u8; 32] {
+        *self.heads
+    }
+
+    pub fn verify_signature<F>(&self, mut find_id: F) -> crate::Result<()>
+    where
+        F: FnMut(&ContentHash) -> crate::Result<identity::Verified>,
+    {
+        let signed_data = self.signed_part();
+        let addr = &self.meta.signature.signer;
+        let signature = &self.meta.signature.signature;
+        let id =
+            find_id(addr).with_context(|| format!("invalid or non-existent id at {:?}", addr))?;
+        for key in id.identity().keys.values() {
+            if key.verify(&signed_data, signature).is_ok() {
+                return Ok(());
+            }
+        }
+        bail!("signature key not in id at {:?}", addr);
+    }
+
+    pub fn bundle_info(&self) -> &BundleInfo {
+        &self.meta.bundle
+    }
+
+    pub fn bundle_hash(&self) -> &bundle::Hash {
+        &self.meta.bundle.info.hash
+    }
+
+    pub fn bundle_path(&self, prefix: &Path) -> PathBuf {
+        let mut p = prefix.join(self.bundle_hash().to_string());
+        p.set_extension(bundle::FILE_EXTENSION);
+        p
+    }
+
+    pub fn is_encrypted(&self) -> bool {
+        self.meta.bundle.encryption.is_some()
+    }
+
+    pub fn is_snapshot(&self) -> bool {
+        self.topic == *TOPIC_SNAPSHOTS
+    }
+
+    pub fn is_mergepoint(&self) -> bool {
+        self.topic == *TOPIC_MERGES
+    }
+
+    /// Remove traces of a record from the given tree
+    pub(crate) fn remove_from(tree: &mut git2::TreeBuilder) -> crate::Result<()> {
+        if tree.get(Heads::BLOB_NAME)?.is_some() {
+            tree.remove(Heads::BLOB_NAME)?;
+        }
+        if tree.get(Meta::BLOB_NAME)?.is_some() {
+            tree.remove(Meta::BLOB_NAME)?;
+        }
+
+        Ok(())
+    }
+}
diff --git a/src/patches/state.rs b/src/patches/state.rs
new file mode 100644
index 0000000..220971d
--- /dev/null
+++ b/src/patches/state.rs
@@ -0,0 +1,231 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    io,
+    ops::Range,
+};
+
+use anyhow::{
+    anyhow,
+    ensure,
+    Context,
+};
+use log::warn;
+
+use super::{
+    Record,
+    TrackingBranch,
+};
+use crate::{
+    git::{
+        self,
+        if_not_found_none,
+        refs::{
+            self,
+            LockedRef,
+        },
+        Refname,
+    },
+    keys::VerificationKey,
+    metadata::{
+        self,
+        git::FromGit,
+        identity,
+    },
+    Result,
+};
+
+/// Somewhat ad-hoc view of the tip of a drop
+pub struct DropHead<'a> {
+    pub tip: git2::Reference<'a>,
+    pub ids: git2::Tree<'a>,
+    pub meta: metadata::drop::Verified,
+}
+
+impl<'a> DropHead<'a> {
+    pub fn from_refname<S: AsRef<str>>(repo: &'a git2::Repository, name: S) -> crate::Result<Self> {
+        let tip = repo.find_reference(name.as_ref())?;
+        let root = tip.peel_to_tree()?;
+        let ids = root
+            .get_name("ids")
+            .ok_or_else(|| anyhow!("invalid drop: 'ids' tree not found"))?
+            .to_object(repo)?
+            .into_tree()
+            .map_err(|_| anyhow!("invalid drop: 'ids' tree is not a tree"))?;
+        let meta = metadata::Drop::from_tree(repo, &root)
+            .context("error loading drop metadata")?
+            .verified(metadata::git::find_parent(repo), |id| {
+                metadata::identity::find_in_tree(repo, &ids, id)
+                    .map(|verified| verified.into_parts().1.keys)
+                    .map_err(|e| io::Error::new(io::ErrorKind::Other, e))
+            })?;
+
+        Ok(Self { tip, ids, meta })
+    }
+}
+
+pub fn unbundle(
+    odb: &git2::Odb,
+    tx: &mut refs::Transaction,
+    ref_prefix: &str,
+    record: &Record,
+) -> Result<Vec<(Refname, git2::Oid)>> {
+    let reflog = format!("it: storing head from {}", record.bundle_hash());
+
+    let mut updated = Vec::with_capacity(record.meta.bundle.references.len());
+    for (name, oid) in &record.meta.bundle.references {
+        let oid = git2::Oid::try_from(oid)?;
+        ensure!(odb.exists(oid), "ref not actually in bundle: {oid} {name}");
+
+        let by_heads = unbundled_ref(ref_prefix, record, name)?;
+        tx.lock_ref(by_heads.clone())?
+            .set_target(oid, reflog.clone());
+        updated.push((by_heads, oid));
+    }
+
+    Ok(updated)
+}
+
+pub fn unbundled_ref(prefix: &str, record: &Record, name: &Refname) -> Result<Refname> {
+    format!(
+        "{}/{}/{}",
+        prefix.trim_matches('/'),
+        record.heads,
+        name.trim_start_matches("refs/")
+    )
+    .try_into()
+    .map_err(Into::into)
+}
+
+pub fn merge_notes(
+    repo: &git2::Repository,
+    submitter: &identity::Verified,
+    topics_ref: &LockedRef,
+    record: &Record,
+) -> Result<()> {
+    let theirs: git2::Oid = record
+        .meta
+        .bundle
+        .references
+        .get(topics_ref.name())
+        .ok_or_else(|| anyhow!("invalid record: missing '{topics_ref}'"))?
+        .try_into()?;
+
+    let tree = git::empty_tree(repo)?;
+    let usr = repo.signature()?;
+    let theirs_commit = repo.find_commit(theirs)?;
+    match if_not_found_none(repo.find_reference(topics_ref.name()))? {
+        None => {
+            let msg = format!(
+                "Create topic from '{theirs}'\n\n{}",
+                record.heads.as_trailer()
+            );
+            let oid = repo.commit(None, &usr, &usr, &msg, &tree, &[&theirs_commit])?;
+            topics_ref.set_target(oid, "it: create topic");
+        },
+        Some(ours_ref) => {
+            let ours_commit = ours_ref.peel_to_commit()?;
+            let ours = ours_commit.id();
+
+            ensure!(ours != theirs, "illegal state: theirs equals ours ({ours})");
+
+            let base = repo
+                .merge_base(ours, theirs)
+                .with_context(|| format!("{topics_ref}: {theirs} diverges from {ours}"))?;
+            let theirs_commit = repo.find_commit(theirs)?;
+
+            verify_commit_range(repo, submitter, theirs_commit.id()..base)?;
+
+            let msg = format!(
+                "Merge '{theirs}' into {}\n\n{}",
+                record.topic,
+                record.heads.as_trailer()
+            );
+            let oid = repo.commit(
+                None,
+                &usr,
+                &usr,
+                &msg,
+                &tree,
+                &[&ours_commit, &theirs_commit],
+            )?;
+            let reflog = format!("it: auto-merge from {theirs}");
+            topics_ref.set_target(oid, reflog);
+        },
+    }
+
+    Ok(())
+}
+
+pub fn update_branches(
+    repo: &git2::Repository,
+    tx: &mut refs::Transaction,
+    submitter: &identity::Verified,
+    meta: &metadata::drop::Verified,
+    record: &Record,
+) -> Result<()> {
+    let branches = meta
+        .roles
+        .branches
+        .iter()
+        .filter_map(|(name, role)| role.role.ids.contains(submitter.id()).then_some(name));
+    for branch in branches {
+        let sandboxed = match TrackingBranch::try_from(branch) {
+            Ok(tracking) => tracking.into_refname(),
+            Err(e) => {
+                warn!("Skipping invalid branch {branch}: {e}");
+                continue;
+            },
+        };
+
+        if let Some(target) = record.meta.bundle.references.get(branch) {
+            let target = git2::Oid::try_from(target)?;
+            let locked = tx.lock_ref(sandboxed.clone())?;
+            let reflog = format!(
+                "it: update tip from {} by {}",
+                record.bundle_hash(),
+                submitter.id()
+            );
+            match if_not_found_none(repo.refname_to_id(&sandboxed))? {
+                Some(ours) => {
+                    ensure!(
+                        repo.graph_descendant_of(target, ours)?,
+                        "checkpoint branch {branch} diverges from previously recorded tip {target}"
+                    );
+                    locked.set_target(target, reflog);
+                },
+                None => locked.set_target(target, reflog),
+            }
+
+            if repo.is_bare() {
+                tx.lock_ref(branch.clone())?
+                    .set_symbolic_target(sandboxed, "it: symref auto-updated branch".to_owned());
+            }
+        }
+    }
+
+    Ok(())
+}
+
+fn verify_commit_range(
+    repo: &git2::Repository,
+    allowed: &identity::Verified,
+    Range { start, end }: Range<git2::Oid>,
+) -> Result<()> {
+    let mut walk = repo.revwalk()?;
+    walk.push(start)?;
+    walk.hide(end)?;
+    walk.simplify_first_parent()?;
+    walk.set_sorting(git2::Sort::TOPOLOGICAL)?;
+    for id in walk {
+        let pk = git::verify_commit_signature(repo, &id?)?;
+        let keyid = VerificationKey::from(pk).keyid();
+        ensure!(
+            allowed.identity().keys.contains_key(&keyid),
+            "good signature by unknown signer"
+        );
+    }
+
+    Ok(())
+}
diff --git a/src/patches/submit.rs b/src/patches/submit.rs
new file mode 100644
index 0000000..bca428b
--- /dev/null
+++ b/src/patches/submit.rs
@@ -0,0 +1,574 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    path::{
+        Path,
+        PathBuf,
+    },
+    str::FromStr,
+};
+
+use anyhow::{
+    anyhow,
+    bail,
+    ensure,
+    Context,
+};
+use globset::{
+    Glob,
+    GlobBuilder,
+    GlobSet,
+    GlobSetBuilder,
+};
+use log::info;
+use once_cell::sync::Lazy;
+use thiserror::Error;
+use tiny_http::Request;
+use url::Url;
+
+use super::{
+    bundle::Bundle,
+    record::{
+        self,
+        Heads,
+        Signature,
+    },
+    state,
+    Record,
+    Seen,
+    Topic,
+    HTTP_HEADER_SIGNATURE,
+    MAX_LEN_BUNDLE,
+    REF_IT_BUNDLES,
+    REF_IT_TOPICS,
+    TOPIC_MERGES,
+};
+use crate::{
+    bundle,
+    git::{
+        self,
+        if_not_found_none,
+        refs,
+    },
+    metadata::{
+        self,
+        git::{
+            FromGit,
+            GitMeta,
+            META_FILE_ID,
+        },
+        identity,
+        ContentHash,
+        Signed,
+        Verified,
+    },
+    Result,
+};
+
+pub static GLOB_HEADS: Lazy<Glob> = Lazy::new(|| Glob::new("refs/heads/**").unwrap());
+pub static GLOB_TAGS: Lazy<Glob> = Lazy::new(|| Glob::new("refs/tags/**").unwrap());
+pub static GLOB_NOTES: Lazy<Glob> = Lazy::new(|| Glob::new("refs/notes/**").unwrap());
+
+pub static GLOB_IT_TOPICS: Lazy<Glob> = Lazy::new(|| {
+    GlobBuilder::new(&format!("{}/*", REF_IT_TOPICS))
+        .literal_separator(true)
+        .build()
+        .unwrap()
+});
+pub static GLOB_IT_IDS: Lazy<Glob> = Lazy::new(|| {
+    GlobBuilder::new("refs/it/ids/*")
+        .literal_separator(true)
+        .build()
+        .unwrap()
+});
+pub static GLOB_IT_BUNDLES: Lazy<Glob> =
+    Lazy::new(|| Glob::new(&format!("{}/**", REF_IT_BUNDLES)).unwrap());
+
+pub static ALLOWED_REFS: Lazy<GlobSet> = Lazy::new(|| {
+    GlobSetBuilder::new()
+        .add(GLOB_HEADS.clone())
+        .add(GLOB_TAGS.clone())
+        .add(GLOB_NOTES.clone())
+        .add(GLOB_IT_TOPICS.clone())
+        .add(GLOB_IT_IDS.clone())
+        .build()
+        .unwrap()
+});
+
+pub struct AcceptArgs<'a, S> {
+    /// The prefix under which to store the refs contained in the bundle
+    pub unbundle_prefix: &'a str,
+    /// The refname of the drop history
+    pub drop_ref: &'a str,
+    /// The refname anchoring the seen objects tree
+    pub seen_ref: &'a str,
+    /// The repo to operate on
+    pub repo: &'a git2::Repository,
+    /// The signer for the drop history
+    pub signer: &'a mut S,
+    /// IPFS API address
+    pub ipfs_api: Option<&'a Url>,
+    /// Options
+    pub options: AcceptOptions,
+}
+
+pub struct AcceptOptions {
+    /// Allow bundles to convey "fat" packs, ie. packs which do not have any
+    /// prerequisites
+    ///
+    /// Default: false
+    pub allow_fat_pack: bool,
+    /// Allow encrypted bundles
+    ///
+    /// Default: false
+    pub allow_encrypted: bool,
+    /// Allowed ref name patterns
+    ///
+    /// Default:
+    ///
+    /// - refs/heads/**
+    /// - refs/tags/**
+    /// - refs/notes/**
+    /// - refs/it/topics/*
+    /// - refs/it/ids/*
+    pub allowed_refs: GlobSet,
+    /// Maximum number of branches the bundle is allowed to carry
+    ///
+    /// A branch is a ref which starts with `refs/heads/`.
+    ///
+    /// Default: 1
+    pub max_branches: usize,
+    /// Maximum number of tags the bundle is allowed to carry
+    ///
+    /// A tag is a ref which starts with `refs/tags/`.
+    ///
+    /// Default: 1
+    pub max_tags: usize,
+    /// Maximum number of git notes refs the bundle is allowed to carry
+    ///
+    /// A notes ref is a ref which starts with `refs/notes/`.
+    ///
+    /// Default: 1
+    pub max_notes: usize,
+    /// Maximum number of refs in the bundle, considering all refs
+    ///
+    /// Default: 10,
+    pub max_refs: usize,
+    /// Maximum number of commits a bundle ref can have
+    ///
+    /// Default: 20
+    pub max_commits: usize,
+}
+
+impl Default for AcceptOptions {
+    fn default() -> Self {
+        Self {
+            allow_fat_pack: false,
+            allow_encrypted: false,
+            allowed_refs: ALLOWED_REFS.clone(),
+            max_branches: 1,
+            max_tags: 1,
+            max_notes: 1,
+            max_refs: 10,
+            max_commits: 20,
+        }
+    }
+}
+
+pub struct Submission {
+    pub signature: Signature,
+    pub bundle: Bundle,
+}
+
+impl Submission {
+    pub fn from_http<P>(bundle_dir: P, req: &mut Request) -> Result<Self>
+    where
+        P: AsRef<Path>,
+    {
+        let len = req
+            .body_length()
+            .ok_or_else(|| anyhow!("chunked body not permitted"))?;
+        ensure!(
+            len <= MAX_LEN_BUNDLE,
+            "submitted patch bundle exceeds {MAX_LEN_BUNDLE}",
+        );
+
+        let mut signature = None;
+
+        for hdr in req.headers() {
+            if hdr.field.equiv(HTTP_HEADER_SIGNATURE) {
+                let sig = Signature::try_from(hdr)?;
+                signature = Some(sig);
+                break;
+            }
+        }
+
+        #[derive(Debug, Error)]
+        #[error("missing header {0}")]
+        struct Missing(&'static str);
+
+        let signature = signature.ok_or(Missing(HTTP_HEADER_SIGNATURE))?;
+        let bundle = Bundle::copy(req.as_reader(), bundle_dir)?;
+
+        Ok(Self { signature, bundle })
+    }
+
+    pub fn submit(self, mut base_url: Url) -> Result<Record> {
+        base_url
+            .path_segments_mut()
+            .map_err(|()| anyhow!("invalid url"))?
+            .push("patches");
+        let tiny_http::Header {
+            field: sig_hdr,
+            value: sig,
+        } = self.signature.into();
+        let req = ureq::request_url("POST", &base_url)
+            .set("Content-Length", &self.bundle.info.len.to_string())
+            .set(sig_hdr.as_str().as_str(), sig.as_str());
+        let res = req.send(self.bundle.reader()?)?;
+
+        Ok(res.into_json()?)
+    }
+
+    pub fn try_accept<S>(
+        &mut self,
+        AcceptArgs {
+            unbundle_prefix,
+            drop_ref,
+            seen_ref,
+            repo,
+            signer,
+            ipfs_api,
+            options,
+        }: AcceptArgs<S>,
+    ) -> Result<Record>
+    where
+        S: crate::keys::Signer,
+    {
+        ensure!(
+            unbundle_prefix.starts_with("refs/"),
+            "prefix must start with 'refs/'"
+        );
+        ensure!(
+            !self.bundle.is_encrypted() || options.allow_encrypted,
+            "encrypted bundle rejected"
+        );
+
+        let header = &self.bundle.header;
+
+        ensure!(
+            matches!(header.object_format, bundle::ObjectFormat::Sha1),
+            "object-format {} not (yet) supported",
+            header.object_format
+        );
+        ensure!(
+            !header.prerequisites.is_empty() || options.allow_fat_pack,
+            "thin pack required"
+        );
+        ensure!(
+            header.references.len() <= options.max_refs,
+            "max number of refs exceeded"
+        );
+        let topic = {
+            let mut topic: Option<Topic> = None;
+
+            let mut heads = 0;
+            let mut tags = 0;
+            let mut notes = 0;
+            static GIT_IT: Lazy<GlobSet> = Lazy::new(|| {
+                GlobSetBuilder::new()
+                    .add(GLOB_HEADS.clone())
+                    .add(GLOB_TAGS.clone())
+                    .add(GLOB_NOTES.clone())
+                    .add(GLOB_IT_TOPICS.clone())
+                    .build()
+                    .unwrap()
+            });
+            let mut matches = Vec::with_capacity(1);
+            for r in header.references.keys() {
+                let cand = globset::Candidate::new(r);
+                ensure!(
+                    options.allowed_refs.is_match_candidate(&cand),
+                    "unconventional ref rejected: {r}"
+                );
+                GIT_IT.matches_candidate_into(&cand, &mut matches);
+                match &matches[..] {
+                    [] => {},
+                    [0] => heads += 1,
+                    [1] => tags += 1,
+                    [2] => notes += 1,
+                    [3] => {
+                        ensure!(topic.is_none(), "more than one topic");
+                        match r.split('/').next_back() {
+                            None => bail!("invalid notes '{r}': missing topic"),
+                            Some(s) => {
+                                let t = Topic::from_str(s).context("invalid topic")?;
+                                topic = Some(t);
+                            },
+                        }
+                    },
+                    x => unreachable!("impossible match: {x:?}"),
+                }
+            }
+            ensure!(
+                heads <= options.max_branches,
+                "max number of git branches exceeded"
+            );
+            ensure!(tags <= options.max_tags, "max number of git tags exceeded");
+            ensure!(
+                notes <= options.max_notes,
+                "max number of git notes exceeded"
+            );
+
+            topic.ok_or_else(|| anyhow!("missing '{}'", GLOB_IT_TOPICS.glob()))?
+        };
+        let heads = Heads::from(header);
+
+        let mut tx = refs::Transaction::new(repo)?;
+        let seen_ref = tx.lock_ref(seen_ref.parse()?)?;
+        let seen_tree = match if_not_found_none(repo.find_reference(seen_ref.name()))? {
+            Some(seen) => seen.peel_to_tree()?,
+            None => git::empty_tree(repo)?,
+        };
+        ensure!(!heads.in_tree(&seen_tree)?, "submission already exists");
+
+        // In a bare drop, indexing the pack is enough to detect missing
+        // prerequisites (ie. delta bases). Otherwise, or if the bundle is
+        // encrypted, we need to look for merge bases from the previously
+        // accepted patches.
+        if !repo.is_bare() || self.bundle.is_encrypted() {
+            let mut prereqs = header
+                .prerequisites
+                .iter()
+                .map(git2::Oid::try_from)
+                .collect::<std::result::Result<Vec<_>, _>>()?;
+
+            for r in repo.references_glob(GLOB_IT_BUNDLES.glob())? {
+                let commit = r?.peel_to_commit()?.id();
+                for (i, id) in prereqs.clone().into_iter().enumerate() {
+                    if if_not_found_none(repo.merge_base(commit, id))?.is_some() {
+                        prereqs.swap_remove(i);
+                    }
+                }
+                if prereqs.is_empty() {
+                    break;
+                }
+            }
+
+            ensure!(
+                prereqs.is_empty(),
+                "prerequisite commits not found, try checkpointing a branch or \
+                base the patch on a previous one: {}",
+                prereqs
+                    .iter()
+                    .map(ToString::to_string)
+                    .collect::<Vec<_>>()
+                    .join(", ")
+            );
+        }
+
+        let odb = repo.odb()?;
+        if !self.bundle.is_encrypted() {
+            let mut pack = self.bundle.packdata()?;
+            pack.index(&odb)?;
+
+            let prereqs = header
+                .prerequisites
+                .iter()
+                .map(git2::Oid::try_from)
+                .collect::<std::result::Result<Vec<_>, _>>()?;
+            let mut walk = repo.revwalk()?;
+            for (name, oid) in &header.references {
+                walk.push(oid.try_into()?)?;
+                for hide in &prereqs {
+                    walk.hide(*hide)?;
+                }
+                let mut cnt = 0;
+                for x in &mut walk {
+                    let _ = x?;
+                    cnt += 1;
+                    ensure!(
+                        cnt <= options.max_commits,
+                        "{name} exceeds configured max number of commits ({})",
+                        options.max_commits
+                    );
+                }
+                walk.reset()?;
+            }
+        }
+
+        if let Some(url) = ipfs_api {
+            let ipfs = self.bundle.ipfs_add(url)?;
+            info!("Published bundle to IPFS as {ipfs}");
+        }
+
+        let record = Record {
+            topic,
+            heads,
+            meta: record::Meta {
+                bundle: record::BundleInfo::from(&self.bundle),
+                signature: self.signature.clone(),
+            },
+        };
+
+        let drop_ref = tx.lock_ref(drop_ref.parse()?)?;
+        let mut drop = state::DropHead::from_refname(repo, drop_ref.name())?;
+        ensure!(
+            drop.meta.roles.snapshot.threshold.get() == 1,
+            "threshold signatures for drop snapshots not yet supported"
+        );
+        ensure!(
+            is_signer_eligible(signer, repo, &drop.ids, &drop.meta)?,
+            "supplied signer does not have the 'snapshot' role needed to record patches"
+        );
+
+        let submitter = {
+            let mut id = Identity::find(repo, &drop.ids, &self.signature.signer)?;
+            id.verify_signature(&record.signed_part(), &self.signature)?;
+            if let Some(updated) = id.update(repo, &drop.ids)? {
+                drop.ids = updated;
+            }
+            id.verified
+        };
+
+        let mut seen = repo.treebuilder(Some(&seen_tree))?;
+        let new_head = record.commit(
+            signer,
+            repo,
+            &drop.ids,
+            Some(&drop.tip.peel_to_commit()?),
+            Some(&mut seen),
+        )?;
+        drop_ref.set_target(new_head, format!("commit: {}", record.topic));
+        seen_ref.set_target(seen.write()?, format!("it: update to record {}", new_head));
+
+        if !self.bundle.is_encrypted() {
+            state::unbundle(&odb, &mut tx, unbundle_prefix, &record)?;
+            let topic_ref = tx.lock_ref(record.topic.as_refname())?;
+            state::merge_notes(repo, &submitter, &topic_ref, &record)?;
+            if record.topic == *TOPIC_MERGES {
+                state::update_branches(repo, &mut tx, &submitter, &drop.meta, &record)?;
+            }
+        }
+
+        tx.commit()?;
+
+        Ok(record)
+    }
+}
+
+fn is_signer_eligible<S>(
+    signer: &S,
+    repo: &git2::Repository,
+    ids: &git2::Tree,
+    meta: &Verified<metadata::Drop>,
+) -> Result<bool>
+where
+    S: crate::keys::Signer,
+{
+    let signer_id = metadata::KeyId::from(signer.ident());
+    for id in &meta.roles.snapshot.ids {
+        let s = metadata::identity::find_in_tree(repo, ids, id)?;
+        if s.identity().keys.contains_key(&signer_id) {
+            return Ok(true);
+        }
+    }
+
+    Ok(false)
+}
+
+struct Identity {
+    verified: identity::Verified,
+    to_update: Option<Signed<metadata::Identity>>,
+}
+
+impl Identity {
+    fn find(repo: &git2::Repository, ids: &git2::Tree, hash: &ContentHash) -> Result<Self> {
+        let find_parent = metadata::git::find_parent(repo);
+
+        let (theirs_hash, theirs_signed, theirs) = metadata::Identity::from_content_hash(
+            repo, hash,
+        )
+        .and_then(|GitMeta { hash, signed }| {
+            let signed_dup = signed.clone();
+            let verified = signed.verified(&find_parent)?;
+            Ok((hash, signed_dup, verified))
+        })?;
+
+        let tree_path = PathBuf::from(theirs.id().to_string()).join(META_FILE_ID);
+        let newer = match if_not_found_none(ids.get_path(&tree_path))? {
+            None => Self {
+                verified: theirs,
+                to_update: Some(theirs_signed),
+            },
+            Some(in_tree) if theirs_hash == in_tree.id() => Self {
+                verified: theirs,
+                to_update: None,
+            },
+            Some(in_tree) => {
+                let (ours_hash, ours) = metadata::Identity::from_blob(
+                    &repo.find_blob(in_tree.id())?,
+                )
+                .and_then(|GitMeta { hash, signed }| {
+                    let ours = signed.verified(&find_parent)?;
+                    Ok((hash, ours))
+                })?;
+
+                if ours.identity().has_ancestor(&theirs_hash, &find_parent)? {
+                    Self {
+                        verified: ours,
+                        to_update: None,
+                    }
+                } else if theirs.identity().has_ancestor(&ours_hash, &find_parent)? {
+                    Self {
+                        verified: theirs,
+                        to_update: Some(theirs_signed),
+                    }
+                } else {
+                    bail!(
+                        "provided signer id at {} diverges from known id at {}",
+                        theirs_hash,
+                        ours_hash,
+                    );
+                }
+            },
+        };
+
+        Ok(newer)
+    }
+
+    fn verify_signature(&self, msg: &[u8], sig: &Signature) -> Result<()> {
+        ensure!(
+            self.verified.did_sign(msg, &sig.signature),
+            "signature not valid for current keys in id {}, provided signer at {}",
+            self.verified.id(),
+            sig.signer
+        );
+        Ok(())
+    }
+
+    fn update<'a>(
+        &mut self,
+        repo: &'a git2::Repository,
+        root: &git2::Tree,
+    ) -> Result<Option<git2::Tree<'a>>> {
+        if let Some(meta) = self.to_update.take() {
+            let mut new_root = repo.treebuilder(Some(root))?;
+            let mut id_tree = repo.treebuilder(None)?;
+            metadata::identity::fold_to_tree(repo, &mut id_tree, meta)?;
+            new_root.insert(
+                self.verified.id().to_string().as_str(),
+                id_tree.write()?,
+                git2::FileMode::Tree.into(),
+            )?;
+
+            let oid = new_root.write()?;
+            let tree = repo.find_tree(oid).map(Some)?;
+
+            return Ok(tree);
+        }
+
+        Ok(None)
+    }
+}
diff --git a/src/patches/traits.rs b/src/patches/traits.rs
new file mode 100644
index 0000000..ef9ae61
--- /dev/null
+++ b/src/patches/traits.rs
@@ -0,0 +1,165 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    io,
+    path::{
+        Path,
+        PathBuf,
+    },
+};
+
+use super::error;
+use crate::git::{
+    self,
+    if_not_found_none,
+};
+
+pub trait BlobData: Sized {
+    type Error;
+
+    const MAX_BYTES: usize;
+
+    fn from_blob(data: &[u8]) -> Result<Self, Self::Error>;
+    fn write_blob<W: io::Write>(&self, writer: W) -> io::Result<()>;
+}
+
+pub trait TreeData: BlobData {
+    const BLOB_NAME: &'static str;
+}
+
+pub struct Blob<T> {
+    pub oid: git2::Oid,
+    pub content: T,
+}
+
+impl<T> Blob<T>
+where
+    T: TreeData,
+    T::Error: Into<crate::Error>,
+{
+    pub fn from_tree<'a>(
+        repo: &'a git2::Repository,
+        tree: &git2::Tree<'a>,
+    ) -> Result<Blob<T>, error::FromTree> {
+        use error::FromTree::NotFound;
+
+        let entry = tree
+            .get_name(T::BLOB_NAME)
+            .ok_or(NotFound { name: T::BLOB_NAME })?;
+        Self::from_entry(repo, entry)
+    }
+
+    pub fn from_entry<'a>(
+        repo: &'a git2::Repository,
+        entry: git2::TreeEntry<'a>,
+    ) -> Result<Self, error::FromTree> {
+        use error::FromTree::{
+            BlobSize,
+            TypeConversion,
+            TypeMismatch,
+        };
+
+        let blob = entry
+            .to_object(repo)?
+            .into_blob()
+            .map_err(|obj| TypeMismatch {
+                name: T::BLOB_NAME,
+                kind: obj.kind(),
+            })?;
+        let sz = blob.size();
+        if sz > T::MAX_BYTES {
+            return Err(BlobSize {
+                max: T::MAX_BYTES,
+                found: sz,
+            });
+        }
+        let content = T::from_blob(blob.content())
+            .map_err(Into::into)
+            .map_err(TypeConversion)?;
+
+        Ok(Self {
+            oid: entry.id(),
+            content,
+        })
+    }
+}
+
+pub trait Foldable {
+    fn folded_name(&self) -> String;
+}
+
+pub trait Seen {
+    fn in_odb(&self, odb: &git2::Odb) -> git::Result<bool>;
+    fn in_tree(&self, tree: &git2::Tree) -> git::Result<bool>;
+}
+
+impl<T> Seen for T
+where
+    T: BlobData + Foldable,
+{
+    fn in_odb(&self, odb: &git2::Odb) -> git::Result<bool> {
+        let hash = blob_hash(self)?;
+        Ok(odb.exists(hash))
+    }
+
+    fn in_tree(&self, tree: &git2::Tree) -> git::Result<bool> {
+        let path = shard_path(&self.folded_name());
+        Ok(if_not_found_none(tree.get_path(&path))?.is_some())
+    }
+}
+
+pub fn to_tree<T: TreeData>(
+    repo: &git2::Repository,
+    tree: &mut git2::TreeBuilder,
+    data: &T,
+) -> git::Result<()> {
+    tree.insert(
+        T::BLOB_NAME,
+        to_blob(repo, data)?,
+        git2::FileMode::Blob.into(),
+    )?;
+    Ok(())
+}
+
+pub fn to_blob<T: BlobData>(repo: &git2::Repository, data: &T) -> git::Result<git2::Oid> {
+    let mut writer = repo.blob_writer(None)?;
+    data.write_blob(&mut writer).map_err(|e| {
+        git2::Error::new(
+            git2::ErrorCode::GenericError,
+            git2::ErrorClass::Object,
+            e.to_string(),
+        )
+    })?;
+    writer.commit()
+}
+
+pub fn blob_hash<T: BlobData>(data: &T) -> git::Result<git2::Oid> {
+    let mut buf = Vec::new();
+    data.write_blob(&mut buf).unwrap();
+    git::blob_hash(&buf)
+}
+
+pub fn write_sharded<F: Foldable>(
+    repo: &git2::Repository,
+    root: &mut git2::TreeBuilder,
+    item: &F,
+    blob: git2::Oid,
+) -> git::Result<()> {
+    let name = item.folded_name();
+    let (pre, suf) = name.split_at(2);
+    let shard = root
+        .get(pre)?
+        .map(|entry| entry.to_object(repo))
+        .transpose()?;
+    let mut sub = repo.treebuilder(shard.as_ref().and_then(git2::Object::as_tree))?;
+    sub.insert(suf, blob, git2::FileMode::Blob.into())?;
+    root.insert(pre, sub.write()?, git2::FileMode::Tree.into())?;
+
+    Ok(())
+}
+
+pub fn shard_path(name: &str) -> PathBuf {
+    let (pre, suf) = name.split_at(2);
+    Path::new(pre).join(suf)
+}
diff --git a/src/serde.rs b/src/serde.rs
new file mode 100644
index 0000000..cbbf6a9
--- /dev/null
+++ b/src/serde.rs
@@ -0,0 +1,28 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+pub mod display {
+    use std::{
+        fmt,
+        str::FromStr,
+    };
+
+    pub fn serialize<T, S>(v: &T, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        T: ToString,
+        S: serde::Serializer,
+    {
+        serializer.serialize_str(&v.to_string())
+    }
+
+    #[allow(unused)]
+    pub fn deserialize<'de, T, D>(deserializer: D) -> Result<T, D::Error>
+    where
+        T: FromStr,
+        T::Err: fmt::Display,
+        D: serde::Deserializer<'de>,
+    {
+        let s: &str = serde::Deserialize::deserialize(deserializer)?;
+        s.parse().map_err(serde::de::Error::custom)
+    }
+}
diff --git a/src/ssh.rs b/src/ssh.rs
new file mode 100644
index 0000000..3019d45
--- /dev/null
+++ b/src/ssh.rs
@@ -0,0 +1,5 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+pub mod agent;
+pub use ssh_key::*;
diff --git a/src/ssh/agent.rs b/src/ssh/agent.rs
new file mode 100644
index 0000000..c29ad62
--- /dev/null
+++ b/src/ssh/agent.rs
@@ -0,0 +1,279 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use std::{
+    env,
+    io::{
+        self,
+        ErrorKind::*,
+    },
+};
+
+use anyhow::Context;
+use ssh_encoding::{
+    CheckedSum,
+    Decode,
+    Encode,
+    Reader,
+    Writer,
+};
+use ssh_key::{
+    public::KeyData,
+    Algorithm,
+    HashAlg,
+    PublicKey,
+    Signature,
+};
+
+#[cfg(unix)]
+pub use std::os::unix::net::UnixStream;
+#[cfg(windows)]
+pub use uds_windows::UnixStram;
+
+const SSH_AUTH_SOCK: &str = "SSH_AUTH_SOCK";
+
+const MAX_AGENT_REPLY_LEN: usize = 256 * 1024;
+
+const SSH_AGENTC_REQUEST_IDENTITIES: u8 = 11;
+const SSH_AGENTC_SIGN_REQUEST: u8 = 13;
+const SSH_AGENT_FAILURE: u8 = 5;
+const SSH_AGENT_IDENTITIES_ANSWER: u8 = 12;
+const SSH_AGENT_RSA_SHA2_256: u32 = 2;
+const SSH_AGENT_RSA_SHA2_512: u32 = 4;
+const SSH_AGENT_SIGN_RESPONSE: u8 = 14;
+
+pub struct Client<T> {
+    conn: T,
+}
+
+impl Client<UnixStream> {
+    pub fn from_env() -> io::Result<Self> {
+        let path = env::var_os(SSH_AUTH_SOCK).ok_or_else(|| {
+            io::Error::new(
+                io::ErrorKind::AddrNotAvailable,
+                "SSH_AUTH_SOCK environment variable not set",
+            )
+        })?;
+        UnixStream::connect(path).map(Self::from)
+    }
+}
+
+impl From<UnixStream> for Client<UnixStream> {
+    fn from(conn: UnixStream) -> Self {
+        Self { conn }
+    }
+}
+
+impl<'a> From<&'a UnixStream> for Client<&'a UnixStream> {
+    fn from(conn: &'a UnixStream) -> Self {
+        Self { conn }
+    }
+}
+
+impl<T> Client<T>
+where
+    T: io::Read + io::Write,
+{
+    pub fn sign(&mut self, key: &PublicKey, msg: impl AsRef<[u8]>) -> io::Result<Signature> {
+        request(
+            &mut self.conn,
+            SignRequest {
+                key,
+                msg: msg.as_ref(),
+            },
+        )
+        .map(|SignResponse { sig }| sig)
+    }
+
+    pub fn list_keys(&mut self) -> io::Result<Vec<PublicKey>> {
+        request(&mut self.conn, RequestIdentities).map(|IdentitiesAnswer { keys }| keys)
+    }
+}
+
+trait Request: Encode<Error = crate::Error> {
+    type Response: Response;
+}
+
+trait Response: Decode<Error = crate::Error> {
+    const SUCCESS: u8;
+}
+
+fn request<I, T>(mut io: I, req: T) -> io::Result<T::Response>
+where
+    I: io::Read + io::Write,
+    T: Request,
+{
+    send(&mut io, req)?;
+    let resp = recv(&mut io)?;
+    let mut reader = resp.as_slice();
+    match u8::decode(&mut reader).map_err(|_| unknown_response())? {
+        x if x == T::Response::SUCCESS => T::Response::decode(&mut reader).map_err(decode),
+        SSH_AGENT_FAILURE => Err(agent_error()),
+        _ => Err(unknown_response()),
+    }
+}
+
+fn send<W, T>(mut io: W, req: T) -> io::Result<()>
+where
+    W: io::Write,
+    T: Encode<Error = crate::Error>,
+{
+    let len = req.encoded_len_prefixed().map_err(encode)?;
+    let mut buf = Vec::with_capacity(len);
+    req.encode_prefixed(&mut buf).map_err(encode)?;
+
+    io.write_all(&buf)?;
+    io.flush()?;
+
+    Ok(())
+}
+
+fn recv<R: io::Read>(mut io: R) -> io::Result<Vec<u8>> {
+    let want = {
+        let mut buf = [0; 4];
+        io.read_exact(&mut buf)?;
+        u32::from_be_bytes(buf) as usize
+    };
+
+    if want < 1 {
+        return Err(incomplete_response());
+    }
+    if want > MAX_AGENT_REPLY_LEN {
+        return Err(reponse_too_large());
+    }
+
+    let mut buf = vec![0; want];
+    io.read_exact(&mut buf)?;
+
+    Ok(buf)
+}
+
+struct SignRequest<'a> {
+    key: &'a PublicKey,
+    msg: &'a [u8],
+}
+
+impl Request for SignRequest<'_> {
+    type Response = SignResponse;
+}
+
+impl Encode for SignRequest<'_> {
+    type Error = crate::Error;
+
+    fn encoded_len(&self) -> Result<usize, Self::Error> {
+        Ok([
+            self.key.key_data().encoded_len_prefixed()?,
+            self.msg.encoded_len()?,
+            SSH_AGENTC_SIGN_REQUEST.encoded_len()?,
+            4, // flags
+        ]
+        .checked_sum()?)
+    }
+
+    fn encode(&self, writer: &mut impl Writer) -> Result<(), Self::Error> {
+        SSH_AGENTC_SIGN_REQUEST.encode(writer)?;
+        self.key.key_data().encode_prefixed(writer)?;
+        self.msg.encode(writer)?;
+        let flags = match self.key.algorithm() {
+            Algorithm::Rsa { hash } => match hash {
+                Some(HashAlg::Sha256) => SSH_AGENT_RSA_SHA2_256,
+                _ => SSH_AGENT_RSA_SHA2_512, // sane default
+            },
+            _ => 0,
+        };
+        flags.encode(writer)?;
+        Ok(())
+    }
+}
+
+struct SignResponse {
+    sig: Signature,
+}
+
+impl Response for SignResponse {
+    const SUCCESS: u8 = SSH_AGENT_SIGN_RESPONSE;
+}
+
+impl Decode for SignResponse {
+    type Error = crate::Error;
+
+    fn decode(reader: &mut impl Reader) -> Result<Self, Self::Error> {
+        let sig = reader.read_prefixed(Signature::decode)?;
+        Ok(Self { sig })
+    }
+}
+
+struct RequestIdentities;
+
+impl Request for RequestIdentities {
+    type Response = IdentitiesAnswer;
+}
+
+impl Encode for RequestIdentities {
+    type Error = crate::Error;
+
+    fn encoded_len(&self) -> Result<usize, Self::Error> {
+        Ok(SSH_AGENTC_REQUEST_IDENTITIES.encoded_len()?)
+    }
+
+    fn encode(&self, writer: &mut impl Writer) -> Result<(), Self::Error> {
+        Ok(SSH_AGENTC_REQUEST_IDENTITIES.encode(writer)?)
+    }
+}
+
+struct IdentitiesAnswer {
+    keys: Vec<PublicKey>,
+}
+
+impl Response for IdentitiesAnswer {
+    const SUCCESS: u8 = SSH_AGENT_IDENTITIES_ANSWER;
+}
+
+impl Decode for IdentitiesAnswer {
+    type Error = crate::Error;
+
+    fn decode(reader: &mut impl Reader) -> Result<Self, Self::Error> {
+        let nkeys = usize::decode(reader).context("nkeys")?;
+        let mut keys = Vec::with_capacity(nkeys);
+
+        for _ in 0..nkeys {
+            let key_data = reader.read_prefixed(KeyData::decode).context("key data")?;
+            let comment = String::decode(reader).context("comment")?;
+            keys.push(PublicKey::new(key_data, comment));
+        }
+
+        Ok(Self { keys })
+    }
+}
+
+fn e(kind: io::ErrorKind, msg: &str) -> io::Error {
+    io::Error::new(kind, msg)
+}
+
+fn ee(kind: io::ErrorKind, e: crate::Error) -> io::Error {
+    io::Error::new(kind, e)
+}
+
+fn incomplete_response() -> io::Error {
+    e(UnexpectedEof, "incomplete response")
+}
+
+fn reponse_too_large() -> io::Error {
+    e(Unsupported, "response payload too large")
+}
+
+fn encode(e: crate::Error) -> io::Error {
+    ee(InvalidData, e.context("failed to encode request"))
+}
+
+fn decode(e: crate::Error) -> io::Error {
+    ee(InvalidData, e.context("failed to decode response"))
+}
+
+fn agent_error() -> io::Error {
+    e(Other, "error response from agent")
+}
+
+fn unknown_response() -> io::Error {
+    e(Unsupported, "unknown response")
+}
diff --git a/src/str.rs b/src/str.rs
new file mode 100644
index 0000000..1825e06
--- /dev/null
+++ b/src/str.rs
@@ -0,0 +1,94 @@
+// Copyright © 2022 Kim Altintop <kim@eagain.io>
+// SPDX-License-Identifier: GPL-2.0-only WITH openvpn-openssl-exception
+
+use core::fmt;
+use std::{
+    ops::Deref,
+    str::FromStr,
+};
+
+use anyhow::ensure;
+
+// A variable-length string type with a maximum length `N`.
+#[derive(Clone, Debug, Eq, Ord, PartialEq, PartialOrd, serde::Serialize)]
+pub struct Varchar<T, const N: usize>(T);
+
+impl<T, const N: usize> Varchar<T, N>
+where
+    T: AsRef<str>,
+{
+    pub fn len(&self) -> usize {
+        self.0.as_ref().len()
+    }
+
+    pub fn is_empty(&self) -> bool {
+        self.0.as_ref().is_empty()
+    }
+
+    fn try_from_t(t: T) -> crate::Result<Self> {
+        let len = t.as_ref().len();
+        ensure!(len <= N, "string length exceeds {N}: {len}");
+        Ok(Self(t))
+    }
+}
+
+impl<const N: usize> Varchar<String, N> {
+    pub const fn new() -> Self {
+        Self(String::new())
+    }
+}
+
+impl<const N: usize> TryFrom<String> for Varchar<String, N> {
+    type Error = crate::Error;
+
+    fn try_from(s: String) -> Result<Self, Self::Error> {
+        Self::try_from_t(s)
+    }
+}
+
+impl<const N: usize> FromStr for Varchar<String, N> {
+    type Err = crate::Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Self::try_from(s.to_owned())
+    }
+}
+
+impl<'a, const N: usize> TryFrom<&'a str> for Varchar<&'a str, N> {
+    type Error = crate::Error;
+
+    fn try_from(s: &'a str) -> Result<Self, Self::Error> {
+        Self::try_from_t(s)
+    }
+}
+
+impl<T, const N: usize> Deref for Varchar<T, N> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<T, const N: usize> fmt::Display for Varchar<T, N>
+where
+    T: AsRef<str>,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(self.0.as_ref())
+    }
+}
+
+impl<'de, T, const N: usize> serde::Deserialize<'de> for Varchar<T, N>
+where
+    T: serde::Deserialize<'de> + TryInto<Self>,
+    <T as TryInto<Self>>::Error: fmt::Display,
+{
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let t = T::deserialize(deserializer)?;
+        t.try_into().map_err(serde::de::Error::custom)
+    }
+}
author	Kim Altintop <kim@eagain.io>	2023-01-09 13:18:33 +0100
committer	Kim Altintop <kim@eagain.io>	2023-01-09 13:18:33 +0100
commit	d2f423521ec76406944ad83098ec33afe20c692b (patch)
tree	afd86bcb088eebdd61ba4e52fa666ff0f41c42a2