docs: start a release document

Change-Id: I884639665c020338ec9ceeb1add5c3b862583674

docs/release-process.md

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+# repo release process
+
+This is the process for creating a new release of repo, as well as all the
+related topics and flows.
+
+[TOC]
+
+## Launcher script
+
+The main repo script serves as a standalone program and is often referred to as
+the "launcher script".
+This makes it easy to copy around and install as you don't have to install any
+other files from the git repo.
+
+Whenever major changes are made to the launcher script, you should increment the
+`VERSION` variable in the launcher itself.
+At runtime, repo will check this to see if it needs to be updated (and notify
+the user automatically).
+
+## Key management
+
+Every release has a git tag that is signed with a key that repo recognizes.
+Those keys are hardcoded inside of the repo launcher itself -- look for the
+`KEYRING_VERSION` and `MAINTAINER_KEYS` settings.
+
+Adding new keys to the repo launcher will allow tags to be recognized by new
+keys, but only people using that updated version will be able to.
+Since the majority of users will be using an official launcher version, their
+version will simply ignore any new signed tags.
+
+If you want to add new keys, it's best to register them long ahead of time,
+and then wait for that updated launcher to make its way out to everyone.
+Even then, there will be a long tail of users with outdated launchers, so be
+prepared for people asking questions.
+
+### Registering a new key
+
+The process of actually adding a new key is quite simple.
+
+1.  Add the public half of the key to `MAINTAINER_KEYS`.
+2.  Increment `KEYRING_VERSION` so repo knows it needs to update.
+3.  Wait a long time after that version is in a release (~months) before trying
+    to create a new release using those new keys.
+
+## Self update algorithm
+
+When creating a new repo checkout with `repo init`, there are a few options that
+control how repo finds updates:
+
+*   `--repo-url`: This tells repo where to clone the full repo project itself.
+    It defaults to the official project (`REPO_URL` in the launcher script).
+*   `--repo-branch`: This tells repo which branch to use for the full project.
+    It defaults to the `stable` branch (`REPO_REV` in the launcher script).
+
+Whenever `repo sync` is run, repo will check to see if an update is available.
+It fetches the latest repo-branch from the repo-url.
+Then it verifies that the latest commit in the branch has a valid signed tag
+using `git tag -v` (which uses gpg).
+If the tag is valid, then repo will update its internal checkout to it.
+
+If the latest commit doesn't have a signed tag, repo will fall back to the
+most recent tag it can find (via `git describe`).
+If that tag is valid, then repo will warn and use that commit instead.
+
+If that tag cannot be verified, it gives up and forces the user to resolve.
+
+## Branch management
+
+All development happens on the `master` branch and should generally be stable.
+
+Since the repo launcher defaults to tracking the `stable` branch, it is not
+normally updated until a new release is available.
+If something goes wrong with a new release, an older release can be force pushed
+and clients will automatically downgrade.
+
+The `maint` branch is used to track the previous major release of repo.
+It is not normally meant to be used by people as `stable` should be good enough.
+Once a new major release is pushed to the `stable` branch, then the previous
+major release can be pushed to `maint`.
+For example, when `stable` moves from `v1.10.x` to `v1.11.x`, then the `maint`
+branch will be updated from `v1.9.x` to `v1.10.x`.
+
+We don't have parallel release branches/series.
+Typically all tags are made against the `master` branch and then pushed to the
+`stable` branch to make it available to the rest of the world.
+Since repo doesn't typically see a lot of changes, this tends to be OK.
+
+## Creating a new release
+
+When you want to create a new release, you'll need to select a good version and
+create a signed tag using a key registered in repo itself.
+Typically we just tag the latest version of the `master` branch.
+The tag could be pushed now, but it won't be used by clients normally (since the
+default `repo-branch` setting is `stable`).
+This would allow some early testing on systems who explicitly select `master`.
+
+### Creating a signed tag
+
+Lets assume your keys live in a dedicated directory, e.g. `~/.gnupg/repo/`.
+
+*** note
+If you need access to the official keys, check out the internal documentation
+at [go/repo-release].
+Note that only official maintainers of repo will have access as it describes
+internal processes for accessing the restricted keys.
+***
+
+```sh
+# Set the gpg key directory.
+$ export GNUPGHOME=~/.gnupg/repo/
+
+# Verify the listed key is “Repo Maintainer”.
+$ gpg -K
+
+# Pick whatever branch or commit you want to tag.
+$ r=master
+
+# Pick the new version.
+$ t=1.12.10
+
+# Create the signed tag.
+$ git tag -s v$t -u "Repo Maintainer <repo@android.kernel.org>" -m "repo $t" $r
+
+# Verify the signed tag.
+$ git show v$t
+```
+
+### Push the new release
+
+Once you're ready to make the release available to everyone, push it to the
+`stable` branch.
+
+Make sure you never push the tag itself to the stable branch!
+Only push the commit -- notice the use of `$t` and `$r` below.
+
+```sh
+$ git push https://gerrit-review.googlesource.com/git-repo v$t
+$ git push https://gerrit-review.googlesource.com/git-repo $r:stable
+```
+
+If something goes horribly wrong, you can force push the previous version to the
+`stable` branch and people should automatically recover.
+Again, make sure you never push the tag itself!
+
+```sh
+$ oldrev="whatever-old-commit"
+$ git push https://gerrit-review.googlesource.com/git-repo $oldrev:stable --force
+```
+
+### Announce the release
+
+Once you do push a new release to `stable`, make sure to announce it on the
+[repo-discuss@googlegroups.com] group.
+Here is an [example announcement].
+
+You can create a short changelog using the command:
+
+```sh
+# If you haven't pushed to the stable branch yet, you can use origin/stable.
+# If you have pushed, change origin/stable to the previous release tag.
+$ git log --format="%h (%aN) %s" --no-merges origin/stable..$r
+```
+
+
+[example announcement]: https://groups.google.com/d/topic/repo-discuss/UGBNismWo1M/discussion
+[repo-discuss@googlegroups.com]: https://groups.google.com/forum/#!forum/repo-discuss
+[go/repo-release]: https://goto.google.com/repo-release