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diff --git a/docs/dev/git.md b/docs/dev/git.md new file mode 100644 index 0000000000..b747ff20c9 --- /dev/null +++ b/docs/dev/git.md @@ -0,0 +1,148 @@ +Some notes on how we use git +============================ + +On keeping the commit history clean +----------------------------------- + +In an ideal world, our git commit history would be a linear progression of +commits each of which contains a single change building on what came +before. Here, by way of an arbitrary example, is the top of `git log --graph +b2dba0607`: + +<img src="git/clean.png" alt="clean git graph" width="500px"> + +Note how the commit comment explains clearly what is changing and why. Also +note the *absence* of merge commits, as well as the absence of commits called +things like (to pick a few culprits): +[“pep8”](https://github.com/matrix-org/synapse/commit/84691da6c), [“fix broken +test”](https://github.com/matrix-org/synapse/commit/474810d9d), +[“oops”](https://github.com/matrix-org/synapse/commit/c9d72e457), +[“typo”](https://github.com/matrix-org/synapse/commit/836358823), or [“Who's +the president?”](https://github.com/matrix-org/synapse/commit/707374d5d). + +There are a number of reasons why keeping a clean commit history is a good +thing: + + * From time to time, after a change lands, it turns out to be necessary to + revert it, or to backport it to a release branch. Those operations are + *much* easier when the change is contained in a single commit. + + * Similarly, it's much easier to answer questions like “is the fix for + `/publicRooms` on the release branch?” if that change consists of a single + commit. + + * Likewise: “what has changed on this branch in the last week?” is much + clearer without merges and “pep8” commits everywhere. + + * Sometimes we need to figure out where a bug got introduced, or some + behaviour changed. One way of doing that is with `git bisect`: pick an + arbitrary commit between the known good point and the known bad point, and + see how the code behaves. However, that strategy fails if the commit you + chose is the middle of someone's epic branch in which they broke the world + before putting it back together again. + +One counterargument is that it is sometimes useful to see how a PR evolved as +it went through review cycles. This is true, but that information is always +available via the GitHub UI (or via the little-known [refs/pull +namespace](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/checking-out-pull-requests-locally)). + + +Of course, in reality, things are more complicated than that. We have release +branches as well as `develop` and `master`, and we deliberately merge changes +between them. Bugs often slip through and have to be fixed later. That's all +fine: this not a cast-iron rule which must be obeyed, but an ideal to aim +towards. + +Merges, squashes, rebases: wtf? +------------------------------- + +Ok, so that's what we'd like to achieve. How do we achieve it? + +The TL;DR is: when you come to merge a pull request, you *probably* want to +“squash and merge”: + +. + +(This applies whether you are merging your own PR, or that of another +contributor.) + +“Squash and merge”<sup id="a1">[1](#f1)</sup> takes all of the changes in the +PR, and bundles them into a single commit. GitHub gives you the opportunity to +edit the commit message before you confirm, and normally you should do so, +because the default will be useless (again: `* woops typo` is not a useful +thing to keep in the historical record). + +The main problem with this approach comes when you have a series of pull +requests which build on top of one another: as soon as you squash-merge the +first PR, you'll end up with a stack of conflicts to resolve in all of the +others. In general, it's best to avoid this situation in the first place by +trying not to have multiple related PRs in flight at the same time. Still, +sometimes that's not possible and doing a regular merge is the lesser evil. + +Another occasion in which a regular merge makes more sense is a PR where you've +deliberately created a series of commits each of which makes sense in its own +right. For example: [a PR which gradually propagates a refactoring operation +through the codebase](https://github.com/matrix-org/synapse/pull/6837), or [a +PR which is the culmination of several other +PRs](https://github.com/matrix-org/synapse/pull/5987). In this case the ability +to figure out when a particular change/bug was introduced could be very useful. + +Ultimately: **this is not a hard-and-fast-rule**. If in doubt, ask yourself “do +each of the commits I am about to merge make sense in their own right”, but +remember that we're just doing our best to balance “keeping the commit history +clean” with other factors. + +Git branching model +------------------- + +A [lot](https://nvie.com/posts/a-successful-git-branching-model/) +[of](http://scottchacon.com/2011/08/31/github-flow.html) +[words](https://www.endoflineblog.com/gitflow-considered-harmful) have been +written in the past about git branching models (no really, [a +lot](https://martinfowler.com/articles/branching-patterns.html)). I tend to +think the whole thing is overblown. Fundamentally, it's not that +complicated. Here's how we do it. + +Let's start with a picture: + + + +It looks complicated, but it's really not. There's one basic rule: *anyone* is +free to merge from *any* more-stable branch to *any* less-stable branch at +*any* time<sup id="a2">[2](#f2)</sup>. (The principle behind this is that if a +change is good enough for the more-stable branch, then it's also good enough go +put in a less-stable branch.) + +Meanwhile, merging (or squashing, as per the above) from a less-stable to a +more-stable branch is a deliberate action in which you want to publish a change +or a set of changes to (some subset of) the world: for example, this happens +when a PR is landed, or as part of our release process. + +So, what counts as a more- or less-stable branch? A little reflection will show +that our active branches are ordered thus, from more-stable to less-stable: + + * `master` (tracks our last release). + * `release-vX.Y.Z` (the branch where we prepare the next release)<sup + id="a3">[3](#f3)</sup>. + * PR branches which are targeting the release. + * `develop` (our "mainline" branch containing our bleeding-edge). + * regular PR branches. + +The corollary is: if you have a bugfix that needs to land in both +`release-vX.Y.Z` *and* `develop`, then you should base your PR on +`release-vX.Y.Z`, get it merged there, and then merge from `release-vX.Y.Z` to +`develop`. (If a fix lands in `develop` and we later need it in a +release-branch, we can of course cherry-pick it, but landing it in the release +branch first helps reduce the chance of annoying conflicts.) + +--- + +<b id="f1">[1]</b>: “Squash and merge” is GitHub's term for this +operation. Given that there is no merge involved, I'm not convinced it's the +most intuitive name. [^](#a1) + +<b id="f2">[2]</b>: Well, anyone with commit access.[^](#a2) + +<b id="f3">[3]</b>: Very, very occasionally (I think this has happened once in +the history of Synapse), we've had two releases in flight at once. Obviously, +`release-v1.2.3` is more-stable than `release-v1.3.0`. [^](#a3) diff --git a/docs/dev/git/branches.jpg b/docs/dev/git/branches.jpg new file mode 100644 index 0000000000..715ecc8cd0 --- /dev/null +++ b/docs/dev/git/branches.jpg Binary files differdiff --git a/docs/dev/git/clean.png b/docs/dev/git/clean.png new file mode 100644 index 0000000000..3accd7ccef --- /dev/null +++ b/docs/dev/git/clean.png Binary files differdiff --git a/docs/dev/git/squash.png b/docs/dev/git/squash.png new file mode 100644 index 0000000000..234caca3e4 --- /dev/null +++ b/docs/dev/git/squash.png Binary files differ |
