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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:

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”, “fix broken +test”, +“oops”, +“typo”, or “Who's +the president?”.

There are a number of reasons why keeping a clean commit history is a good +thing:

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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.
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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.
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Likewise: “what has changed on this branch in the last week?” is much +clearer without merges and “pep8” commits everywhere.
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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.
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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).

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”:

squash and merge .

(This applies whether you are merging your own PR, or that of another +contributor.)

“Squash and merge”¹ 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, or a +PR which is the culmination of several other +PRs. 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 +of +words have been +written in the past about git branching models (no really, a +lot). 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:

branching model

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 time². (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 (the branch where we prepare the next release)³.
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 and develop, then you should base your PR on +release-vX.Y, get it merged there, and then merge from release-vX.Y 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.)

[1]: “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. ^

[2]: Well, anyone with commit access.^

[3]: 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 is more-stable than release-v1.3. ^

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