forked from MirrorHub/synapse
148 lines
7 KiB
Markdown
148 lines
7 KiB
Markdown
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="img/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` (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` *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.)
|
|
|
|
---
|
|
|
|
<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` is more-stable than `release-v1.3`. [^](#a3)
|