deploy: 7c805f00a787b53b0eacddb9cb4b5470b9c8d135

author: erikjohnston <erikjohnston@users.noreply.github.com> 2024-02-06 16:35:44 +0000
committer: erikjohnston <erikjohnston@users.noreply.github.com> 2024-02-06 16:35:44 +0000
commit: d2d9645a32a0eb7b5b97db02ebab27105974e050 (patch)
tree: 81655a778d2b6d065667cc6a84ea5c038afd5fce /v1.101/development/git.html
parent: deploy: 93edd0932e636252528ca4b95802f35b1a513b6e (diff)
download: synapse-d2d9645a32a0eb7b5b97db02ebab27105974e050.tar.xz
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+                <ol class="chapter"><li class="chapter-item expanded affix "><li class="part-title">Introduction</li><li class="chapter-item expanded "><a href="../welcome_and_overview.html">Welcome and Overview</a></li><li class="chapter-item expanded affix "><li class="part-title">Setup</li><li class="chapter-item expanded "><a href="../setup/installation.html">Installation</a></li><li class="chapter-item expanded "><a href="../postgres.html">Using Postgres</a></li><li class="chapter-item expanded "><a href="../reverse_proxy.html">Configuring a Reverse Proxy</a></li><li class="chapter-item expanded "><a href="../setup/forward_proxy.html">Configuring a Forward/Outbound Proxy</a></li><li class="chapter-item expanded "><a href="../turn-howto.html">Configuring a Turn Server</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../setup/turn/coturn.html">coturn TURN server</a></li><li class="chapter-item expanded "><a href="../setup/turn/eturnal.html">eturnal TURN 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+
+                        <h1 id="some-notes-on-how-we-use-git"><a class="header" href="#some-notes-on-how-we-use-git">Some notes on how we use git</a></h1>
+<h2 id="on-keeping-the-commit-history-clean"><a class="header" href="#on-keeping-the-commit-history-clean">On keeping the commit history clean</a></h2>
+<p>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 <code>git log --graph b2dba0607</code>:</p>
+<img src="img/git/clean.png" alt="clean git graph" width="500px">
+<p>Note how the commit comment explains clearly what is changing and why. Also
+note the <em>absence</em> of merge commits, as well as the absence of commits called
+things like (to pick a few culprits):
+<a href="https://github.com/element-hq/synapse/commit/84691da6c">“pep8”</a>, <a href="https://github.com/element-hq/synapse/commit/474810d9d">“fix broken
+test”</a>,
+<a href="https://github.com/element-hq/synapse/commit/c9d72e457">“oops”</a>,
+<a href="https://github.com/element-hq/synapse/commit/836358823">“typo”</a>, or <a href="https://github.com/element-hq/synapse/commit/707374d5d">“Who's
+the president?”</a>.</p>
+<p>There are a number of reasons why keeping a clean commit history is a good
+thing:</p>
+<ul>
+<li>
+<p>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
+<em>much</em> easier when the change is contained in a single commit.</p>
+</li>
+<li>
+<p>Similarly, it's much easier to answer questions like “is the fix for
+<code>/publicRooms</code> on the release branch?” if that change consists of a single
+commit.</p>
+</li>
+<li>
+<p>Likewise: “what has changed on this branch in the last week?” is much
+clearer without merges and “pep8” commits everywhere.</p>
+</li>
+<li>
+<p>Sometimes we need to figure out where a bug got introduced, or some
+behaviour changed. One way of doing that is with <code>git bisect</code>: 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.</p>
+</li>
+</ul>
+<p>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 <a href="https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/checking-out-pull-requests-locally">refs/pull
+namespace</a>).</p>
+<p>Of course, in reality, things are more complicated than that. We have release
+branches as well as <code>develop</code> and <code>master</code>, 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.</p>
+<h2 id="merges-squashes-rebases-wtf"><a class="header" href="#merges-squashes-rebases-wtf">Merges, squashes, rebases: wtf?</a></h2>
+<p>Ok, so that's what we'd like to achieve. How do we achieve it?</p>
+<p>The TL;DR is: when you come to merge a pull request, you <em>probably</em> want to
+“squash and merge”:</p>
+<p><img src="img/git/squash.png" alt="squash and merge" />.</p>
+<p>(This applies whether you are merging your own PR, or that of another
+contributor.)</p>
+<p>“Squash and merge”<sup id="a1"><a href="#f1">1</a></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: <code>* woops typo</code> is not a useful
+thing to keep in the historical record).</p>
+<p>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.</p>
+<p>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 href="https://github.com/matrix-org/synapse/pull/6837">a PR which gradually propagates a refactoring operation
+through the codebase</a>, or <a href="https://github.com/matrix-org/synapse/pull/5987">a
+PR which is the culmination of several other
+PRs</a>. In this case the ability
+to figure out when a particular change/bug was introduced could be very useful.</p>
+<p>Ultimately: <strong>this is not a hard-and-fast-rule</strong>. 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.</p>
+<h2 id="git-branching-model"><a class="header" href="#git-branching-model">Git branching model</a></h2>
+<p>A <a href="https://nvie.com/posts/a-successful-git-branching-model/">lot</a>
+<a href="http://scottchacon.com/2011/08/31/github-flow.html">of</a>
+<a href="https://www.endoflineblog.com/gitflow-considered-harmful">words</a> have been
+written in the past about git branching models (no really, <a href="https://martinfowler.com/articles/branching-patterns.html">a
+lot</a>). I tend to
+think the whole thing is overblown. Fundamentally, it's not that
+complicated. Here's how we do it.</p>
+<p>Let's start with a picture:</p>
+<p><img src="img/git/branches.jpg" alt="branching model" /></p>
+<p>It looks complicated, but it's really not. There's one basic rule: <em>anyone</em> is
+free to merge from <em>any</em> more-stable branch to <em>any</em> less-stable branch at
+<em>any</em> time<sup id="a2"><a href="#f2">2</a></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.)</p>
+<p>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.</p>
+<p>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:</p>
+<ul>
+<li><code>master</code> (tracks our last release).</li>
+<li><code>release-vX.Y</code> (the branch where we prepare the next release)<sup
+id="a3"><a href="#f3">3</a></sup>.</li>
+<li>PR branches which are targeting the release.</li>
+<li><code>develop</code> (our &quot;mainline&quot; branch containing our bleeding-edge).</li>
+<li>regular PR branches.</li>
+</ul>
+<p>The corollary is: if you have a bugfix that needs to land in both
+<code>release-vX.Y</code> <em>and</em> <code>develop</code>, then you should base your PR on
+<code>release-vX.Y</code>, get it merged there, and then merge from <code>release-vX.Y</code> to
+<code>develop</code>. (If a fix lands in <code>develop</code> 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.)</p>
+<hr />
+<p><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. <a href="#a1">^</a></p>
+<p><b id="f2">[2]</b>: Well, anyone with commit access.<a href="#a2">^</a></p>
+<p><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,
+<code>release-v1.2</code> is more-stable than <code>release-v1.3</code>. <a href="#a3">^</a></p>
+
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author	erikjohnston <erikjohnston@users.noreply.github.com>	2024-02-06 16:35:44 +0000
committer	erikjohnston <erikjohnston@users.noreply.github.com>	2024-02-06 16:35:44 +0000
commit	d2d9645a32a0eb7b5b97db02ebab27105974e050 (patch)
tree	81655a778d2b6d065667cc6a84ea5c038afd5fce /v1.101/development/git.html
parent	deploy: 93edd0932e636252528ca4b95802f35b1a513b6e (diff)
download	synapse-d2d9645a32a0eb7b5b97db02ebab27105974e050.tar.xz