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380 lines
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<h1 class="menu-title">Synapse</h1>
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<span class=fa-svg id="print-button"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M128 0C92.7 0 64 28.7 64 64v96h64V64H354.7L384 93.3V160h64V93.3c0-17-6.7-33.3-18.7-45.3L400 18.7C388 6.7 371.7 0 354.7 0H128zM384 352v32 64H128V384 368 352H384zm64 32h32c17.7 0 32-14.3 32-32V256c0-35.3-28.7-64-64-64H64c-35.3 0-64 28.7-64 64v96c0 17.7 14.3 32 32 32H64v64c0 35.3 28.7 64 64 64H384c35.3 0 64-28.7 64-64V384zm-16-88c-13.3 0-24-10.7-24-24s10.7-24 24-24s24 10.7 24 24s-10.7 24-24 24z"/></svg></span>
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<a href="https://github.com/element-hq/synapse" title="Git repository" aria-label="Git repository">
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<span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 496 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M165.9 397.4c0 2-2.3 3.6-5.2 3.6-3.3.3-5.6-1.3-5.6-3.6 0-2 2.3-3.6 5.2-3.6 3-.3 5.6 1.3 5.6 3.6zm-31.1-4.5c-.7 2 1.3 4.3 4.3 4.9 2.6 1 5.6 0 6.2-2s-1.3-4.3-4.3-5.2c-2.6-.7-5.5.3-6.2 2.3zm44.2-1.7c-2.9.7-4.9 2.6-4.6 4.9.3 2 2.9 3.3 5.9 2.6 2.9-.7 4.9-2.6 4.6-4.6-.3-1.9-3-3.2-5.9-2.9zM244.8 8C106.1 8 0 113.3 0 252c0 110.9 69.8 205.8 169.5 239.2 12.8 2.3 17.3-5.6 17.3-12.1 0-6.2-.3-40.4-.3-61.4 0 0-70 15-84.7-29.8 0 0-11.4-29.1-27.8-36.6 0 0-22.9-15.7 1.6-15.4 0 0 24.9 2 38.6 25.8 21.9 38.6 58.6 27.5 72.9 20.9 2.3-16 8.8-27.1 16-33.7-55.9-6.2-112.3-14.3-112.3-110.5 0-27.5 7.6-41.3 23.6-58.9-2.6-6.5-11.1-33.3 2.6-67.9 20.9-6.5 69 27 69 27 20-5.6 41.5-8.5 62.8-8.5s42.8 2.9 62.8 8.5c0 0 48.1-33.6 69-27 13.7 34.7 5.2 61.4 2.6 67.9 16 17.7 25.8 31.5 25.8 58.9 0 96.5-58.9 104.2-114.8 110.5 9.2 7.9 17 22.9 17 46.4 0 33.7-.3 75.4-.3 83.6 0 6.5 4.6 14.4 17.3 12.1C428.2 457.8 496 362.9 496 252 496 113.3 383.5 8 244.8 8zM97.2 352.9c-1.3 1-1 3.3.7 5.2 1.6 1.6 3.9 2.3 5.2 1 1.3-1 1-3.3-.7-5.2-1.6-1.6-3.9-2.3-5.2-1zm-10.8-8.1c-.7 1.3.3 2.9 2.3 3.9 1.6 1 3.6.7 4.3-.7.7-1.3-.3-2.9-2.3-3.9-2-.6-3.6-.3-4.3.7zm32.4 35.6c-1.6 1.3-1 4.3 1.3 6.2 2.3 2.3 5.2 2.6 6.5 1 1.3-1.3.7-4.3-1.3-6.2-2.2-2.3-5.2-2.6-6.5-1zm-11.4-14.7c-1.6 1-1.6 3.6 0 5.9 1.6 2.3 4.3 3.3 5.6 2.3 1.6-1.3 1.6-3.9 0-6.2-1.4-2.3-4-3.3-5.6-2z"/></svg></span>
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<main>
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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>
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||
<p>In an ideal world, our git commit history would be a linear progression of
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commits each of which contains a single change building on what came
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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">
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||
<p>Note how the commit comment explains clearly what is changing and why. Also
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||
note the <em>absence</em> of merge commits, as well as the absence of commits called
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||
things like (to pick a few culprits):
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<a href="https://github.com/element-hq/synapse/commit/84691da6c">“pep8”</a>, <a href="https://github.com/element-hq/synapse/commit/474810d9d">“fix broken
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||
test”</a>,
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<a href="https://github.com/element-hq/synapse/commit/c9d72e457">“oops”</a>,
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<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
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the president?”</a>.</p>
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<p>There are a number of reasons why keeping a clean commit history is a good
|
||
thing:</p>
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||
<ul>
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||
<li>
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<p>From time to time, after a change lands, it turns out to be necessary to
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revert it, or to backport it to a release branch. Those operations are
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||
<em>much</em> easier when the change is contained in a single commit.</p>
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</li>
|
||
<li>
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<p>Similarly, it’s much easier to answer questions like “is the fix for
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<code>/publicRooms</code> on the release branch?” if that change consists of a single
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commit.</p>
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</li>
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<li>
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<p>Likewise: “what has changed on this branch in the last week?” is much
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clearer without merges and “pep8” commits everywhere.</p>
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</li>
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<li>
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<p>Sometimes we need to figure out where a bug got introduced, or some
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behaviour changed. One way of doing that is with <code>git bisect</code>: pick an
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arbitrary commit between the known good point and the known bad point, and
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||
see how the code behaves. However, that strategy fails if the commit you
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chose is the middle of someone’s epic branch in which they broke the world
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before putting it back together again.</p>
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||
</li>
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</ul>
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<p>One counterargument is that it is sometimes useful to see how a PR evolved as
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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 “mainline” 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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