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  74. <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 class="chapter-item expanded "><a href="../delegate.html">Delegation</a></li><li class="chapter-item expanded affix "><li class="part-title">Upgrading</li><li class="chapter-item expanded "><a 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interface</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../workers.html">Workers</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../synctl_workers.html">Using synctl with Workers</a></li><li class="chapter-item expanded "><a href="../systemd-with-workers/index.html">Systemd</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../usage/administration/index.html">Administration</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../usage/administration/admin_api/index.html">Admin API</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../admin_api/account_validity.html">Account Validity</a></li><li class="chapter-item expanded "><a href="../usage/administration/admin_api/background_updates.html">Background Updates</a></li><li class="chapter-item expanded "><a href="../admin_api/event_reports.html">Event Reports</a></li><li class="chapter-item expanded "><a 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  149. <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>
  150. <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>
  151. <p>In an ideal world, our git commit history would be a linear progression of
  152. commits each of which contains a single change building on what came
  153. before. Here, by way of an arbitrary example, is the top of <code>git log --graph b2dba0607</code>:</p>
  154. <img src="img/git/clean.png" alt="clean git graph" width="500px">
  155. <p>Note how the commit comment explains clearly what is changing and why. Also
  156. note the <em>absence</em> of merge commits, as well as the absence of commits called
  157. things like (to pick a few culprits):
  158. <a href="https://github.com/matrix-org/synapse/commit/84691da6c">“pep8”</a>, <a href="https://github.com/matrix-org/synapse/commit/474810d9d">“fix broken
  159. test”</a>,
  160. <a href="https://github.com/matrix-org/synapse/commit/c9d72e457">“oops”</a>,
  161. <a href="https://github.com/matrix-org/synapse/commit/836358823">“typo”</a>, or <a href="https://github.com/matrix-org/synapse/commit/707374d5d">“Who's
  162. the president?”</a>.</p>
  163. <p>There are a number of reasons why keeping a clean commit history is a good
  164. thing:</p>
  165. <ul>
  166. <li>
  167. <p>From time to time, after a change lands, it turns out to be necessary to
  168. revert it, or to backport it to a release branch. Those operations are
  169. <em>much</em> easier when the change is contained in a single commit.</p>
  170. </li>
  171. <li>
  172. <p>Similarly, it's much easier to answer questions like “is the fix for
  173. <code>/publicRooms</code> on the release branch?” if that change consists of a single
  174. commit.</p>
  175. </li>
  176. <li>
  177. <p>Likewise: “what has changed on this branch in the last week?” is much
  178. clearer without merges and “pep8” commits everywhere.</p>
  179. </li>
  180. <li>
  181. <p>Sometimes we need to figure out where a bug got introduced, or some
  182. behaviour changed. One way of doing that is with <code>git bisect</code>: pick an
  183. arbitrary commit between the known good point and the known bad point, and
  184. see how the code behaves. However, that strategy fails if the commit you
  185. chose is the middle of someone's epic branch in which they broke the world
  186. before putting it back together again.</p>
  187. </li>
  188. </ul>
  189. <p>One counterargument is that it is sometimes useful to see how a PR evolved as
  190. it went through review cycles. This is true, but that information is always
  191. 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
  192. namespace</a>).</p>
  193. <p>Of course, in reality, things are more complicated than that. We have release
  194. branches as well as <code>develop</code> and <code>master</code>, and we deliberately merge changes
  195. between them. Bugs often slip through and have to be fixed later. That's all
  196. fine: this not a cast-iron rule which must be obeyed, but an ideal to aim
  197. towards.</p>
  198. <h2 id="merges-squashes-rebases-wtf"><a class="header" href="#merges-squashes-rebases-wtf">Merges, squashes, rebases: wtf?</a></h2>
  199. <p>Ok, so that's what we'd like to achieve. How do we achieve it?</p>
  200. <p>The TL;DR is: when you come to merge a pull request, you <em>probably</em> want to
  201. “squash and merge”:</p>
  202. <p><img src="img/git/squash.png" alt="squash and merge" />.</p>
  203. <p>(This applies whether you are merging your own PR, or that of another
  204. contributor.)</p>
  205. <p>“Squash and merge”<sup id="a1"><a href="#f1">1</a></sup> takes all of the changes in the
  206. PR, and bundles them into a single commit. GitHub gives you the opportunity to
  207. edit the commit message before you confirm, and normally you should do so,
  208. because the default will be useless (again: <code>* woops typo</code> is not a useful
  209. thing to keep in the historical record).</p>
  210. <p>The main problem with this approach comes when you have a series of pull
  211. requests which build on top of one another: as soon as you squash-merge the
  212. first PR, you'll end up with a stack of conflicts to resolve in all of the
  213. others. In general, it's best to avoid this situation in the first place by
  214. trying not to have multiple related PRs in flight at the same time. Still,
  215. sometimes that's not possible and doing a regular merge is the lesser evil.</p>
  216. <p>Another occasion in which a regular merge makes more sense is a PR where you've
  217. deliberately created a series of commits each of which makes sense in its own
  218. right. For example: <a href="https://github.com/matrix-org/synapse/pull/6837">a PR which gradually propagates a refactoring operation
  219. through the codebase</a>, or <a href="https://github.com/matrix-org/synapse/pull/5987">a
  220. PR which is the culmination of several other
  221. PRs</a>. In this case the ability
  222. to figure out when a particular change/bug was introduced could be very useful.</p>
  223. <p>Ultimately: <strong>this is not a hard-and-fast-rule</strong>. If in doubt, ask yourself “do
  224. each of the commits I am about to merge make sense in their own right”, but
  225. remember that we're just doing our best to balance “keeping the commit history
  226. clean” with other factors.</p>
  227. <h2 id="git-branching-model"><a class="header" href="#git-branching-model">Git branching model</a></h2>
  228. <p>A <a href="https://nvie.com/posts/a-successful-git-branching-model/">lot</a>
  229. <a href="http://scottchacon.com/2011/08/31/github-flow.html">of</a>
  230. <a href="https://www.endoflineblog.com/gitflow-considered-harmful">words</a> have been
  231. written in the past about git branching models (no really, <a href="https://martinfowler.com/articles/branching-patterns.html">a
  232. lot</a>). I tend to
  233. think the whole thing is overblown. Fundamentally, it's not that
  234. complicated. Here's how we do it.</p>
  235. <p>Let's start with a picture:</p>
  236. <p><img src="img/git/branches.jpg" alt="branching model" /></p>
  237. <p>It looks complicated, but it's really not. There's one basic rule: <em>anyone</em> is
  238. free to merge from <em>any</em> more-stable branch to <em>any</em> less-stable branch at
  239. <em>any</em> time<sup id="a2"><a href="#f2">2</a></sup>. (The principle behind this is that if a
  240. change is good enough for the more-stable branch, then it's also good enough go
  241. put in a less-stable branch.)</p>
  242. <p>Meanwhile, merging (or squashing, as per the above) from a less-stable to a
  243. more-stable branch is a deliberate action in which you want to publish a change
  244. or a set of changes to (some subset of) the world: for example, this happens
  245. when a PR is landed, or as part of our release process.</p>
  246. <p>So, what counts as a more- or less-stable branch? A little reflection will show
  247. that our active branches are ordered thus, from more-stable to less-stable:</p>
  248. <ul>
  249. <li><code>master</code> (tracks our last release).</li>
  250. <li><code>release-vX.Y</code> (the branch where we prepare the next release)<sup
  251. id="a3"><a href="#f3">3</a></sup>.</li>
  252. <li>PR branches which are targeting the release.</li>
  253. <li><code>develop</code> (our &quot;mainline&quot; branch containing our bleeding-edge).</li>
  254. <li>regular PR branches.</li>
  255. </ul>
  256. <p>The corollary is: if you have a bugfix that needs to land in both
  257. <code>release-vX.Y</code> <em>and</em> <code>develop</code>, then you should base your PR on
  258. <code>release-vX.Y</code>, get it merged there, and then merge from <code>release-vX.Y</code> to
  259. <code>develop</code>. (If a fix lands in <code>develop</code> and we later need it in a
  260. release-branch, we can of course cherry-pick it, but landing it in the release
  261. branch first helps reduce the chance of annoying conflicts.)</p>
  262. <hr />
  263. <p><b id="f1">[1]</b>: “Squash and merge” is GitHub's term for this
  264. operation. Given that there is no merge involved, I'm not convinced it's the
  265. most intuitive name. <a href="#a1">^</a></p>
  266. <p><b id="f2">[2]</b>: Well, anyone with commit access.<a href="#a2">^</a></p>
  267. <p><b id="f3">[3]</b>: Very, very occasionally (I think this has happened once in
  268. the history of Synapse), we've had two releases in flight at once. Obviously,
  269. <code>release-v1.2</code> is more-stable than <code>release-v1.3</code>. <a href="#a3">^</a></p>
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