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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 server</a></li></ol></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 href="../upgrade.html">Upgrading between Synapse Versions</a></li><li class="chapter-item expanded affix "><li class="part-title">Usage</li><li class="chapter-item expanded "><a href="../federate.html">Federation</a></li><li class="chapter-item expanded "><a href="../usage/configuration/index.html">Configuration</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../usage/configuration/config_documentation.html">Configuration Manual</a></li><li class="chapter-item expanded "><a href="../usage/configuration/homeserver_sample_config.html">Homeserver Sample Config File</a></li><li class="chapter-item expanded "><a href="../usage/configuration/logging_sample_config.html">Logging Sample Config File</a></li><li class="chapter-item expanded "><a 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Providers</a></li><li class="chapter-item expanded "><a href="../jwt.html">JSON Web Tokens</a></li><li class="chapter-item expanded "><a href="../usage/configuration/user_authentication/refresh_tokens.html">Refresh Tokens</a></li></ol></li><li class="chapter-item expanded "><a href="../CAPTCHA_SETUP.html">Registration Captcha</a></li><li class="chapter-item expanded "><a href="../application_services.html">Application Services</a></li><li class="chapter-item expanded "><a href="../server_notices.html">Server Notices</a></li><li class="chapter-item expanded "><a href="../consent_tracking.html">Consent Tracking</a></li><li class="chapter-item expanded "><a href="../user_directory.html">User Directory</a></li><li class="chapter-item expanded "><a href="../message_retention_policies.html">Message Retention Policies</a></li><li class="chapter-item expanded "><a href="../modules/index.html">Pluggable Modules</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../modules/writing_a_module.html">Writing a module</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../modules/spam_checker_callbacks.html">Spam checker callbacks</a></li><li class="chapter-item expanded "><a href="../modules/third_party_rules_callbacks.html">Third-party rules callbacks</a></li><li class="chapter-item expanded "><a href="../modules/presence_router_callbacks.html">Presence router callbacks</a></li><li class="chapter-item expanded "><a href="../modules/account_validity_callbacks.html">Account validity callbacks</a></li><li class="chapter-item expanded "><a href="../modules/password_auth_provider_callbacks.html">Password auth provider callbacks</a></li><li class="chapter-item expanded "><a href="../modules/background_update_controller_callbacks.html">Background update controller callbacks</a></li><li class="chapter-item expanded "><a href="../modules/account_data_callbacks.html">Account data callbacks</a></li><li class="chapter-item expanded "><a href="../modules/porting_legacy_module.html">Porting a legacy module to the new 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 href="../admin_api/media_admin_api.html">Media</a></li><li class="chapter-item expanded "><a href="../admin_api/purge_history_api.html">Purge History</a></li><li class="chapter-item expanded "><a href="../admin_api/register_api.html">Register Users</a></li><li class="chapter-item expanded "><a href="../usage/administration/admin_api/registration_tokens.html">Registration Tokens</a></li><li class="chapter-item expanded "><a href="../admin_api/room_membership.html">Manipulate Room Membership</a></li><li class="chapter-item expanded "><a href="../admin_api/rooms.html">Rooms</a></li><li class="chapter-item expanded "><a href="../admin_api/server_notices.html">Server Notices</a></li><li class="chapter-item expanded "><a href="../admin_api/statistics.html">Statistics</a></li><li class="chapter-item expanded "><a href="../admin_api/user_admin_api.html">Users</a></li><li class="chapter-item expanded "><a href="../admin_api/version_api.html">Server Version</a></li><li class="chapter-item expanded "><a href="../usage/administration/admin_api/federation.html">Federation</a></li></ol></li><li class="chapter-item expanded "><a href="../manhole.html">Manhole</a></li><li class="chapter-item expanded "><a href="../metrics-howto.html">Monitoring</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../usage/administration/monitoring/reporting_homeserver_usage_statistics.html">Reporting Homeserver Usage Statistics</a></li></ol></li><li class="chapter-item expanded "><a href="../usage/administration/monthly_active_users.html">Monthly Active Users</a></li><li class="chapter-item expanded "><a href="../usage/administration/understanding_synapse_through_grafana_graphs.html">Understanding Synapse Through Grafana Graphs</a></li><li class="chapter-item expanded "><a href="../usage/administration/useful_sql_for_admins.html">Useful SQL for Admins</a></li><li class="chapter-item expanded "><a href="../usage/administration/database_maintenance_tools.html">Database Maintenance Tools</a></li><li class="chapter-item expanded "><a href="../usage/administration/state_groups.html">State Groups</a></li><li class="chapter-item expanded "><a href="../usage/administration/request_log.html">Request log format</a></li><li class="chapter-item expanded "><a href="../usage/administration/admin_faq.html">Admin FAQ</a></li><li class="chapter-item expanded "><div>Scripts</div></li></ol></li><li class="chapter-item expanded "><li class="part-title">Development</li><li class="chapter-item expanded "><a href="../development/contributing_guide.html">Contributing Guide</a></li><li class="chapter-item expanded "><a href="../code_style.html">Code Style</a></li><li class="chapter-item expanded "><a href="../development/reviews.html">Reviewing Code</a></li><li class="chapter-item expanded "><a href="../development/releases.html">Release Cycle</a></li><li class="chapter-item expanded "><a href="../development/git.html" class="active">Git Usage</a></li><li class="chapter-item expanded "><div>Testing</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../development/demo.html">Demo scripts</a></li></ol></li><li class="chapter-item expanded "><a href="../opentracing.html">OpenTracing</a></li><li class="chapter-item expanded "><a href="../development/database_schema.html">Database Schemas</a></li><li class="chapter-item expanded "><a href="../development/experimental_features.html">Experimental features</a></li><li class="chapter-item expanded "><a href="../development/dependencies.html">Dependency management</a></li><li class="chapter-item expanded "><div>Synapse Architecture</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../development/synapse_architecture/cancellation.html">Cancellation</a></li><li class="chapter-item expanded "><a href="../log_contexts.html">Log Contexts</a></li><li class="chapter-item expanded "><a href="../replication.html">Replication</a></li><li class="chapter-item expanded "><a href="../tcp_replication.html">TCP Replication</a></li><li class="chapter-item expanded "><a href="../development/synapse_architecture/faster_joins.html">Faster remote joins</a></li></ol></li><li class="chapter-item expanded "><a href="../development/internal_documentation/index.html">Internal Documentation</a></li><li><ol class="section"><li class="chapter-item expanded "><div>Single Sign-On</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../development/saml.html">SAML</a></li><li class="chapter-item expanded "><a href="../development/cas.html">CAS</a></li></ol></li><li class="chapter-item expanded "><a href="../development/room-dag-concepts.html">Room DAG concepts</a></li><li class="chapter-item expanded "><div>State Resolution</div></li><li><ol class="section"><li class="chapter-item expanded "><a href="../auth_chain_difference_algorithm.html">The Auth Chain Difference Algorithm</a></li></ol></li><li class="chapter-item expanded "><a href="../media_repository.html">Media Repository</a></li><li class="chapter-item expanded "><a href="../room_and_user_statistics.html">Room and User Statistics</a></li></ol></li><li class="chapter-item expanded "><div>Scripts</div></li><li class="chapter-item expanded affix "><li class="part-title">Other</li><li class="chapter-item expanded "><a href="../deprecation_policy.html">Dependency Deprecation Policy</a></li><li class="chapter-item expanded "><a href="../other/running_synapse_on_single_board_computers.html">Running Synapse on a Single-Board Computer</a></li></ol>
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<h1 class="menu-title">Synapse</h1>
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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>
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<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>
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<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/matrix-org/synapse/commit/84691da6c">“pep8”</a>, <a href="https://github.com/matrix-org/synapse/commit/474810d9d">“fix broken
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test”</a>,
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<a href="https://github.com/matrix-org/synapse/commit/c9d72e457">“oops”</a>,
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<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
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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
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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>
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<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
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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
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namespace</a>).</p>
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<p>Of course, in reality, things are more complicated than that. We have release
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branches as well as <code>develop</code> and <code>master</code>, and we deliberately merge changes
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between them. Bugs often slip through and have to be fixed later. That's all
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fine: this not a cast-iron rule which must be obeyed, but an ideal to aim
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towards.</p>
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<h2 id="merges-squashes-rebases-wtf"><a class="header" href="#merges-squashes-rebases-wtf">Merges, squashes, rebases: wtf?</a></h2>
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<p>Ok, so that's what we'd like to achieve. How do we achieve it?</p>
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<p>The TL;DR is: when you come to merge a pull request, you <em>probably</em> want to
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“squash and merge”:</p>
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<p><img src="img/git/squash.png" alt="squash and merge" />.</p>
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<p>(This applies whether you are merging your own PR, or that of another
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contributor.)</p>
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<p>“Squash and merge”<sup id="a1"><a href="#f1">1</a></sup> takes all of the changes in the
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PR, and bundles them into a single commit. GitHub gives you the opportunity to
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edit the commit message before you confirm, and normally you should do so,
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because the default will be useless (again: <code>* woops typo</code> is not a useful
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thing to keep in the historical record).</p>
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<p>The main problem with this approach comes when you have a series of pull
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requests which build on top of one another: as soon as you squash-merge the
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first PR, you'll end up with a stack of conflicts to resolve in all of the
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others. In general, it's best to avoid this situation in the first place by
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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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