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@@ -77,7 +77,7 @@
 
         <nav id="sidebar" class="sidebar" aria-label="Table of contents">
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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 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 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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 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 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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>
+                <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 href="upgrade.html">Upgrading 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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 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 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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></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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@@ -17331,6 +17331,353 @@ but one should be used if unsure.</p>
 <p>New experimental configuration flags should be added under the <code>experimental</code>
 configuration key (see the <code>synapse.config.experimental</code> file) and either explain
 (briefly) what is being enabled, or include the MSC number.</p>
+<div style="break-before: page; page-break-before: always;"></div><h1 id="cancellation"><a class="header" href="#cancellation">Cancellation</a></h1>
+<p>Sometimes, requests take a long time to service and clients disconnect
+before Synapse produces a response. To avoid wasting resources, Synapse
+can cancel request processing for select endpoints marked with the
+<code>@cancellable</code> decorator.</p>
+<p>Synapse makes use of Twisted's <code>Deferred.cancel()</code> feature to make
+cancellation work. The <code>@cancellable</code> decorator does nothing by itself
+and merely acts as a flag, signalling to developers and other code alike
+that a method can be cancelled.</p>
+<h2 id="enabling-cancellation-for-an-endpoint"><a class="header" href="#enabling-cancellation-for-an-endpoint">Enabling cancellation for an endpoint</a></h2>
+<ol>
+<li>Check that the endpoint method, and any <code>async</code> functions in its call
+tree handle cancellation correctly. See
+<a href="development/synapse_architecture/cancellation.html#handling-cancellation-correctly">Handling cancellation correctly</a>
+for a list of things to look out for.</li>
+<li>Add the <code>@cancellable</code> decorator to the <code>on_GET/POST/PUT/DELETE</code>
+method. It's not recommended to make non-<code>GET</code> methods cancellable,
+since cancellation midway through some database updates is less
+likely to be handled correctly.</li>
+</ol>
+<h2 id="mechanics"><a class="header" href="#mechanics">Mechanics</a></h2>
+<p>There are two stages to cancellation: downward propagation of a
+<code>cancel()</code> call, followed by upwards propagation of a <code>CancelledError</code>
+out of a blocked <code>await</code>.
+Both Twisted and asyncio have a cancellation mechanism.</p>
+<table><thead><tr><th></th><th>Method</th><th>Exception</th><th>Exception inherits from</th></tr></thead><tbody>
+<tr><td>Twisted</td><td><code>Deferred.cancel()</code></td><td><code>twisted.internet.defer.CancelledError</code></td><td><code>Exception</code> (!)</td></tr>
+<tr><td>asyncio</td><td><code>Task.cancel()</code></td><td><code>asyncio.CancelledError</code></td><td><code>BaseException</code></td></tr>
+</tbody></table>
+<h3 id="deferredcancel"><a class="header" href="#deferredcancel">Deferred.cancel()</a></h3>
+<p>When Synapse starts handling a request, it runs the async method
+responsible for handling it using <code>defer.ensureDeferred</code>, which returns
+a <code>Deferred</code>. For example:</p>
+<pre><code class="language-python">def do_something() -&gt; Deferred[None]:
+    ...
+
+@cancellable
+async def on_GET() -&gt; Tuple[int, JsonDict]:
+    d = make_deferred_yieldable(do_something())
+    await d
+    return 200, {}
+
+request = defer.ensureDeferred(on_GET())
+</code></pre>
+<p>When a client disconnects early, Synapse checks for the presence of the
+<code>@cancellable</code> decorator on <code>on_GET</code>. Since <code>on_GET</code> is cancellable,
+<code>Deferred.cancel()</code> is called on the <code>Deferred</code> from
+<code>defer.ensureDeferred</code>, ie. <code>request</code>. Twisted knows which <code>Deferred</code>
+<code>request</code> is waiting on and passes the <code>cancel()</code> call on to <code>d</code>.</p>
+<p>The <code>Deferred</code> being waited on, <code>d</code>, may have its own handling for
+<code>cancel()</code> and pass the call on to other <code>Deferred</code>s.</p>
+<p>Eventually, a <code>Deferred</code> handles the <code>cancel()</code> call by resolving itself
+with a <code>CancelledError</code>.</p>
+<h3 id="cancellederror"><a class="header" href="#cancellederror">CancelledError</a></h3>
+<p>The <code>CancelledError</code> gets raised out of the <code>await</code> and bubbles up, as
+per normal Python exception handling.</p>
+<h2 id="handling-cancellation-correctly"><a class="header" href="#handling-cancellation-correctly">Handling cancellation correctly</a></h2>
+<p>In general, when writing code that might be subject to cancellation, two
+things must be considered:</p>
+<ul>
+<li>The effect of <code>CancelledError</code>s raised out of <code>await</code>s.</li>
+<li>The effect of <code>Deferred</code>s being <code>cancel()</code>ed.</li>
+</ul>
+<p>Examples of code that handles cancellation incorrectly include:</p>
+<ul>
+<li><code>try-except</code> blocks which swallow <code>CancelledError</code>s.</li>
+<li>Code that shares the same <code>Deferred</code>, which may be cancelled, between
+multiple requests.</li>
+<li>Code that starts some processing that's exempt from cancellation, but
+uses a logging context from cancellable code. The logging context
+will be finished upon cancellation, while the uncancelled processing
+is still using it.</li>
+</ul>
+<p>Some common patterns are listed below in more detail.</p>
+<h3 id="async-function-calls"><a class="header" href="#async-function-calls"><code>async</code> function calls</a></h3>
+<p>Most functions in Synapse are relatively straightforward from a
+cancellation standpoint: they don't do anything with <code>Deferred</code>s and
+purely call and <code>await</code> other <code>async</code> functions.</p>
+<p>An <code>async</code> function handles cancellation correctly if its own code
+handles cancellation correctly and all the async function it calls
+handle cancellation correctly. For example:</p>
+<pre><code class="language-python">async def do_two_things() -&gt; None:
+    check_something()
+    await do_something()
+    await do_something_else()
+</code></pre>
+<p><code>do_two_things</code> handles cancellation correctly if <code>do_something</code> and
+<code>do_something_else</code> handle cancellation correctly.</p>
+<p>That is, when checking whether a function handles cancellation
+correctly, its implementation and all its <code>async</code> function calls need to
+be checked, recursively.</p>
+<p>As <code>check_something</code> is not <code>async</code>, it does not need to be checked.</p>
+<h3 id="cancellederrors"><a class="header" href="#cancellederrors">CancelledErrors</a></h3>
+<p>Because Twisted's <code>CancelledError</code>s are <code>Exception</code>s, it's easy to
+accidentally catch and suppress them. Care must be taken to ensure that
+<code>CancelledError</code>s are allowed to propagate upwards.</p>
+<table width="100%">
+<tr>
+<td width="50%" valign="top">
+<p><strong>Bad</strong>:</p>
+<pre><code class="language-python">try:
+    await do_something()
+except Exception:
+    # `CancelledError` gets swallowed here.
+    logger.info(...)
+</code></pre>
+</td>
+<td width="50%" valign="top">
+<p><strong>Good</strong>:</p>
+<pre><code class="language-python">try:
+    await do_something()
+except CancelledError:
+    raise
+except Exception:
+    logger.info(...)
+</code></pre>
+</td>
+</tr>
+<tr>
+<td width="50%" valign="top">
+<p><strong>OK</strong>:</p>
+<pre><code class="language-python">try:
+    check_something()
+    # A `CancelledError` won't ever be raised here.
+except Exception:
+    logger.info(...)
+</code></pre>
+</td>
+<td width="50%" valign="top">
+<p><strong>Good</strong>:</p>
+<pre><code class="language-python">try:
+    await do_something()
+except ValueError:
+    logger.info(...)
+</code></pre>
+</td>
+</tr>
+</table>
+<h4 id="defergatherresults"><a class="header" href="#defergatherresults">defer.gatherResults</a></h4>
+<p><code>defer.gatherResults</code> produces a <code>Deferred</code> which:</p>
+<ul>
+<li>broadcasts <code>cancel()</code> calls to every <code>Deferred</code> being waited on.</li>
+<li>wraps the first exception it sees in a <code>FirstError</code>.</li>
+</ul>
+<p>Together, this means that <code>CancelledError</code>s will be wrapped in
+a <code>FirstError</code> unless unwrapped. Such <code>FirstError</code>s are liable to be
+swallowed, so they must be unwrapped.</p>
+<table width="100%">
+<tr>
+<td width="50%" valign="top">
+<p><strong>Bad</strong>:</p>
+<pre><code class="language-python">async def do_something() -&gt; None:
+    await make_deferred_yieldable(
+        defer.gatherResults([...], consumeErrors=True)
+    )
+
+try:
+    await do_something()
+except CancelledError:
+    raise
+except Exception:
+    # `FirstError(CancelledError)` gets swallowed here.
+    logger.info(...)
+</code></pre>
+</td>
+<td width="50%" valign="top">
+<p><strong>Good</strong>:</p>
+<pre><code class="language-python">async def do_something() -&gt; None:
+    await make_deferred_yieldable(
+        defer.gatherResults([...], consumeErrors=True)
+    ).addErrback(unwrapFirstError)
+
+try:
+    await do_something()
+except CancelledError:
+    raise
+except Exception:
+    logger.info(...)
+</code></pre>
+</td>
+</tr>
+</table>
+<h3 id="creation-of-deferreds"><a class="header" href="#creation-of-deferreds">Creation of <code>Deferred</code>s</a></h3>
+<p>If a function creates a <code>Deferred</code>, the effect of cancelling it must be considered. <code>Deferred</code>s that get shared are likely to have unintended behaviour when cancelled.</p>
+<table width="100%">
+<tr>
+<td width="50%" valign="top">
+<p><strong>Bad</strong>:</p>
+<pre><code class="language-python">cache: Dict[str, Deferred[None]] = {}
+
+def wait_for_room(room_id: str) -&gt; Deferred[None]:
+    deferred = cache.get(room_id)
+    if deferred is None:
+        deferred = Deferred()
+        cache[room_id] = deferred
+    # `deferred` can have multiple waiters.
+    # All of them will observe a `CancelledError`
+    # if any one of them is cancelled.
+    return make_deferred_yieldable(deferred)
+
+# Request 1
+await wait_for_room(&quot;!aAAaaAaaaAAAaAaAA:matrix.org&quot;)
+# Request 2
+await wait_for_room(&quot;!aAAaaAaaaAAAaAaAA:matrix.org&quot;)
+</code></pre>
+</td>
+<td width="50%" valign="top">
+<p><strong>Good</strong>:</p>
+<pre><code class="language-python">cache: Dict[str, Deferred[None]] = {}
+
+def wait_for_room(room_id: str) -&gt; Deferred[None]:
+    deferred = cache.get(room_id)
+    if deferred is None:
+        deferred = Deferred()
+        cache[room_id] = deferred
+    # `deferred` will never be cancelled now.
+    # A `CancelledError` will still come out of
+    # the `await`.
+    # `delay_cancellation` may also be used.
+    return make_deferred_yieldable(stop_cancellation(deferred))
+
+# Request 1
+await wait_for_room(&quot;!aAAaaAaaaAAAaAaAA:matrix.org&quot;)
+# Request 2
+await wait_for_room(&quot;!aAAaaAaaaAAAaAaAA:matrix.org&quot;)
+</code></pre>
+</td>
+</tr>
+<tr>
+<td width="50%" valign="top">
+</td>
+<td width="50%" valign="top">
+<p><strong>Good</strong>:</p>
+<pre><code class="language-python">cache: Dict[str, List[Deferred[None]]] = {}
+
+def wait_for_room(room_id: str) -&gt; Deferred[None]:
+    if room_id not in cache:
+        cache[room_id] = []
+    # Each request gets its own `Deferred` to wait on.
+    deferred = Deferred()
+    cache[room_id]].append(deferred)
+    return make_deferred_yieldable(deferred)
+
+# Request 1
+await wait_for_room(&quot;!aAAaaAaaaAAAaAaAA:matrix.org&quot;)
+# Request 2
+await wait_for_room(&quot;!aAAaaAaaaAAAaAaAA:matrix.org&quot;)
+</code></pre>
+</td>
+</table>
+<h3 id="uncancelled-processing"><a class="header" href="#uncancelled-processing">Uncancelled processing</a></h3>
+<p>Some <code>async</code> functions may kick off some <code>async</code> processing which is
+intentionally protected from cancellation, by <code>stop_cancellation</code> or
+other means. If the <code>async</code> processing inherits the logcontext of the
+request which initiated it, care must be taken to ensure that the
+logcontext is not finished before the <code>async</code> processing completes.</p>
+<table width="100%">
+<tr>
+<td width="50%" valign="top">
+<p><strong>Bad</strong>:</p>
+<pre><code class="language-python">cache: Optional[ObservableDeferred[None]] = None
+
+async def do_something_else(
+    to_resolve: Deferred[None]
+) -&gt; None:
+    await ...
+    logger.info(&quot;done!&quot;)
+    to_resolve.callback(None)
+
+async def do_something() -&gt; None:
+    if not cache:
+        to_resolve = Deferred()
+        cache = ObservableDeferred(to_resolve)
+        # `do_something_else` will never be cancelled and
+        # can outlive the `request-1` logging context.
+        run_in_background(do_something_else, to_resolve)
+
+    await make_deferred_yieldable(cache.observe())
+
+with LoggingContext(&quot;request-1&quot;):
+    await do_something()
+</code></pre>
+</td>
+<td width="50%" valign="top">
+<p><strong>Good</strong>:</p>
+<pre><code class="language-python">cache: Optional[ObservableDeferred[None]] = None
+
+async def do_something_else(
+    to_resolve: Deferred[None]
+) -&gt; None:
+    await ...
+    logger.info(&quot;done!&quot;)
+    to_resolve.callback(None)
+
+async def do_something() -&gt; None:
+    if not cache:
+        to_resolve = Deferred()
+        cache = ObservableDeferred(to_resolve)
+        run_in_background(do_something_else, to_resolve)
+        # We'll wait until `do_something_else` is
+        # done before raising a `CancelledError`.
+        await make_deferred_yieldable(
+            delay_cancellation(cache.observe())
+        )
+    else:
+        await make_deferred_yieldable(cache.observe())
+
+with LoggingContext(&quot;request-1&quot;):
+    await do_something()
+</code></pre>
+</td>
+</tr>
+<tr>
+<td width="50%">
+<p><strong>OK</strong>:</p>
+<pre><code class="language-python">cache: Optional[ObservableDeferred[None]] = None
+
+async def do_something_else(
+    to_resolve: Deferred[None]
+) -&gt; None:
+    await ...
+    logger.info(&quot;done!&quot;)
+    to_resolve.callback(None)
+
+async def do_something() -&gt; None:
+    if not cache:
+        to_resolve = Deferred()
+        cache = ObservableDeferred(to_resolve)
+        # `do_something_else` will get its own independent
+        # logging context. `request-1` will not count any
+        # metrics from `do_something_else`.
+        run_as_background_process(
+            &quot;do_something_else&quot;,
+            do_something_else,
+            to_resolve,
+        )
+
+    await make_deferred_yieldable(cache.observe())
+
+with LoggingContext(&quot;request-1&quot;):
+    await do_something()
+</code></pre>
+</td>
+<td width="50%">
+</td>
+</tr>
+</table>
 <div style="break-before: page; page-break-before: always;"></div><h1 id="log-contexts"><a class="header" href="#log-contexts">Log Contexts</a></h1>
 <p>To help track the processing of individual requests, synapse uses a
 '<code>log context</code>' to track which request it is handling at any given