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                        <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="#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>

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