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<h1 id="cancellation"><a class="header" href="#cancellation">Cancellation</a></h1>
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<p>Sometimes, requests take a long time to service and clients disconnect
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before Synapse produces a response. To avoid wasting resources, Synapse
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can cancel request processing for select endpoints marked with the
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<code>@cancellable</code> decorator.</p>
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<p>Synapse makes use of Twisted's <code>Deferred.cancel()</code> feature to make
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cancellation work. The <code>@cancellable</code> decorator does nothing by itself
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and merely acts as a flag, signalling to developers and other code alike
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that a method can be cancelled.</p>
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<h2 id="enabling-cancellation-for-an-endpoint"><a class="header" href="#enabling-cancellation-for-an-endpoint">Enabling cancellation for an endpoint</a></h2>
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<ol>
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<li>Check that the endpoint method, and any <code>async</code> functions in its call
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tree handle cancellation correctly. See
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<a href="#handling-cancellation-correctly">Handling cancellation correctly</a>
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for a list of things to look out for.</li>
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<li>Add the <code>@cancellable</code> decorator to the <code>on_GET/POST/PUT/DELETE</code>
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method. It's not recommended to make non-<code>GET</code> methods cancellable,
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since cancellation midway through some database updates is less
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likely to be handled correctly.</li>
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</ol>
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<h2 id="mechanics"><a class="header" href="#mechanics">Mechanics</a></h2>
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<p>There are two stages to cancellation: downward propagation of a
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<code>cancel()</code> call, followed by upwards propagation of a <code>CancelledError</code>
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out of a blocked <code>await</code>.
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Both Twisted and asyncio have a cancellation mechanism.</p>
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<table><thead><tr><th></th><th>Method</th><th>Exception</th><th>Exception inherits from</th></tr></thead><tbody>
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<tr><td>Twisted</td><td><code>Deferred.cancel()</code></td><td><code>twisted.internet.defer.CancelledError</code></td><td><code>Exception</code> (!)</td></tr>
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<tr><td>asyncio</td><td><code>Task.cancel()</code></td><td><code>asyncio.CancelledError</code></td><td><code>BaseException</code></td></tr>
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</tbody></table>
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<h3 id="deferredcancel"><a class="header" href="#deferredcancel">Deferred.cancel()</a></h3>
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<p>When Synapse starts handling a request, it runs the async method
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responsible for handling it using <code>defer.ensureDeferred</code>, which returns
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a <code>Deferred</code>. For example:</p>
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<pre><code class="language-python">def do_something() -> Deferred[None]:
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...
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@cancellable
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async def on_GET() -> Tuple[int, JsonDict]:
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d = make_deferred_yieldable(do_something())
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await d
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return 200, {}
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request = defer.ensureDeferred(on_GET())
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</code></pre>
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<p>When a client disconnects early, Synapse checks for the presence of the
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<code>@cancellable</code> decorator on <code>on_GET</code>. Since <code>on_GET</code> is cancellable,
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<code>Deferred.cancel()</code> is called on the <code>Deferred</code> from
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<code>defer.ensureDeferred</code>, ie. <code>request</code>. Twisted knows which <code>Deferred</code>
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<code>request</code> is waiting on and passes the <code>cancel()</code> call on to <code>d</code>.</p>
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<p>The <code>Deferred</code> being waited on, <code>d</code>, may have its own handling for
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<code>cancel()</code> and pass the call on to other <code>Deferred</code>s.</p>
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<p>Eventually, a <code>Deferred</code> handles the <code>cancel()</code> call by resolving itself
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with a <code>CancelledError</code>.</p>
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<h3 id="cancellederror"><a class="header" href="#cancellederror">CancelledError</a></h3>
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<p>The <code>CancelledError</code> gets raised out of the <code>await</code> and bubbles up, as
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per normal Python exception handling.</p>
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<h2 id="handling-cancellation-correctly"><a class="header" href="#handling-cancellation-correctly">Handling cancellation correctly</a></h2>
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<p>In general, when writing code that might be subject to cancellation, two
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things must be considered:</p>
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<ul>
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<li>The effect of <code>CancelledError</code>s raised out of <code>await</code>s.</li>
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<li>The effect of <code>Deferred</code>s being <code>cancel()</code>ed.</li>
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</ul>
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<p>Examples of code that handles cancellation incorrectly include:</p>
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<ul>
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<li><code>try-except</code> blocks which swallow <code>CancelledError</code>s.</li>
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<li>Code that shares the same <code>Deferred</code>, which may be cancelled, between
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multiple requests.</li>
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<li>Code that starts some processing that's exempt from cancellation, but
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uses a logging context from cancellable code. The logging context
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will be finished upon cancellation, while the uncancelled processing
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is still using it.</li>
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</ul>
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<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() -> 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() -> 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() -> 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) -> 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("!aAAaaAaaaAAAaAaAA:matrix.org")
|
|
# Request 2
|
|
await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
|
|
</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) -> 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("!aAAaaAaaaAAAaAaAA:matrix.org")
|
|
# Request 2
|
|
await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
|
|
</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) -> 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("!aAAaaAaaaAAAaAaAA:matrix.org")
|
|
# Request 2
|
|
await wait_for_room("!aAAaaAaaaAAAaAaAA:matrix.org")
|
|
</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]
|
|
) -> None:
|
|
await ...
|
|
logger.info("done!")
|
|
to_resolve.callback(None)
|
|
|
|
async def do_something() -> 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("request-1"):
|
|
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]
|
|
) -> None:
|
|
await ...
|
|
logger.info("done!")
|
|
to_resolve.callback(None)
|
|
|
|
async def do_something() -> 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("request-1"):
|
|
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]
|
|
) -> None:
|
|
await ...
|
|
logger.info("done!")
|
|
to_resolve.callback(None)
|
|
|
|
async def do_something() -> 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(
|
|
"do_something_else",
|
|
do_something_else,
|
|
to_resolve,
|
|
)
|
|
|
|
await make_deferred_yieldable(cache.observe())
|
|
|
|
with LoggingContext("request-1"):
|
|
await do_something()
|
|
</code></pre>
|
|
</td>
|
|
<td width="50%">
|
|
</td>
|
|
</tr>
|
|
</table>
|
|
|
|
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