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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() -> Deferred[None]:
...
@cancellable
async def on_GET() -> 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() -> 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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