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cancellation.md

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Cancellation

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 @cancellable decorator.

Synapse makes use of Twisted's Deferred.cancel() feature to make cancellation work. The @cancellable decorator does nothing by itself and merely acts as a flag, signalling to developers and other code alike that a method can be cancelled.

Enabling cancellation for an endpoint

  1. Check that the endpoint method, and any async functions in its call tree handle cancellation correctly. See Handling cancellation correctly for a list of things to look out for.
  2. Add the @cancellable decorator to the on_GET/POST/PUT/DELETE method. It's not recommended to make non-GET methods cancellable, since cancellation midway through some database updates is less likely to be handled correctly.

Mechanics

There are two stages to cancellation: downward propagation of a cancel() call, followed by upwards propagation of a CancelledError out of a blocked await. Both Twisted and asyncio have a cancellation mechanism.

Method Exception Exception inherits from
Twisted Deferred.cancel() twisted.internet.defer.CancelledError Exception (!)
asyncio Task.cancel() asyncio.CancelledError BaseException

Deferred.cancel()

When Synapse starts handling a request, it runs the async method responsible for handling it using defer.ensureDeferred, which returns a Deferred. For example:

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())

When a client disconnects early, Synapse checks for the presence of the @cancellable decorator on on_GET. Since on_GET is cancellable, Deferred.cancel() is called on the Deferred from defer.ensureDeferred, ie. request. Twisted knows which Deferred request is waiting on and passes the cancel() call on to d.

The Deferred being waited on, d, may have its own handling for cancel() and pass the call on to other Deferreds.

Eventually, a Deferred handles the cancel() call by resolving itself with a CancelledError.

CancelledError

The CancelledError gets raised out of the await and bubbles up, as per normal Python exception handling.

Handling cancellation correctly

In general, when writing code that might be subject to cancellation, two things must be considered:

  • The effect of CancelledErrors raised out of awaits.
  • The effect of Deferreds being cancel()ed.

Examples of code that handles cancellation incorrectly include:

  • try-except blocks which swallow CancelledErrors.
  • Code that shares the same Deferred, which may be cancelled, between multiple requests.
  • 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.

Some common patterns are listed below in more detail.

async function calls

Most functions in Synapse are relatively straightforward from a cancellation standpoint: they don't do anything with Deferreds and purely call and await other async functions.

An async function handles cancellation correctly if its own code handles cancellation correctly and all the async function it calls handle cancellation correctly. For example:

async def do_two_things() -> None:
    check_something()
    await do_something()
    await do_something_else()

do_two_things handles cancellation correctly if do_something and do_something_else handle cancellation correctly.

That is, when checking whether a function handles cancellation correctly, its implementation and all its async function calls need to be checked, recursively.

As check_something is not async, it does not need to be checked.

CancelledErrors

Because Twisted's CancelledErrors are Exceptions, it's easy to accidentally catch and suppress them. Care must be taken to ensure that CancelledErrors are allowed to propagate upwards.

Bad:

try:
    await do_something()
except Exception:
    # `CancelledError` gets swallowed here.
    logger.info(...)

Good:

try:
    await do_something()
except CancelledError:
    raise
except Exception:
    logger.info(...)

OK:

try:
    check_something()
    # A `CancelledError` won't ever be raised here.
except Exception:
    logger.info(...)

Good:

try:
    await do_something()
except ValueError:
    logger.info(...)

defer.gatherResults

defer.gatherResults produces a Deferred which:

  • broadcasts cancel() calls to every Deferred being waited on.
  • wraps the first exception it sees in a FirstError.

Together, this means that CancelledErrors will be wrapped in a FirstError unless unwrapped. Such FirstErrors are liable to be swallowed, so they must be unwrapped.

Bad:

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(...)

Good:

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(...)

Creation of Deferreds

If a function creates a Deferred, the effect of cancelling it must be considered. Deferreds that get shared are likely to have unintended behaviour when cancelled.

Bad:

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")

Good:

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")

Good:

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")

Uncancelled processing

Some async functions may kick off some async processing which is intentionally protected from cancellation, by stop_cancellation or other means. If the async processing inherits the logcontext of the request which initiated it, care must be taken to ensure that the logcontext is not finished before the async processing completes.

Bad:

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()

Good:

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()

OK:

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()