+ +

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

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.
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 `Deferred`s

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()
+

+ +

How do faster joins work?

This is a work-in-progress set of notes with two goals:

act as a reference, explaining how Synapse implements faster joins; and
record the rationale behind our choices.

Overview: processing events in a partially-joined room

The response to a partial join consists of

the requested join event J,
a list of the servers in the room (according to the state before J),
a subset of the state of the room before J,
the full auth chain of that state subset.

Synapse marks the room as partially joined by adding a row to the database table +partial_state_rooms. It also marks the join event J as "partially stated", +meaning that we have neither received nor computed the full state before/after +J. This is done by adding a row to partial_state_events.

DB schema

matrix=> \d partial_state_events
+Table "matrix.partial_state_events"
+  Column  │ Type │ Collation │ Nullable │ Default
+══════════╪══════╪═══════════╪══════════╪═════════
+ room_id  │ text │           │ not null │
+ event_id │ text │           │ not null │
+ 
+matrix=> \d partial_state_rooms
+                Table "matrix.partial_state_rooms"
+         Column         │  Type  │ Collation │ Nullable │ Default 
+════════════════════════╪════════╪═══════════╪══════════╪═════════
+ room_id                │ text   │           │ not null │ 
+ device_lists_stream_id │ bigint │           │ not null │ 0
+ join_event_id          │ text   │           │          │ 
+ joined_via             │ text   │           │          │ 
+
+matrix=> \d partial_state_rooms_servers
+     Table "matrix.partial_state_rooms_servers"
+   Column    │ Type │ Collation │ Nullable │ Default 
+═════════════╪══════╪═══════════╪══════════╪═════════
+ room_id     │ text │           │ not null │ 
+ server_name │ text │           │ not null │ 
+

Indices, foreign-keys and check constraints are omitted for brevity.

While partially joined to a room, Synapse receives events E from remote +homeservers as normal, and can create events at the request of its local users. +However, we run into trouble when we enforce the checks on an event.

+
+
Is a valid event, otherwise it is dropped. For an event to be valid, it +must contain a room_id, and it must comply with the event format of that +room version.
+
Passes signature checks, otherwise it is dropped.
+
Passes hash checks, otherwise it is redacted before being processed further.
+
Passes authorization rules based on the event’s auth events, otherwise it +is rejected.
+
Passes authorization rules based on the state before the event, otherwise +it is rejected.
+
Passes authorization rules based on the current state of the room, +otherwise it is “soft failed”.
+
+

We can enforce checks 1--4 without any problems. +But we cannot enforce checks 5 or 6 with complete certainty, since Synapse does +not know the full state before E, nor that of the room.

Partial state

Instead, we make a best-effort approximation. +While the room is considered partially joined, Synapse tracks the "partial +state" before events. +This works in a similar way as regular state:

The partial state before J is that given to us by the partial join response.
The partial state before an event E is the resolution of the partial states +after each of E's prev_events.
If E is rejected or a message event, the partial state after E is the +partial state before E.
Otherwise, the partial state after E is the partial state before E, plus +E itself.

More concisely, partial state propagates just like full state; the only +difference is that we "seed" it with an incomplete initial state. +Synapse records that we have only calculated partial state for this event with +a row in partial_state_events.

While the room remains partially stated, check 5 on incoming events to that +room becomes:

+
+
Passes authorization rules based on the resolution between the partial +state before E and E's auth events. If the event fails to pass +authorization rules, it is rejected.
+
+

Additionally, check 6 is deleted: no soft-failures are enforced.

While partially joined, the current partial state of the room is defined as the +resolution across the partial states after all forward extremities in the room.

Remark. Events with partial state are not considered +outliers.

Approximation error

Using partial state means the auth checks can fail in a few different ways¹.

¹ +

Is this exhaustive?

We may erroneously accept an incoming event in check 5 based on partial state +when it would have been rejected based on full state, or vice versa.
This means that an event could erroneously be added to the current partial +state of the room when it would not be present in the full state of the room, +or vice versa.
Additionally, we may have skipped soft-failing an event that would have been +soft-failed based on full state.

(Note that the discrepancies described in the last two bullets are user-visible.)

This means that we have to be very careful when we want to lookup pieces of room +state in a partially-joined room. Our approximation of the state may be +incorrect or missing. But we can make some educated guesses. If

our partial state is likely to be correct, or
the consequences of our partial state being incorrect are minor,

then we proceed as normal, and let the resync process fix up any mistakes (see +below).

When is our partial state likely to be correct?

It's more accurate the closer we are to the partial join event. (So we should +ideally complete the resync as soon as possible.)
Non-member events: we will have received them as part of the partial join +response, if they were part of the room state at that point. We may +incorrectly accept or reject updates to that state (at first because we lack +remote membership information; later because of compounding errors), so these +can become incorrect over time.
Local members' memberships: we are the only ones who can create join and +knock events for our users. We can't be completely confident in the +correctness of bans, invites and kicks from other homeservers, but the resync +process should correct any mistakes.
Remote members' memberships: we did not receive these in the /send_join +response, so we have essentially no idea if these are correct or not.

In short, we deem it acceptable to trust the partial state for non-membership +and local membership events. For remote membership events, we wait for the +resync to complete, at which point we have the full state of the room and can +proceed as normal.

Fixing the approximation with a resync

The partial-state approximation is only a temporary affair. In the background, +synapse beings a "resync" process. This is a continuous loop, starting at the +partial join event and proceeding downwards through the event graph. For each +E seen in the room since partial join, Synapse will fetch

the event ids in the state of the room before E, via +/state_ids;
the event ids in the full auth chain of E, included in the /state_ids +response; and
any events from the previous two bullets that Synapse hasn't persisted, via +`/state.

This means Synapse has (or can compute) the full state before E, which allows +Synapse to properly authorise or reject E. At this point ,the event +is considered to have "full state" rather than "partial state". We record this +by removing E from the partial_state_events table.

[TODO: Does Synapse persist a new state group for the full state +before E, or do we alter the (partial-)state group in-place? Are state groups +ever marked as partially-stated? ]

This scheme means it is possible for us to have accepted and sent an event to +clients, only to reject it during the resync. From a client's perspective, the +effect is similar to a retroactive +state change due to state resolution---i.e. a "state reset".²

² +

Clients should refresh caches to detect such a change. Rumour has it that +sliding sync will fix this.

When all events since the join J have been fully-stated, the room resync +process is complete. We record this by removing the room from +partial_state_rooms.

Faster joins on workers

For the time being, the resync process happens on the master worker. +A new replication stream un_partial_stated_room is added. Whenever a resync +completes and a partial-state room becomes fully stated, a new message is sent +into that stream containing the room ID.

Notes on specific cases

+
NB. The notes below are rough. Some of them are hidden under <details> +disclosures because they have yet to be implemented in mainline Synapse.
+

Creating events during a partial join

When sending out messages during a partial join, we assume our partial state is +accurate and proceed as normal. For this to have any hope of succeeding at all, +our partial state must contain an entry for each of the (type, state key) pairs +specified by the auth rules:

m.room.create
m.room.join_rules
m.room.power_levels
m.room.third_party_invite
m.room.member

The first four of these should be present in the state before J that is given +to us in the partial join response; only membership events are omitted. In order +for us to consider the user joined, we must have their membership event. That +means the only possible omission is the target's membership in an invite, kick +or ban.

The worst possibility is that we locally invite someone who is banned according to +the full state, because we lack their ban in our current partial state. The rest +of the federation---at least, those who are fully joined---should correctly +enforce the membership transition constraints. So any the erroneous invite should be ignored by fully-joined +homeservers and resolved by the resync for partially-joined homeservers.

In more generality, there are two problems we're worrying about here:

We might create an event that is valid under our partial state, only to later +find out that is actually invalid according to the full state.
Or: we might refuse to create an event that is invalid under our partial +state, even though it would be perfectly valid under the full state.

However we expect such problems to be unlikely in practise, because

We trust that the room has sensible power levels, e.g. that bad actors with +high power levels are demoted before their ban.
We trust that the resident server provides us up-to-date power levels, join +rules, etc.
State changes in rooms are relatively infrequent, and the resync period is +relatively quick.

Sending out the event over federation

TODO: needs prose fleshing out.

Normally: send out in a fed txn to all HSes in the room. +We only know that some HSes were in the room at some point. Wat do. +Send it out to the list of servers from the first join. +TODO what do we do here if we have full state? +If the prev event was created by us, we can risk sending it to the wrong HS. (Motivation: privacy concern of the content. Not such a big deal for a public room or an encrypted room. But non-encrypted invite-only...) +But don't want to send out sensitive data in other HS's events in this way.

Suppose we discover after resync that we shouldn't have sent out one our events (not a prev_event) to a target HS. Not much we can do. +What about if we didn't send them an event but shouldn't've? +E.g. what if someone joined from a new HS shortly after you did? We wouldn't talk to them. +Could imagine sending out the "Missed" events after the resync but... painful to work out what they shuld have seen if they joined/left. +Instead, just send them the latest event (if they're still in the room after resync) and let them backfill.(?)

Don't do this currently.
If anyone who has received our messages sends a message to a HS we missed, they can backfill our messages
Gap: rooms which are infrequently used and take a long time to resync.

Joining after a partial join

NB. Not yet implemented.

TODO: needs prose fleshing out. Liase with Matthieu. Explain why /send_join +(Rich was surprised we didn't just create it locally. Answer: to try and avoid +a join which then gets rejected after resync.)

We don't know for sure that any join we create would be accepted. +E.g. the joined user might have been banned; the join rules might have changed in a way that we didn't realise... some way in which the partial state was mistaken. +Instead, do another partial make-join/send-join handshake to confirm that the join works.

Probably going to get a bunch of duplicate state events and auth events.... but the point of partial joins is that these should be small. Many are already persisted = good.
What if the second send_join response includes a different list of reisdent HSes? Could ignore it. +
- Could even have a special flag that says "just make me a join", i.e. don't bother giving me state or servers in room. Deffo want the auth chain tho.
+
SQ: wrt device lists it's a lot safer to ignore it!!!!!
What if the state at the second join is inconsistent with what we have? Ignore it?

Leaving (and kicks and bans) after a partial join

NB. Not yet implemented.

When you're fully joined to a room, to have U leave a room their homeserver +needs to

create a new leave event for U which will be accepted by other homeservers, +and
send that event U out to the homeservers in the federation.

When is a leave event accepted? See +v10 auth rules:

+
+
If type is m.room.member: [...] +> +> 5. If membership is leave: +> +> 1. If the sender matches state_key, allow if and only if that user’s current membership state is invite, join, or knock. +2. [...]
+
+

I think this means that (well-formed!) self-leaves are governed entirely by +4.5.1. This means that if we correctly calculate state which says that U is +invited, joined or knocked and include it in the leave's auth events, our event +is accepted by checks 4 and 5 on incoming events.

+
+
Passes authorization rules based on the event’s auth events, otherwise +> it is rejected.
+
Passes authorization rules based on the state before the event, otherwise +> it is rejected.
+
+

The only way to fail check 6 is if the receiving server's current state of the +room says that U is banned, has left, or has no membership event. But this is +fine: the receiving server already thinks that U isn't in the room.

+
+
Passes authorization rules based on the current state of the room, +> otherwise it is “soft failed”.
+
+

For the second point (publishing the leave event), the best thing we can do is +to is publish to all HSes we know to be currently in the room. If they miss that +event, they might send us traffic in the room that we don't care about. This is +a problem with leaving after a "full" join; we don't seek to fix this with +partial joins.

(With that said: there's nothing machine-readable in the /send response. I don't +think we can deduce "destination has left the room" from a failure to /send an +event into that room?)

Can we still do this during a partial join?

We can create leave events and can choose what gets included in our auth events, +so we can be sure that we pass check 4 on incoming events. For check 5, we might +have an incorrect view of the state before an event. +The only way we might erroneously think a leave is valid is if

the partial state before the leave has U joined, invited or knocked, but
the full state before the leave has U banned, left or not present,

in which case the leave doesn't make anything worse: other HSes already consider +us as not in the room, and will continue to do so after seeing the leave.

The remaining obstacle is then: can we safely broadcast the leave event? We may +miss servers or incorrectly think that a server is in the room. Or the +destination server may be offline and miss the transaction containing our leave +event.This should self-heal when they see an event whose prev_events descends +from our leave.

Another option we considered was to use federation /send_leave to ask a +fully-joined server to send out the event on our behalf. But that introduces +complexity without much benefit. Besides, as Rich put it,

+
sending out leaves is pretty best-effort currently
+

so this is probably good enough as-is.

Cleanup after the last leave

TODO: what cleanup is necessary? Is it all just nice-to-have to save unused +work?

+ +