user_left_room distributor signal is unreliable

[squahtx]

In Synapse, there is a class called Distributor, which maintains a list of callbacks for a named signal and calls them all when the signal is fired: https://github.com/matrix-org/synapse/blob/develop/synapse/util/distributor.py#L43

There's only one signal defined currently and it's user_left_room, which is used in two places:

1. DeviceHandler, to mark a remote user's device list as unsubscribed
   

   synapse/synapse/handlers/device.py

   Lines 569 to 575 in 1a209ef

   	async def user_left_room(self, user: UserID, room_id: str) -> None:
   	user_id = user.to_string()
   	room_ids = await self.store.get_rooms_for_user(user_id)
   	if not room_ids:
   	# We no longer share rooms with this user, so we'll no longer
   	# receive device updates. Mark this in DB.
   	await self.store.mark_remote_user_device_list_as_unsubscribed(user_id)

2. TypingWriterHandler, to stop a user's typing indicator when they leave a room.
   

   synapse/synapse/handlers/typing.py

   Lines 311 to 315 in 1a209ef

   	def user_left_room(self, user: UserID, room_id: str) -> None:
   	user_id = user.to_string()
   	if self.is_mine_id(user_id):
   	member = RoomMember(room_id=room_id, user_id=user_id)
   	self._stopped_typing(member)

Unfortunately, the user_left_room signal is currently only reliable when local users leave a room normally (and not via a ban!).
This manifested as #13651 for device lists. Once #13651 is fixed, device list tracking will no longer use the signal.

We should look into whether the typing listener is necessary and if it can be moved elsewhere, so that we can eliminate this bit of architecture that's only used by one tiny part of synapse.

[DMRobertson]

The Distributor sounds a lot like the Notifier:

synapse/synapse/notifier.py

Line 210 in 0eb7e69

class Notifier:

[squahtx]

There's only one signal defined currently and it's user_left_room, which is used in two places:
2. TypingWriterHandler, to stop a user's typing indicator when they leave a room.

related:

• we should terminate typing notifs for users who have left a room rather than wait for them to timeout, as fun as it is visualising them as smoke hanging on the air #3878