Faster joins: handle presence correctly

[richvdh]

The presence code really doesn't take much account of faster joins currently. We need to think about how it ought to work, and make it so.

synapse/synapse/storage/controllers/state.py

Lines 454 to 461 in 7c6b220

	# FIXME(faster_joins): what do we do here?
	# https://github.com/matrix-org/synapse/issues/12814
	# https://github.com/matrix-org/synapse/issues/12815
	# https://github.com/matrix-org/synapse/issues/13008
	return await self.stores.main.get_partial_current_state_deltas(
	prev_stream_id, max_stream_id
	)

[richvdh]

See also presence.py which includes a bunch of calls to get_users_in_room, many of which assume a complete list of federated users.

[richvdh]

We think:

• incorrect presence during a partial join is acceptable anyway
• you won't get correct presence for users in a faster-joins room when you join. We could maybe improve this, but...
• it should sort itself out within 5 minutes anyway?

[richvdh]

So the work here is to get correct presence at the point we transition into full state (rather than 5 minutes later).

[richvdh]

We consider the existing behaviour is probably good enough for now.