Faster joins: room alias server list may be incomplete

[richvdh]

If a room alias is added on a server that is doing a faster join, the list of servers associated with the alias may be incomplete until the faster join completes.

[richvdh]

Currently we probably return just ourselves, which is useless. We should probably return the list of servers we got from the /send_join, which will be good enough.

[richvdh]

(This is a bit of an edge-case as it's unlikely someone will add an alias (and then someone else will join through it) during a faster join)

[DMRobertson]

The current situation is that we block waiting for full state:

  synapse_main |   File "/usr/local/lib/python3.9/site-packages/synapse/rest/client/directory.py", line 91, in on_PUT
  synapse_main |     await self.directory_handler.create_association(
  synapse_main |   File "/usr/local/lib/python3.9/site-packages/synapse/handlers/directory.py", line 175, in create_association
  synapse_main |     await self._create_association(room_alias, room_id, servers, creator=user_id)
  synapse_main |   File "/usr/local/lib/python3.9/site-packages/synapse/handlers/directory.py", line 88, in _create_association
  synapse_main |     servers = await self._storage_controllers.state.get_current_hosts_in_room(
  synapse_main |   File "/usr/local/lib/python3.9/site-packages/synapse/storage/controllers/state.py", line 539, in get_current_hosts_in_room
  synapse_main |     await self._partial_state_room_tracker.await_full_state(room_id)
  synapse_main |   File "/usr/local/lib/python3.9/site-packages/synapse/logging/opentracing.py", line 889, in _wrapper
  synapse_main |     return await func(*args, **kwargs)  # type: ignore[misc]
  synapse_main |   File "/usr/local/lib/python3.9/site-packages/synapse/storage/util/partial_state_events_tracker.py", line 172, in await_full_state
  synapse_main |     logger.info(

So we need to change that behaviour.

Also: do we persist the list of servers we get from a send_join response? Presumably this is what partial_state_rooms_servers is for?

Edit: ahh yes, get_partial_state_servers_at_join:

synapse/synapse/storage/databases/main/room.py

Lines 1152 to 1167 in c3a4780

	@cached(iterable=True)
	async def get_partial_state_servers_at_join(self, room_id: str) -> Sequence[str]:
	"""Gets the list of servers in a partial state room at the time we joined it.
	Returns:
	The `servers_in_room` list from the `/send_join` response for partial state
	rooms. May not be accurate or complete, as it comes from a remote
	homeserver.
	An empty list for full state rooms.
	"""
	return await self.db_pool.simple_select_onecol(
	"partial_state_rooms_servers",
	keyvalues={"room_id": room_id},
	retcol="server_name",
	desc="get_partial_state_servers_at_join",
	)

[DMRobertson]

Presumably once we have completed the resync we need to update the alias tables to reflect the full list of servers we now know about?

[DMRobertson]

Presumably once we have completed the resync we need to update the alias tables to reflect the full list of servers we now know about?

On reflection, I don't think this is correct. Quoting MSC3706:

servers_in_room: A new field of type [string], listing the servers active in the room (ie, those with joined members) before the join.

We are trusting the resident server to accurately provide that list of servers in the roomand the state at our join event. The resync gives us the state at that join event, so there shouldn't suddenly be a different set of servers in the room to deal with... if the resident server is well-behaved. (Do we want to check that?)

What I think this means is: we have to react to new servers joining the room after a partial join in exactly the same way as after a full join?