Speed up remote invite rejection database call (#8815)

This is another PR that grew out of #6739.

The existing code for checking whether a user is currently invited to a room when they want to leave the room looks like the following:

f737368a26/synapse/handlers/room_member.py (L518-L540)

It calls `get_invite_for_local_user_in_room`, which will actually query *all* rooms the user has been invited to, before iterating over them and matching via the room ID. It will then return a tuple of a lot of information which we pull the event ID out of.

I need to do a similar check for knocking, but this code wasn't very efficient. I then tried to write a different implementation using `StateHandler.get_current_state` but this actually didn't work as we haven't *joined* the room yet - we've only been invited to it. That means that only certain tables in Synapse have our desired `invite` membership state. One of those tables is `local_current_membership`.

So I wrote a store method that just queries that table instead

synapse/handlers/room_member.py

@@ -31,7 +31,6 @@ from synapse.api.errors import (
 from synapse.api.ratelimiting import Ratelimiter
 from synapse.events import EventBase
 from synapse.events.snapshot import EventContext
-from synapse.storage.roommember import RoomsForUser
 from synapse.types import JsonDict, Requester, RoomAlias, RoomID, StateMap, UserID
 from synapse.util.async_helpers import Linearizer
 from synapse.util.distributor import user_left_room
@@ -515,10 +514,16 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
         elif effective_membership_state == Membership.LEAVE:
             if not is_host_in_room:
                 # perhaps we've been invited
-                invite = await self.store.get_invite_for_local_user_in_room(
-                    user_id=target.to_string(), room_id=room_id
-                )  # type: Optional[RoomsForUser]
-                if not invite:
+                (
+                    current_membership_type,
+                    current_membership_event_id,
+                ) = await self.store.get_local_current_membership_for_user_in_room(
+                    target.to_string(), room_id
+                )
+                if (
+                    current_membership_type != Membership.INVITE
+                    or not current_membership_event_id
+                ):
                     logger.info(
                         "%s sent a leave request to %s, but that is not an active room "
                         "on this server, and there is no pending invite",
@@ -528,6 +533,7 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
 
                     raise SynapseError(404, "Not a known room")
 
+                invite = await self.store.get_event(current_membership_event_id)
                 logger.info(
                     "%s rejects invite to %s from %s", target, room_id, invite.sender
                 )