Skip soft fail checks for rooms with partial state (#13354)

When a room has the partial state flag, we may not have an accurate
`m.room.member` event for event senders in the room's current state, and
so cannot perform soft fail checks correctly. Skip the soft fail check
entirely in this case.

As an alternative, we could block until we have full state, but that
would prevent us from receiving incoming events over federation, which
is undesirable.

Signed-off-by: Sean Quah <seanq@matrix.org>

changelog.d/13354.misc

@@ -0,0 +1 @@
+Faster room joins: skip soft fail checks while Synapse only has partial room state, since the current membership of event senders may not be accurately known.

synapse/handlers/federation_event.py

@@ -1664,11 +1664,21 @@ async def _check_for_soft_fail(
         """Checks if we should soft fail the event; if so, marks the event as
         such.
 
+        Does nothing for events in rooms with partial state, since we may not have an
+        accurate membership event for the sender in the current state.
+
         Args:
             event
             state_ids: The state at the event if we don't have all the event's prev events
             origin: The host the event originates from.
         """
+        if await self._store.is_partial_state_room(event.room_id):
+            # We might not know the sender's membership in the current state, so don't
+            # soft fail anything. Even if we do have a membership for the sender in the
+            # current state, it may have been derived from state resolution between
+            # partial and full state and may not be accurate.
+            return
+
         extrem_ids_list = await self._store.get_latest_event_ids_in_room(event.room_id)
         extrem_ids = set(extrem_ids_list)
         prev_event_ids = set(event.prev_event_ids())