Faster joins: persist to database (#12012)

When we get a partial_state response from send_join, store information in the
database about it:
 * store a record about the room as a whole having partial state, and stash the
   list of member servers too.
 * flag the join event itself as having partial state
 * also, for any new events whose prev-events are partial-stated, note that
   they will *also* be partial-stated.

We don't yet make any attempt to interpret this data, so API calls (and a bunch
of other things) are just going to get incorrect data.

synapse/handlers/federation_event.py

@@ -397,6 +397,7 @@ class FederationEventHandler:
         state: List[EventBase],
         event: EventBase,
         room_version: RoomVersion,
+        partial_state: bool,
     ) -> int:
         """Persists the events returned by a send_join
 
@@ -412,6 +413,7 @@ class FederationEventHandler:
             event
             room_version: The room version we expect this room to have, and
                 will raise if it doesn't match the version in the create event.
+            partial_state: True if the state omits non-critical membership events
 
         Returns:
             The stream ID after which all events have been persisted.
@@ -453,10 +455,14 @@ class FederationEventHandler:
         )
 
         # and now persist the join event itself.
-        logger.info("Peristing join-via-remote %s", event)
+        logger.info(
+            "Peristing join-via-remote %s (partial_state: %s)", event, partial_state
+        )
         with nested_logging_context(suffix=event.event_id):
             context = await self._state_handler.compute_event_context(
-                event, old_state=state
+                event,
+                old_state=state,
+                partial_state=partial_state,
             )
 
             context = await self._check_event_auth(origin, event, context)
@@ -698,6 +704,8 @@ class FederationEventHandler:
 
         try:
             state = await self._resolve_state_at_missing_prevs(origin, event)
+            # TODO(faster_joins): make sure that _resolve_state_at_missing_prevs does
+            #   not return partial state
             await self._process_received_pdu(
                 origin, event, state=state, backfilled=backfilled
             )
@@ -1791,6 +1799,7 @@ class FederationEventHandler:
             prev_state_ids=prev_state_ids,
             prev_group=prev_group,
             delta_ids=state_updates,
+            partial_state=context.partial_state,
         )
 
     async def _run_push_actions_and_persist_event(