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/state/__init__.py

@@ -258,7 +258,10 @@ class StateHandler:
         return await self.store.get_joined_hosts(room_id, entry)
 
     async def compute_event_context(
-        self, event: EventBase, old_state: Optional[Iterable[EventBase]] = None
+        self,
+        event: EventBase,
+        old_state: Optional[Iterable[EventBase]] = None,
+        partial_state: bool = False,
     ) -> EventContext:
         """Build an EventContext structure for a non-outlier event.
 
@@ -273,6 +276,8 @@ class StateHandler:
                 calculated from existing events. This is normally only specified
                 when receiving an event from federation where we don't have the
                 prev events for, e.g. when backfilling.
+            partial_state: True if `old_state` is partial and omits non-critical
+                membership events
         Returns:
             The event context.
         """
@@ -295,8 +300,28 @@ class StateHandler:
 
         else:
             # otherwise, we'll need to resolve the state across the prev_events.
-            logger.debug("calling resolve_state_groups from compute_event_context")
 
+            # partial_state should not be set explicitly in this case:
+            # we work it out dynamically
+            assert not partial_state
+
+            # if any of the prev-events have partial state, so do we.
+            # (This is slightly racy - the prev-events might get fixed up before we use
+            # their states - but I don't think that really matters; it just means we
+            # might redundantly recalculate the state for this event later.)
+            prev_event_ids = event.prev_event_ids()
+            incomplete_prev_events = await self.store.get_partial_state_events(
+                prev_event_ids
+            )
+            if any(incomplete_prev_events.values()):
+                logger.debug(
+                    "New/incoming event %s refers to prev_events %s with partial state",
+                    event.event_id,
+                    [k for (k, v) in incomplete_prev_events.items() if v],
+                )
+                partial_state = True
+
+            logger.debug("calling resolve_state_groups from compute_event_context")
             entry = await self.resolve_state_groups_for_events(
                 event.room_id, event.prev_event_ids()
             )
@@ -342,6 +367,7 @@ class StateHandler:
                 prev_state_ids=state_ids_before_event,
                 prev_group=state_group_before_event_prev_group,
                 delta_ids=deltas_to_state_group_before_event,
+                partial_state=partial_state,
             )
 
         #
@@ -373,6 +399,7 @@ class StateHandler:
             prev_state_ids=state_ids_before_event,
             prev_group=state_group_before_event,
             delta_ids=delta_ids,
+            partial_state=partial_state,
         )
 
     @measure_func()