Refactor EventContext (#12689)

Refactor how the `EventContext` class works, with the intention of reducing the amount of state we fetch from the DB during event processing.

The idea here is to get rid of the cached `current_state_ids` and `prev_state_ids` that live in the `EventContext`, and instead defer straight to the database (and its caching). 

One change that may have a noticeable effect is that we now no longer prefill the `get_current_state_ids` cache on a state change. However, that query is relatively light, since its just a case of reading a table from the DB (unlike fetching state at an event which is more heavyweight). For deployments with workers this cache isn't even used.


Part of #12684

synapse/storage/persist_events.py

@@ -487,12 +487,6 @@ class EventsPersistenceStorage:
             # extremities in each room
             new_forward_extremities: Dict[str, Set[str]] = {}
 
-            # map room_id->(type,state_key)->event_id tracking the full
-            # state in each room after adding these events.
-            # This is simply used to prefill the get_current_state_ids
-            # cache
-            current_state_for_room: Dict[str, StateMap[str]] = {}
-
             # map room_id->(to_delete, to_insert) where to_delete is a list
             # of type/state keys to remove from current state, and to_insert
             # is a map (type,key)->event_id giving the state delta in each
@@ -628,14 +622,8 @@ class EventsPersistenceStorage:
 
                             state_delta_for_room[room_id] = delta
 
-                        # If we have the current_state then lets prefill
-                        # the cache with it.
-                        if current_state is not None:
-                            current_state_for_room[room_id] = current_state
-
             await self.persist_events_store._persist_events_and_state_updates(
                 chunk,
-                current_state_for_room=current_state_for_room,
                 state_delta_for_room=state_delta_for_room,
                 new_forward_extremities=new_forward_extremities,
                 use_negative_stream_ordering=backfilled,
@@ -733,7 +721,8 @@ class EventsPersistenceStorage:
 
             The first state map is the full new current state and the second
             is the delta to the existing current state. If both are None then
-            there has been no change.
+            there has been no change. Either or neither can be None if there
+            has been a change.
 
             The function may prune some old entries from the set of new
             forward extremities if it's safe to do so.
@@ -743,9 +732,6 @@ class EventsPersistenceStorage:
             the new current state is only returned if we've already calculated
             it.
         """
-        # map from state_group to ((type, key) -> event_id) state map
-        state_groups_map = {}
-
         # Map from (prev state group, new state group) -> delta state dict
         state_group_deltas = {}
 
@@ -759,16 +745,6 @@ class EventsPersistenceStorage:
                     )
                 continue
 
-            if ctx.state_group in state_groups_map:
-                continue
-
-            # We're only interested in pulling out state that has already
-            # been cached in the context. We'll pull stuff out of the DB later
-            # if necessary.
-            current_state_ids = ctx.get_cached_current_state_ids()
-            if current_state_ids is not None:
-                state_groups_map[ctx.state_group] = current_state_ids
-
             if ctx.prev_group:
                 state_group_deltas[(ctx.prev_group, ctx.state_group)] = ctx.delta_ids
 
@@ -826,18 +802,14 @@ class EventsPersistenceStorage:
             delta_ids = state_group_deltas.get((old_state_group, new_state_group), None)
             if delta_ids is not None:
                 # We have a delta from the existing to new current state,
-                # so lets just return that. If we happen to already have
-                # the current state in memory then lets also return that,
-                # but it doesn't matter if we don't.
-                new_state = state_groups_map.get(new_state_group)
-                return new_state, delta_ids, new_latest_event_ids
+                # so lets just return that.
+                return None, delta_ids, new_latest_event_ids
 
         # Now that we have calculated new_state_groups we need to get
         # their state IDs so we can resolve to a single state set.
-        missing_state = new_state_groups - set(state_groups_map)
-        if missing_state:
-            group_to_state = await self.state_store._get_state_for_groups(missing_state)
-            state_groups_map.update(group_to_state)
+        state_groups_map = await self.state_store._get_state_for_groups(
+            new_state_groups
+        )
 
         if len(new_state_groups) == 1:
             # If there is only one state group, then we know what the current