Pull less state out if we fail to backfill (#16788)

Sometimes we fail to fetch events during backfill due to missing state,
and we often end up querying the same bad events periodically (as people
backpaginate). In such cases its likely we will continue to fail to get
the state, and therefore we should try *before* loading the state that
we have from the DB (as otherwise it's wasted DB and memory).

---------

Co-authored-by: reivilibre <oliverw@matrix.org>

changelog.d/16788.misc

@@ -0,0 +1 @@
+Pull less state out of the DB when we retry fetching old events during backfill.

synapse/handlers/federation_event.py

@@ -1141,16 +1141,8 @@ async def _compute_event_context_with_maybe_missing_prevs(
             partial_state_flags = await self._store.get_partial_state_events(seen)
             partial_state = any(partial_state_flags.values())
 
-            # Get the state of the events we know about
-            ours = await self._state_storage_controller.get_state_groups_ids(
-                room_id, seen, await_full_state=False
-            )
-
             # state_maps is a list of mappings from (type, state_key) to event_id
-            state_maps: List[StateMap[str]] = list(ours.values())
-
-            # we don't need this any more, let's delete it.
-            del ours
+            state_maps: List[StateMap[str]] = []
 
             # Ask the remote server for the states we don't
             # know about
@@ -1169,6 +1161,17 @@ async def _compute_event_context_with_maybe_missing_prevs(
 
                     state_maps.append(remote_state_map)
 
+            # Get the state of the events we know about. We do this *after*
+            # trying to fetch missing state over federation as that might fail
+            # and then we can skip loading the local state.
+            ours = await self._state_storage_controller.get_state_groups_ids(
+                room_id, seen, await_full_state=False
+            )
+            state_maps.extend(ours.values())
+
+            # we don't need this any more, let's delete it.
+            del ours
+
             room_version = await self._store.get_room_version_id(room_id)
             state_map = await self._state_resolution_handler.resolve_events_with_store(
                 room_id,