Pull less state out if we fail to backfill

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,