Fix using MSC2716 batch sending with event persistence workers

changelog.d/11220.bugfix

@@ -0,0 +1 @@
+Fix using MSC2716 batch sending in combination with event persistence workers. Contributed by @tulir at Beeper.

synapse/app/generic_worker.py

@@ -113,6 +113,7 @@
 )
 from synapse.storage.databases.main.presence import PresenceStore
 from synapse.storage.databases.main.room import RoomWorkerStore
+from synapse.storage.databases.main.room_batch import RoomBatchStore
 from synapse.storage.databases.main.search import SearchStore
 from synapse.storage.databases.main.session import SessionStore
 from synapse.storage.databases.main.stats import StatsStore
@@ -240,6 +241,7 @@ class GenericWorkerSlavedStore(
     SlavedEventStore,
     SlavedKeyStore,
     RoomWorkerStore,
+    RoomBatchStore,
     DirectoryStore,
     SlavedApplicationServiceStore,
     SlavedRegistrationStore,

synapse/events/snapshot.py

@@ -322,6 +322,11 @@ async def _fill_out_state(self) -> None:
         attributes by loading from the database.
         """
         if self.state_group is None:
+            # No state group means the event is an outlier. Usually the state_ids dicts are also
+            # pre-set to empty dicts, but they get reset when the context is serialized, so set
+            # them to empty dicts again here.
+            self._current_state_ids = {}
+            self._prev_state_ids = {}
             return
 
         current_state_ids = await self._storage.state.get_state_ids_for_group(