Return an immutable value from get_latest_event_ids_in_room. (#16326)

2 files changed, 6 insertions, 4 deletions

diff --git a/synapse/storage/databases/main/event_federation.py b/synapse/storage/databases/main/event_federation.py
index 09de8f55e2..afffa54985 100644
--- a/synapse/storage/databases/main/event_federation.py
+++ b/synapse/storage/databases/main/event_federation.py
@@ -19,6 +19,7 @@ from typing import (
     TYPE_CHECKING,
     Collection,
     Dict,
+    FrozenSet,
     Iterable,
     List,
     Optional,
@@ -47,7 +48,7 @@ from synapse.storage.database import (
 from synapse.storage.databases.main.events_worker import EventsWorkerStore
 from synapse.storage.databases.main.signatures import SignatureWorkerStore
 from synapse.storage.engines import PostgresEngine, Sqlite3Engine
-from synapse.types import JsonDict, StrCollection, StrSequence
+from synapse.types import JsonDict, StrCollection
 from synapse.util import json_encoder
 from synapse.util.caches.descriptors import cached
 from synapse.util.caches.lrucache import LruCache
@@ -1179,13 +1180,14 @@ class EventFederationWorkerStore(SignatureWorkerStore, EventsWorkerStore, SQLBas
         )
 
     @cached(max_entries=5000, iterable=True)
-    async def get_latest_event_ids_in_room(self, room_id: str) -> StrSequence:
-        return await self.db_pool.simple_select_onecol(
+    async def get_latest_event_ids_in_room(self, room_id: str) -> FrozenSet[str]:
+        event_ids = await self.db_pool.simple_select_onecol(
             table="event_forward_extremities",
             keyvalues={"room_id": room_id},
             retcol="event_id",
             desc="get_latest_event_ids_in_room",
         )
+        return frozenset(event_ids)
 
     async def get_min_depth(self, room_id: str) -> Optional[int]:
         """For the given room, get the minimum depth we have seen for it."""
diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index 0c1ed75240..bc8474a589 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -222,7 +222,7 @@ class PersistEventsStore:
 
             for room_id, latest_event_ids in new_forward_extremities.items():
                 self.store.get_latest_event_ids_in_room.prefill(
-                    (room_id,), list(latest_event_ids)
+                    (room_id,), frozenset(latest_event_ids)
                 )
 
     async def _get_events_which_are_prevs(self, event_ids: Iterable[str]) -> List[str]: