Convert more cached return values to immutable types (#16356)

2 files changed, 13 insertions, 3 deletions

diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
index eedde97ab0..7c62cdfaef 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -1538,7 +1538,7 @@ class FederationEventHandler:
             logger.exception("Failed to resync device for %s", sender)
 
     async def backfill_event_id(
-        self, destinations: List[str], room_id: str, event_id: str
+        self, destinations: StrCollection, room_id: str, event_id: str
     ) -> PulledPduInfo:
         """Backfill a single event and persist it as a non-outlier which means
         we also pull in all of the state and auth events necessary for it.
diff --git a/synapse/handlers/relations.py b/synapse/handlers/relations.py
index db97f7aede..9b13448cdd 100644
--- a/synapse/handlers/relations.py
+++ b/synapse/handlers/relations.py
@@ -13,7 +13,17 @@
 # limitations under the License.
 import enum
 import logging
-from typing import TYPE_CHECKING, Collection, Dict, FrozenSet, Iterable, List, Optional
+from typing import (
+    TYPE_CHECKING,
+    Collection,
+    Dict,
+    FrozenSet,
+    Iterable,
+    List,
+    Mapping,
+    Optional,
+    Sequence,
+)
 
 import attr
 
@@ -245,7 +255,7 @@ class RelationsHandler:
 
     async def get_references_for_events(
         self, event_ids: Collection[str], ignored_users: FrozenSet[str] = frozenset()
-    ) -> Dict[str, List[_RelatedEvent]]:
+    ) -> Mapping[str, Sequence[_RelatedEvent]]:
         """Get a list of references to the given events.
 
         Args: