Return read-only collections from `@cached` methods (#13755)

It's important that collections returned from `@cached` methods are not
modified, otherwise future retrievals from the cache will return the
modified collection.

This applies to the return values from `@cached` methods and the values
inside the dictionaries returned by `@cachedList` methods. It's not
necessary for the dictionaries returned by `@cachedList` methods
themselves to be read-only.

Signed-off-by: Sean Quah <seanq@matrix.org>
Co-authored-by: David Robertson <davidr@element.io>

1 files changed, 3 insertions, 3 deletions

diff --git a/synapse/events/builder.py b/synapse/events/builder.py

index 94dd1298e1..c82745275f 100644
--- a/synapse/events/builder.py
+++ b/synapse/events/builder.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import logging
-from typing import TYPE_CHECKING, Any, Dict, List, Optional, Tuple, Union
+from typing import TYPE_CHECKING, Any, Collection, Dict, List, Optional, Tuple, Union
 
 import attr
 from signedjson.types import SigningKey
@@ -103,7 +103,7 @@ class EventBuilder:
 
     async def build(
         self,
-        prev_event_ids: List[str],
+        prev_event_ids: Collection[str],
         auth_event_ids: Optional[List[str]],
         depth: Optional[int] = None,
     ) -> EventBase:
@@ -136,7 +136,7 @@ class EventBuilder:
 
         format_version = self.room_version.event_format
         # The types of auth/prev events changes between event versions.
-        prev_events: Union[List[str], List[Tuple[str, Dict[str, str]]]]
+        prev_events: Union[Collection[str], List[Tuple[str, Dict[str, str]]]]
         auth_events: Union[List[str], List[Tuple[str, Dict[str, str]]]]
         if format_version == EventFormatVersions.ROOM_V1_V2:
             auth_events = await self._store.add_event_hashes(auth_event_ids)