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, 2 insertions, 2 deletions

diff --git a/synapse/handlers/receipts.py b/synapse/handlers/receipts.py

index 04c61ae3dd..2bacdebfb5 100644
--- a/synapse/handlers/receipts.py
+++ b/synapse/handlers/receipts.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, Iterable, List, Optional, Tuple
+from typing import TYPE_CHECKING, Iterable, List, Optional, Sequence, Tuple
 
 from synapse.api.constants import EduTypes, ReceiptTypes
 from synapse.appservice import ApplicationService
@@ -189,7 +189,7 @@ class ReceiptEventSource(EventSource[int, JsonDict]):
 
     @staticmethod
     def filter_out_private_receipts(
-        rooms: List[JsonDict], user_id: str
+        rooms: Sequence[JsonDict], user_id: str
     ) -> List[JsonDict]:
         """
         Filters a list of serialized receipts (as returned by /sync and /initialSync)