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/storage/databases/main/directory.py b/synapse/storage/databases/main/directory.py
index 5903fdaf00..44aa181174 100644
--- a/synapse/storage/databases/main/directory.py
+++ b/synapse/storage/databases/main/directory.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Iterable, List, Optional, Tuple
+from typing import Iterable, List, Optional, Sequence, Tuple
 
 import attr
 
@@ -74,7 +74,7 @@ class DirectoryWorkerStore(CacheInvalidationWorkerStore):
         )
 
     @cached(max_entries=5000)
-    async def get_aliases_for_room(self, room_id: str) -> List[str]:
+    async def get_aliases_for_room(self, room_id: str) -> Sequence[str]:
         return await self.db_pool.simple_select_onecol(
             "room_aliases",
             {"room_id": room_id},