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/storage/databases/main/signatures.py b/synapse/storage/databases/main/signatures.py
index 05da15074a..5dcb1fc0b5 100644
--- a/synapse/storage/databases/main/signatures.py
+++ b/synapse/storage/databases/main/signatures.py
@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from typing import Collection, Dict, List, Tuple
+from typing import Collection, Dict, List, Mapping, Tuple
 
 from unpaddedbase64 import encode_base64
 
@@ -26,7 +26,7 @@ from synapse.util.caches.descriptors import cached, cachedList
 
 class SignatureWorkerStore(EventsWorkerStore):
     @cached()
-    def get_event_reference_hash(self, event_id: str) -> Dict[str, Dict[str, bytes]]:
+    def get_event_reference_hash(self, event_id: str) -> Mapping[str, bytes]:
         # This is a dummy function to allow get_event_reference_hashes
         # to use its cache
         raise NotImplementedError()
@@ -36,7 +36,7 @@ class SignatureWorkerStore(EventsWorkerStore):
     )
     async def get_event_reference_hashes(
         self, event_ids: Collection[str]
-    ) -> Dict[str, Dict[str, bytes]]:
+    ) -> Mapping[str, Mapping[str, bytes]]:
         """Get all hashes for given events.
 
         Args: