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>

4 files changed, 8 insertions, 8 deletions

diff --git a/synapse/handlers/directory.py b/synapse/handlers/directory.py
index d31b0fbb17..a5798e9483 100644
--- a/synapse/handlers/directory.py
+++ b/synapse/handlers/directory.py
@@ -14,7 +14,7 @@
 
 import logging
 import string
-from typing import TYPE_CHECKING, Iterable, List, Optional
+from typing import TYPE_CHECKING, Iterable, List, Optional, Sequence
 
 from typing_extensions import Literal
 
@@ -486,7 +486,7 @@ class DirectoryHandler:
             )
             if canonical_alias:
                 # Ensure we do not mutate room_aliases.
-                room_aliases = room_aliases + [canonical_alias]
+                room_aliases = list(room_aliases) + [canonical_alias]
 
             if not self.config.roomdirectory.is_publishing_room_allowed(
                 user_id, room_id, room_aliases
@@ -529,7 +529,7 @@ class DirectoryHandler:
 
     async def get_aliases_for_room(
         self, requester: Requester, room_id: str
-    ) -> List[str]:
+    ) -> Sequence[str]:
         """
         Get a list of the aliases that currently point to this room on this server
         """
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)
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index 0e759b8a5d..060bbcb181 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -1928,6 +1928,6 @@ class RoomShutdownHandler:
         return {
             "kicked_users": kicked_users,
             "failed_to_kick_users": failed_to_kick_users,
-            "local_aliases": aliases_for_room,
+            "local_aliases": list(aliases_for_room),
             "new_room_id": new_room_id,
         }
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 399685e5b7..4bae46158a 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -1519,7 +1519,7 @@ class SyncHandler:
             one_time_keys_count = await self.store.count_e2e_one_time_keys(
                 user_id, device_id
             )
-            unused_fallback_key_types = (
+            unused_fallback_key_types = list(
                 await self.store.get_e2e_unused_fallback_key_types(user_id, device_id)
             )
 
@@ -2301,7 +2301,7 @@ class SyncHandler:
         sync_result_builder: "SyncResultBuilder",
         room_builder: "RoomSyncResultBuilder",
         ephemeral: List[JsonDict],
-        tags: Optional[Dict[str, Dict[str, Any]]],
+        tags: Optional[Mapping[str, Mapping[str, Any]]],
         account_data: Mapping[str, JsonDict],
         always_include: bool = False,
     ) -> None: