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/controllers/state.py b/synapse/storage/controllers/state.py

index 52efd4a171..9d7a8a792f 100644
--- a/synapse/storage/controllers/state.py
+++ b/synapse/storage/controllers/state.py
@@ -14,6 +14,7 @@
 import logging
 from typing import (
     TYPE_CHECKING,
+    AbstractSet,
     Any,
     Awaitable,
     Callable,
@@ -23,7 +24,6 @@ from typing import (
     List,
     Mapping,
     Optional,
-    Set,
     Tuple,
 )
 
@@ -527,7 +527,7 @@ class StateStorageController:
         )
         return state_map.get(key)
 
-    async def get_current_hosts_in_room(self, room_id: str) -> Set[str]:
+    async def get_current_hosts_in_room(self, room_id: str) -> AbstractSet[str]:
         """Get current hosts in room based on current state.
 
         Blocks until we have full state for the given room. This only happens for rooms
@@ -584,7 +584,7 @@ class StateStorageController:
 
     async def get_users_in_room_with_profiles(
         self, room_id: str
-    ) -> Dict[str, ProfileInfo]:
+    ) -> Mapping[str, ProfileInfo]:
         """
         Get the current users in the room with their profiles.
         If the room is currently partial-stated, this will block until the room has