mypy plugin to check `@cached` return types (#14911)

Co-authored-by: David Robertson <davidr@element.io>
Co-authored-by: Patrick Cloke <patrickc@matrix.org>
Co-authored-by: Erik Johnston <erik@matrix.org>

Assert that the return type of callables wrapped in @cached
and @cachedList are cachable (aka immutable). 

4 files changed, 15 insertions, 10 deletions

diff --git a/synapse/storage/databases/main/event_push_actions.py b/synapse/storage/databases/main/event_push_actions.py
index ba99e63d26..39556481ff 100644
--- a/synapse/storage/databases/main/event_push_actions.py
+++ b/synapse/storage/databases/main/event_push_actions.py
@@ -182,6 +182,7 @@ class UserPushAction(EmailPushAction):
     profile_tag: str
 
 
+# TODO This is used as a cached value and is mutable.
 @attr.s(slots=True, auto_attribs=True)
 class NotifCounts:
     """
@@ -193,7 +194,7 @@ class NotifCounts:
     highlight_count: int = 0
 
 
-@attr.s(slots=True, auto_attribs=True)
+@attr.s(slots=True, frozen=True, auto_attribs=True)
 class RoomNotifCounts:
     """
     The per-user, per-room count of notifications. Used by sync and push.
@@ -201,7 +202,7 @@ class RoomNotifCounts:
 
     main_timeline: NotifCounts
     # Map of thread ID to the notification counts.
-    threads: Dict[str, NotifCounts]
+    threads: Mapping[str, NotifCounts]
 
     @staticmethod
     def empty() -> "RoomNotifCounts":
@@ -483,7 +484,7 @@ class EventPushActionsWorkerStore(ReceiptsWorkerStore, StreamWorkerStore, SQLBas
 
         return room_to_count
 
-    @cached(tree=True, max_entries=5000, iterable=True)
+    @cached(tree=True, max_entries=5000, iterable=True)  # type: ignore[synapse-@cached-mutable]
     async def get_unread_event_push_actions_by_room_for_user(
         self,
         room_id: str,
diff --git a/synapse/storage/databases/main/relations.py b/synapse/storage/databases/main/relations.py
index b67f780c10..9246b418f5 100644
--- a/synapse/storage/databases/main/relations.py
+++ b/synapse/storage/databases/main/relations.py
@@ -458,7 +458,7 @@ class RelationsWorkerStore(SQLBaseStore):
         )
         return result is not None
 
-    @cached()
+    @cached()  # type: ignore[synapse-@cached-mutable]
     async def get_references_for_event(self, event_id: str) -> List[JsonDict]:
         raise NotImplementedError()
 
@@ -512,11 +512,12 @@ class RelationsWorkerStore(SQLBaseStore):
             "_get_references_for_events_txn", _get_references_for_events_txn
         )
 
-    @cached()
+    @cached()  # type: ignore[synapse-@cached-mutable]
     def get_applicable_edit(self, event_id: str) -> Optional[EventBase]:
         raise NotImplementedError()
 
-    @cachedList(cached_method_name="get_applicable_edit", list_name="event_ids")
+    # TODO: This returns a mutable object, which is generally bad.
+    @cachedList(cached_method_name="get_applicable_edit", list_name="event_ids")  # type: ignore[synapse-@cached-mutable]
     async def get_applicable_edits(
         self, event_ids: Collection[str]
     ) -> Mapping[str, Optional[EventBase]]:
@@ -598,11 +599,12 @@ class RelationsWorkerStore(SQLBaseStore):
             for original_event_id in event_ids
         }
 
-    @cached()
+    @cached()  # type: ignore[synapse-@cached-mutable]
     def get_thread_summary(self, event_id: str) -> Optional[Tuple[int, EventBase]]:
         raise NotImplementedError()
 
-    @cachedList(cached_method_name="get_thread_summary", list_name="event_ids")
+    # TODO: This returns a mutable object, which is generally bad.
+    @cachedList(cached_method_name="get_thread_summary", list_name="event_ids")  # type: ignore[synapse-@cached-mutable]
     async def get_thread_summaries(
         self, event_ids: Collection[str]
     ) -> Mapping[str, Optional[Tuple[int, EventBase]]]:
diff --git a/synapse/storage/databases/main/roommember.py b/synapse/storage/databases/main/roommember.py
index 3755773faa..e93573f315 100644
--- a/synapse/storage/databases/main/roommember.py
+++ b/synapse/storage/databases/main/roommember.py
@@ -275,7 +275,7 @@ class RoomMemberWorkerStore(EventsWorkerStore, CacheInvalidationWorkerStore):
             _get_users_in_room_with_profiles,
         )
 
-    @cached(max_entries=100000)
+    @cached(max_entries=100000)  # type: ignore[synapse-@cached-mutable]
     async def get_room_summary(self, room_id: str) -> Mapping[str, MemberSummary]:
         """Get the details of a room roughly suitable for use by the room
         summary extension to /sync. Useful when lazy loading room members.
@@ -1071,7 +1071,8 @@ class RoomMemberWorkerStore(EventsWorkerStore, CacheInvalidationWorkerStore):
         )
         return {row["event_id"]: row["membership"] for row in rows}
 
-    @cached(max_entries=10000)
+    # TODO This returns a mutable object, which is generally confusing when using a cache.
+    @cached(max_entries=10000)  # type: ignore[synapse-@cached-mutable]
     def _get_joined_hosts_cache(self, room_id: str) -> "_JoinedHostsCache":
         return _JoinedHostsCache()
 
diff --git a/synapse/storage/roommember.py b/synapse/storage/roommember.py
index 2500381b7b..cbfb32014c 100644
--- a/synapse/storage/roommember.py
+++ b/synapse/storage/roommember.py
@@ -45,6 +45,7 @@ class ProfileInfo:
     display_name: Optional[str]
 
 
+# TODO This is used as a cached value and is mutable.
 @attr.s(slots=True, frozen=True, weakref_slot=False, auto_attribs=True)
 class MemberSummary:
     # A truncated list of (user_id, event_id) tuples for users of a given