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). 

1 files changed, 7 insertions, 5 deletions

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]]]: