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). 

6 files changed, 72 insertions, 21 deletions

diff --git a/synapse/handlers/room_list.py b/synapse/handlers/room_list.py
index bb0bdb8e6f..36e2db8975 100644
--- a/synapse/handlers/room_list.py
+++ b/synapse/handlers/room_list.py
@@ -33,7 +33,7 @@ from synapse.api.errors import (
     RequestSendFailed,
     SynapseError,
 )
-from synapse.types import JsonDict, ThirdPartyInstanceID
+from synapse.types import JsonDict, JsonMapping, ThirdPartyInstanceID
 from synapse.util.caches.descriptors import _CacheContext, cached
 from synapse.util.caches.response_cache import ResponseCache
 
@@ -256,7 +256,7 @@ class RoomListHandler:
         cache_context: _CacheContext,
         with_alias: bool = True,
         allow_private: bool = False,
-    ) -> Optional[JsonDict]:
+    ) -> Optional[JsonMapping]:
         """Returns the entry for a room
 
         Args:
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
diff --git a/synapse/util/caches/descriptors.py b/synapse/util/caches/descriptors.py
index 8514a75a1c..ce736fdf75 100644
--- a/synapse/util/caches/descriptors.py
+++ b/synapse/util/caches/descriptors.py
@@ -36,6 +36,8 @@ from typing import (
 )
 from weakref import WeakValueDictionary
 
+import attr
+
 from twisted.internet import defer
 from twisted.python.failure import Failure
 
@@ -466,6 +468,35 @@ class _CacheContext:
         )
 
 
+@attr.s(auto_attribs=True, slots=True, frozen=True)
+class _CachedFunctionDescriptor:
+    """Helper for `@cached`, we name it so that we can hook into it with mypy
+    plugin."""
+
+    max_entries: int
+    num_args: Optional[int]
+    uncached_args: Optional[Collection[str]]
+    tree: bool
+    cache_context: bool
+    iterable: bool
+    prune_unread_entries: bool
+    name: Optional[str]
+
+    def __call__(self, orig: F) -> CachedFunction[F]:
+        d = DeferredCacheDescriptor(
+            orig,
+            max_entries=self.max_entries,
+            num_args=self.num_args,
+            uncached_args=self.uncached_args,
+            tree=self.tree,
+            cache_context=self.cache_context,
+            iterable=self.iterable,
+            prune_unread_entries=self.prune_unread_entries,
+            name=self.name,
+        )
+        return cast(CachedFunction[F], d)
+
+
 def cached(
     *,
     max_entries: int = 1000,
@@ -476,9 +507,8 @@ def cached(
     iterable: bool = False,
     prune_unread_entries: bool = True,
     name: Optional[str] = None,
-) -> Callable[[F], CachedFunction[F]]:
-    func = lambda orig: DeferredCacheDescriptor(
-        orig,
+) -> _CachedFunctionDescriptor:
+    return _CachedFunctionDescriptor(
         max_entries=max_entries,
         num_args=num_args,
         uncached_args=uncached_args,
@@ -489,7 +519,26 @@ def cached(
         name=name,
     )
 
-    return cast(Callable[[F], CachedFunction[F]], func)
+
+@attr.s(auto_attribs=True, slots=True, frozen=True)
+class _CachedListFunctionDescriptor:
+    """Helper for `@cachedList`, we name it so that we can hook into it with mypy
+    plugin."""
+
+    cached_method_name: str
+    list_name: str
+    num_args: Optional[int] = None
+    name: Optional[str] = None
+
+    def __call__(self, orig: F) -> CachedFunction[F]:
+        d = DeferredCacheListDescriptor(
+            orig,
+            cached_method_name=self.cached_method_name,
+            list_name=self.list_name,
+            num_args=self.num_args,
+            name=self.name,
+        )
+        return cast(CachedFunction[F], d)
 
 
 def cachedList(
@@ -498,7 +547,7 @@ def cachedList(
     list_name: str,
     num_args: Optional[int] = None,
     name: Optional[str] = None,
-) -> Callable[[F], CachedFunction[F]]:
+) -> _CachedListFunctionDescriptor:
     """Creates a descriptor that wraps a function in a `DeferredCacheListDescriptor`.
 
     Used to do batch lookups for an already created cache. One of the arguments
@@ -527,16 +576,13 @@ def cachedList(
             def batch_do_something(self, first_arg, second_args):
                 ...
     """
-    func = lambda orig: DeferredCacheListDescriptor(
-        orig,
+    return _CachedListFunctionDescriptor(
         cached_method_name=cached_method_name,
         list_name=list_name,
         num_args=num_args,
         name=name,
     )
 
-    return cast(Callable[[F], CachedFunction[F]], func)
-
 
 def _get_cache_key_builder(
     param_names: Sequence[str],