1 files changed, 24 insertions, 0 deletions
diff --git a/synapse/storage/databases/main/roommember.py b/synapse/storage/databases/main/roommember.py
index cc528fcf2d..e222b7bd1f 100644
--- a/synapse/storage/databases/main/roommember.py
+++ b/synapse/storage/databases/main/roommember.py
@@ -670,6 +670,30 @@ class RoomMemberWorkerStore(EventsWorkerStore):
 
         return user_who_share_room
 
+    @cached(cache_context=True, iterable=True)
+    async def get_mutual_rooms_between_users(
+        self, user_ids: FrozenSet[str], cache_context: _CacheContext
+    ) -> FrozenSet[str]:
+        """
+        Returns the set of rooms that all users in `user_ids` share.
+
+        Args:
+            user_ids: A frozen set of all users to investigate and return
+              overlapping joined rooms for.
+            cache_context
+        """
+        shared_room_ids: Optional[FrozenSet[str]] = None
+        for user_id in user_ids:
+            room_ids = await self.get_rooms_for_user(
+                user_id, on_invalidate=cache_context.invalidate
+            )
+            if shared_room_ids is not None:
+                shared_room_ids &= room_ids
+            else:
+                shared_room_ids = room_ids
+
+        return shared_room_ids or frozenset()
+
     async def get_joined_users_from_context(
         self, event: EventBase, context: EventContext
     ) -> Dict[str, ProfileInfo]: