Check appservice user interest against the local users instead of all users (get_users_in_room mis-use) (#13958)

synapse/appservice/__init__.py

@@ -172,12 +172,24 @@ class ApplicationService:
         Returns:
             True if this service would like to know about this room.
         """
-        member_list = await store.get_users_in_room(
+        # We can use `get_local_users_in_room(...)` here because an application service
+        # can only be interested in local users of the server it's on (ignore any remote
+        # users that might match the user namespace regex).
+        #
+        # In the future, we can consider re-using
+        # `store.get_app_service_users_in_room` which is very similar to this
+        # function but has a slightly worse performance than this because we
+        # have an early escape-hatch if we find a single user that the
+        # appservice is interested in. The juice would be worth the squeeze if
+        # `store.get_app_service_users_in_room` was used in more places besides
+        # an experimental MSC. But for now we can avoid doing more work and
+        # barely using it later.
+        local_user_ids = await store.get_local_users_in_room(
             room_id, on_invalidate=cache_context.invalidate
         )
 
         # check joined member events
-        for user_id in member_list:
+        for user_id in local_user_ids:
             if self.is_interested_in_user(user_id):
                 return True
         return False