2 files changed, 25 insertions, 0 deletions
diff --git a/synapse/rest/admin/rooms.py b/synapse/rest/admin/rooms.py
index 9d953d58de..68054ffc28 100644
--- a/synapse/rest/admin/rooms.py
+++ b/synapse/rest/admin/rooms.py
@@ -303,6 +303,7 @@ class RoomRestServlet(RestServlet):
 
         members = await self.store.get_users_in_room(room_id)
         ret["joined_local_devices"] = await self.store.count_devices_by_users(members)
+        ret["forgotten"] = await self.store.is_locally_forgotten_room(room_id)
 
         return HTTPStatus.OK, ret
 
diff --git a/synapse/storage/databases/main/roommember.py b/synapse/storage/databases/main/roommember.py
index 5e5f607a14..827c1f1efd 100644
--- a/synapse/storage/databases/main/roommember.py
+++ b/synapse/storage/databases/main/roommember.py
@@ -1215,6 +1215,30 @@ class RoomMemberWorkerStore(EventsWorkerStore):
             "get_forgotten_rooms_for_user", _get_forgotten_rooms_for_user_txn
         )
 
+    async def is_locally_forgotten_room(self, room_id: str) -> bool:
+        """Returns whether all local users have forgotten this room_id.
+
+        Args:
+            room_id: The room ID to query.
+
+        Returns:
+            Whether the room is forgotten.
+        """
+
+        sql = """
+            SELECT count(*) > 0 FROM local_current_membership
+            INNER JOIN room_memberships USING (room_id, event_id)
+            WHERE
+                room_id = ?
+                AND forgotten = 0;
+        """
+
+        rows = await self.db_pool.execute("is_forgotten_room", None, sql, room_id)
+
+        # `count(*)` returns always an integer
+        # If any rows still exist it means someone has not forgotten this room yet
+        return not rows[0][0]
+
     async def get_rooms_user_has_been_in(self, user_id: str) -> Set[str]:
         """Get all rooms that the user has ever been in.