Add function to get all forgotten rooms for user

This will allow us to efficiently filter out rooms that have been
forgotten in other queries without having to join against the
`room_memberships` table.

synapse/storage/roommember.py

@@ -639,6 +639,39 @@ class RoomMemberWorkerStore(EventsWorkerStore):
         count = yield self.runInteraction("did_forget_membership", f)
         defer.returnValue(count == 0)
 
+    @cached()
+    def get_forgotten_rooms_for_user(self, user_id):
+        """Gets all rooms the user has forgotten.
+
+        Args:
+            user_id (str)
+
+        Returns:
+            Deferred[set[str]]
+        """
+
+        def _get_forgotten_rooms_for_user_txn(txn):
+            # This is a slightly convoluted query that first looks up all rooms
+            # that the user has forgotten in the past, then rechecks that list
+            # to see if any have subsequently been updated. This is done so that
+            # we can use a partial index on `forgotten = 1` on the assumption
+            # that few users will actually forget many rooms.
+            sql = """
+                SELECT room_id, (
+                    SELECT count(*) FROM room_memberships
+                    WHERE room_id = m.room_id AND user_id = m.user_id AND forgotten = 0
+                ) AS count
+                FROM room_memberships AS m
+                WHERE user_id = ? AND forgotten = 1
+                GROUP BY room_id, user_id;
+            """
+            txn.execute(sql, (user_id,))
+            return set(row[0] for row in txn if row[1] == 0)
+
+        return self.runInteraction(
+            "get_forgotten_rooms_for_user", _get_forgotten_rooms_for_user_txn
+        )
+
     @defer.inlineCallbacks
     def get_rooms_user_has_been_in(self, user_id):
         """Get all rooms that the user has ever been in.
@@ -670,6 +703,13 @@ class RoomMemberStore(RoomMemberWorkerStore):
             _CURRENT_STATE_MEMBERSHIP_UPDATE_NAME,
             self._background_current_state_membership,
         )
+        self.register_background_index_update(
+            "room_membership_forgotten_idx",
+            index_name="room_memberships_user_room_forgotten",
+            table="room_memberships",
+            columns=["user_id", "room_id"],
+            where_clause="forgotten = 1",
+        )
 
     def _store_room_members_txn(self, txn, events, backfilled):
         """Store a room member in the database.
@@ -771,6 +811,9 @@ class RoomMemberStore(RoomMemberWorkerStore):
             txn.execute(sql, (user_id, room_id))
 
             self._invalidate_cache_and_stream(txn, self.did_forget, (user_id, room_id))
+            self._invalidate_cache_and_stream(
+                txn, self.get_forgotten_rooms_for_user, (user_id,)
+            )
 
         return self.runInteraction("forget_membership", f)
 

synapse/storage/schema/delta/56/room_membership_idx.sql

@@ -0,0 +1,25 @@
+/* Copyright 2019 The Matrix.org Foundation C.I.C.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+-- We add membership to current state so that we don't need to join against
+-- room_memberships, which can be surprisingly costly (we do such queries
+-- very frequently).
+-- This will be null for non-membership events and the content.membership key
+-- for membership events. (Will also be null for membership events until the
+-- background update job has finished).
+
+-- Adds an index on room_memberships for fetching all forgotten rooms for a user
+INSERT INTO background_updates (update_name, progress_json) VALUES
+  ('room_membership_forgotten_idx', '{}');