Delete event_push_summary_unique_index again. (#14669)

if a Synapse deployment upgraded (from < 1.62.0 to >= 1.70.0) then it
is possible for schema deltas to run before background updates causing
drift in the database schema due to:

1. A delta registered a background update to create an index.
2. A delta dropped the above index if it exists (but it yet exist won't since
  the background job hasn't run).
3. The code assumed the index was dropped.

To fix this we:

1. Cancel the background update which could create the index.
2. Drop the index again.
3. Drop a related index which is dropped by the background update.

changelog.d/14669.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in Synapse 1.70.0 which could cause spurious `UNIQUE constraint failed` errors in the `rotate_notifs` background job.

synapse/storage/databases/main/event_push_actions.py

@@ -274,15 +274,6 @@ def __init__(
                 self._clear_old_push_actions_staging, 30 * 60 * 1000
             )
 
-        self.db_pool.updates.register_background_index_update(
-            "event_push_summary_unique_index",
-            index_name="event_push_summary_unique_index",
-            table="event_push_summary",
-            columns=["user_id", "room_id"],
-            unique=True,
-            replaces_index="event_push_summary_user_rm",
-        )
-
         self.db_pool.updates.register_background_index_update(
             "event_push_summary_unique_index2",
             index_name="event_push_summary_unique_index2",

synapse/storage/schema/main/delta/73/23_fix_thread_index.sql

@@ -0,0 +1,33 @@
+/* Copyright 2022 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.
+ */
+
+-- If a Synapse deployment made a large jump in versions (from < 1.62.0 to >= 1.70.0)
+-- in a single upgrade then it might be possible for the event_push_summary_unique_index
+-- to be created in the background from delta 71/02event_push_summary_unique.sql after
+-- delta 73/06thread_notifications_thread_id_idx.sql is executed, causing it to
+-- not drop the event_push_summary_unique_index index.
+--
+-- See https://github.com/matrix-org/synapse/issues/14641
+
+-- Stop the index from being scheduled for creation in the background.
+DELETE FROM background_updates WHERE update_name = 'event_push_summary_unique_index';
+
+-- The above background job also replaces another index, so ensure that side-effect
+-- is applied.
+DROP INDEX IF EXISTS event_push_summary_user_rm;
+
+-- Fix deployments which ran the 73/06thread_notifications_thread_id_idx.sql delta
+-- before the event_push_summary_unique_index background job was run.
+DROP INDEX IF EXISTS event_push_summary_unique_index;