Fix schema delta error in 1.85 (#15739)

Some users seem to have multiple rows per user / room with a null thread
ID, which we need to handle.

changelog.d/15739.bugfix

@@ -0,0 +1 @@
+Fix bug in schema delta that broke upgrades for some deployments. Introduced in v1.85.0.

synapse/storage/schema/main/delta/77/05thread_notifications_backfill.sql

@@ -23,13 +23,25 @@ UPDATE event_push_actions_staging SET thread_id = 'main' WHERE thread_id IS NULL
 UPDATE event_push_actions SET thread_id = 'main' WHERE thread_id IS NULL;
 
 -- Empirically we can end up with entries in the push summary table with both a
--- `NULL` and `main` thread ID, which causes the update below to fail. We fudge
+-- `NULL` and `main` thread ID, which causes the insert below to fail. We fudge
 -- this by deleting any `NULL` rows that have a corresponding `main`.
 DELETE FROM event_push_summary AS a WHERE thread_id IS NULL AND EXISTS (
     SELECT 1 FROM event_push_summary AS b
     WHERE b.thread_id = 'main' AND a.user_id = b.user_id AND a.room_id = b.room_id
 );
-UPDATE event_push_summary SET thread_id = 'main' WHERE thread_id IS NULL;
+-- Copy the NULL threads to have a 'main' thread ID.
+--
+-- Note: Some people seem to have duplicate rows with a `NULL` thread ID, in
+-- which case we just fudge it with using MAX of the values. The counts *may* be
+-- wrong for such rooms, but a) its an edge case, and b) they'll be fixed when
+-- the user reads the room.
+INSERT INTO event_push_summary (user_id, room_id, notif_count, stream_ordering, unread_count, last_receipt_stream_ordering, thread_id)
+    SELECT user_id, room_id, MAX(notif_count), MAX(stream_ordering), MAX(unread_count), MAX(last_receipt_stream_ordering), 'main'
+    FROM event_push_summary
+    WHERE thread_id IS NULL
+    GROUP BY user_id, room_id, thread_id;
+
+DELETE FROM event_push_summary AS a WHERE thread_id IS NULL;
 
 -- Drop the background updates to calculate the indexes used to find null thread_ids.
 DELETE FROM background_updates WHERE update_name = 'event_push_actions_thread_id_null';