Give the correct next event when the message timestamps are the same - MSC3030 (#13658)

Discovered while working on https://github.com/matrix-org/synapse/pull/13589 and I had all the messages at the same timestamp in the tests.

Part of https://github.com/matrix-org/matrix-spec-proposals/pull/3030

Complement tests: https://github.com/matrix-org/complement/pull/457

changelog.d/13658.bugfix

@@ -0,0 +1 @@
+Fix MSC3030 `/timestamp_to_event` endpoint to return the correct next event when the events have the same timestamp.

synapse/storage/databases/main/events_worker.py

@@ -2111,7 +2111,14 @@ class EventsWorkerStore(SQLBaseStore):
                 AND room_id = ?
                 /* Make sure event is not rejected */
                 AND rejections.event_id IS NULL
-            ORDER BY origin_server_ts %s
+            /**
+             * First sort by the message timestamp. If the message timestamps are the
+             * same, we want the message that logically comes "next" (before/after
+             * the given timestamp) based on the DAG and its topological order (`depth`).
+             * Finally, we can tie-break based on when it was received on the server
+             * (`stream_ordering`).
+             */
+            ORDER BY origin_server_ts %s, depth %s, stream_ordering %s
             LIMIT 1;
         """
 
@@ -2130,7 +2137,8 @@ class EventsWorkerStore(SQLBaseStore):
                 order = "ASC"
 
             txn.execute(
-                sql_template % (comparison_operator, order), (timestamp, room_id)
+                sql_template % (comparison_operator, order, order, order),
+                (timestamp, room_id),
             )
             row = txn.fetchone()
             if row: