Give the correct next event when the message timestamps are the same

Discovered while working on #13589
and I had all the messages at the same timestamp

Part of matrix-org/matrix-spec-proposals#3030

Author: MadLittleMods

synapse/storage/databases/main/events_worker.py

@@ -2111,10 +2111,19 @@ async def get_event_id_for_timestamp(
                 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;
         """
 
+        #
+
         def get_event_id_for_timestamp_txn(txn: LoggingTransaction) -> Optional[str]:
             if direction == "b":
                 # Find closest event *before* a given timestamp. We use descending
@@ -2130,7 +2139,8 @@ def get_event_id_for_timestamp_txn(txn: LoggingTransaction) -> Optional[str]:
                 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: