Add actual guranteed order for UNION

We use `union all` because we don't need any of the deduplication logic
(`union` is really a union + distinct). `UNION ALL`` does preserve the
ordering of the operand queries but there is no actual gurantee that it
has this behavior in all scenarios so we need the extra `ORDER BY` at the
bottom.

See https://dba.stackexchange.com/questions/316818/are-results-from-union-all-clauses-always-appended-in-order/316835#316835

synapse/storage/databases/main/stream.py

@@ -922,6 +922,13 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
             min_stream = end_token.stream
             max_stream = end_token.get_max_stream_pos()
 
+            # We use `union all` because we don't need any of the deduplication logic
+            # (`union` is really a union + distinct). `UNION ALL`` does preserve the
+            # ordering of the operand queries but there is no actual gurantee that it
+            # has this behavior in all scenarios so we need the extra `ORDER BY` at the
+            # bottom.
+            #
+            # We're using the subquery syntax for SQLite compatibility.
             sql = """
                 SELECT * FROM (
                     SELECT instance_name, stream_ordering, topological_ordering, event_id
@@ -945,6 +952,7 @@ class StreamWorkerStore(EventsWorkerStore, SQLBaseStore):
                     ORDER BY stream_ordering DESC
                     LIMIT 1
                 ) AS b
+                ORDER BY stream_ordering DESC
             """
             txn.execute(
                 sql,

tests/storage/test_stream.py

@@ -449,7 +449,7 @@ class GetLastEventInRoomBeforeStreamOrderingTestCase(HomeserverTestCase):
 
         # Assemble a token that encompasses event1 -> event4 on worker1
         end_token = RoomStreamToken(
-            stream=event_pos1.stream,
+            stream=event_pos2.stream,
             instance_map=immutabledict({"worker1": event_pos4.stream}),
         )