Use new order_by argument to get events in stream order

Author: Fizzadar

Diff for: synapse/handlers/message.py

@@ -175,12 +175,11 @@ async def get_state_events(
         state_filter = state_filter or StateFilter.all()
 
         if at_token:
-            # FIXME this claims to get the state at a stream position, but
-            # get_recent_events_for_room operates by topo ordering. This therefore
-            # does not reliably give you the state at the given stream position.
-            # (https://github.com/matrix-org/synapse/issues/3305)
             last_events, _ = await self.store.get_recent_events_for_room(
-                room_id, end_token=at_token.room_key, limit=1
+                room_id,
+                end_token=at_token.room_key,
+                limit=1,
+                order_by="stream",
             )
 
             if not last_events:

Diff for: synapse/handlers/sync.py

@@ -683,12 +683,11 @@ async def get_state_at(
             stream_position: point at which to get state
             state_filter: The state filter used to fetch state from the database.
         """
-        # FIXME this claims to get the state at a stream position, but
-        # get_recent_events_for_room operates by topo ordering. This therefore
-        # does not reliably give you the state at the given stream position.
-        # (https://github.com/matrix-org/synapse/issues/3305)
         last_events, _ = await self.store.get_recent_events_for_room(
-            room_id, end_token=stream_position.room_key, limit=1
+            room_id,
+            end_token=stream_position.room_key,
+            limit=1,
+            order_by="stream",
         )
 
         if last_events: