Fix races in room stats (and other) updates. (#6187)

Hopefully this will fix the occasional failures we were seeing in the room directory.

The problem was that events are not necessarily persisted (and `current_state_delta_stream` updated) in the same order as their stream_id. So for instance current_state_delta 9 might be persisted *before* current_state_delta 8. Then, when the room stats saw stream_id 9, it assumed it had done everything up to 9, and never came back to do stream_id 8.

We can solve this easily by only processing up to the stream_id where we know all events have been persisted.

1 files changed, 7 insertions, 5 deletions

diff --git a/synapse/handlers/stats.py b/synapse/handlers/stats.py

index c62b113115..466daf9202 100644
--- a/synapse/handlers/stats.py
+++ b/synapse/handlers/stats.py
@@ -87,21 +87,23 @@ class StatsHandler(StateDeltasHandler):
             # Be sure to read the max stream_ordering *before* checking if there are any outstanding
             # deltas, since there is otherwise a chance that we could miss updates which arrive
             # after we check the deltas.
-            room_max_stream_ordering = yield self.store.get_room_max_stream_ordering()
+            room_max_stream_ordering = self.store.get_room_max_stream_ordering()
             if self.pos == room_max_stream_ordering:
                 break
 
-            deltas = yield self.store.get_current_state_deltas(self.pos)
+            logger.debug(
+                "Processing room stats %s->%s", self.pos, room_max_stream_ordering
+            )
+            max_pos, deltas = yield self.store.get_current_state_deltas(
+                self.pos, room_max_stream_ordering
+            )
 
             if deltas:
                 logger.debug("Handling %d state deltas", len(deltas))
                 room_deltas, user_deltas = yield self._handle_deltas(deltas)
-
-                max_pos = deltas[-1]["stream_id"]
             else:
                 room_deltas = {}
                 user_deltas = {}
-                max_pos = room_max_stream_ordering
 
             # Then count deltas for total_events and total_event_bytes.
             room_count, user_count = yield self.store.get_changes_room_total_events_and_bytes(