In sync wait for worker to catch up since token (#17215)

Otherwise things will get confused.

An alternative would be to make sure that for lagging stream we don't
return anything (and make sure the returned next_batch token doesn't go
backwards). But that is a faff.

1 files changed, 7 insertions, 0 deletions

diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py
index fd7167904d..f1bd85aa27 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -95,6 +95,10 @@ class DeltaState:
     to_insert: StateMap[str]
     no_longer_in_room: bool = False
 
+    def is_noop(self) -> bool:
+        """Whether this state delta is actually empty"""
+        return not self.to_delete and not self.to_insert and not self.no_longer_in_room
+
 
 class PersistEventsStore:
     """Contains all the functions for writing events to the database.
@@ -1017,6 +1021,9 @@ class PersistEventsStore:
     ) -> None:
         """Update the current state stored in the datatabase for the given room"""
 
+        if state_delta.is_noop():
+            return
+
         async with self._stream_id_gen.get_next() as stream_ordering:
             await self.db_pool.runInteraction(
                 "update_current_state",