Add fast path for replication events stream fetch (#16580)

We can bail early if the from token is greater than or equal to the
current token.

1 files changed, 6 insertions, 0 deletions

diff --git a/synapse/replication/tcp/streams/events.py b/synapse/replication/tcp/streams/events.py
index 38823113d8..57138fea80 100644
--- a/synapse/replication/tcp/streams/events.py
+++ b/synapse/replication/tcp/streams/events.py
@@ -157,6 +157,12 @@ class EventsStream(_StreamFromIdGen):
         current_token: Token,
         target_row_count: int,
     ) -> StreamUpdateResult:
+        # The events stream cannot be "reset", so its safe to return early if
+        # the from token is larger than the current token (the DB query will
+        # trivially return 0 rows anyway).
+        if from_token >= current_token:
+            return [], current_token, False
+
         # the events stream merges together three separate sources:
         #  * new events
         #  * current_state changes