From 408c13801a244a89d23f9c8e8ccce1b3d049abb6 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erikj@matrix.org>
Date: Mon, 30 Oct 2023 14:47:57 +0000
Subject: [PATCH] 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.
---
changelog.d/16580.bugfix | 1 +
synapse/replication/tcp/streams/events.py | 6 ++++++
2 files changed, 7 insertions(+)
create mode 100644 changelog.d/16580.bugfix
diff --git a/changelog.d/16580.bugfix b/changelog.d/16580.bugfix
new file mode 100644
index 0000000000..4f4a0380cd
--- /dev/null
+++ b/changelog.d/16580.bugfix
@@ -0,0 +1 @@
+Fix a long-standing, exceedingly rare edge case where the first event persisted by a new event persister worker might not be sent down `/sync`.
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
--
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