From 166848ad0a510b8bbe7e1b38e5a9c13d03bece1b Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Fri, 27 Oct 2023 15:15:31 +0100
Subject: [PATCH 1/2] Reduce amount of caches POSITIONS we send

---
 synapse/replication/tcp/streams/_base.py | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/synapse/replication/tcp/streams/_base.py b/synapse/replication/tcp/streams/_base.py
index 5c4d228f3d7c..58a44029aa75 100644
--- a/synapse/replication/tcp/streams/_base.py
+++ b/synapse/replication/tcp/streams/_base.py
@@ -161,6 +161,14 @@ async def get_updates(self) -> StreamUpdateResult:
             and `limited` is whether there are more updates to fetch.
         """
         current_token = self.current_token(self.local_instance_name)
+
+        # If the minimum current token for the local instance is less than or
+        # equal to the last thing we published, we know that there are no
+        # updates.
+        if self.last_token >= self.minimal_local_current_token():
+            self.last_token = current_token
+            return [], current_token, False
+
         updates, current_token, limited = await self.get_updates_since(
             self.local_instance_name, self.last_token, current_token
         )
@@ -489,6 +497,8 @@ def current_token(self, instance_name: str) -> Token:
         return self.store.get_cache_stream_token_for_writer(instance_name)
 
     def minimal_local_current_token(self) -> Token:
+        if self.store._cache_id_gen:
+            return self.store._cache_id_gen.get_minimal_local_current_token()
         return self.current_token(self.local_instance_name)
 
 

From d4a7aa4e89e1237f2d01e8b118423638a0ea6b3d Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Fri, 27 Oct 2023 15:16:27 +0100
Subject: [PATCH 2/2] Newsfile

---
 changelog.d/16561.bugfix | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/16561.bugfix

diff --git a/changelog.d/16561.bugfix b/changelog.d/16561.bugfix
new file mode 100644
index 000000000000..4f4a0380cd77
--- /dev/null
+++ b/changelog.d/16561.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`.