From a711ae78a8f8ba406ff122035c8bf096fac9a26c Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Wed, 23 Feb 2022 14:22:22 +0000
Subject: [PATCH] Add logging to `/sync` for debugging #11916 (#12068)

---
 changelog.d/12068.misc   | 1 +
 synapse/handlers/sync.py | 9 +++++++++
 2 files changed, 10 insertions(+)
 create mode 100644 changelog.d/12068.misc

diff --git a/changelog.d/12068.misc b/changelog.d/12068.misc
new file mode 100644
index 000000000..72b211e4f
--- /dev/null
+++ b/changelog.d/12068.misc
@@ -0,0 +1 @@
+Add some logging to `/sync` to try and track down #11916.
diff --git a/synapse/handlers/sync.py b/synapse/handlers/sync.py
index 98eaad331..0aa3052fd 100644
--- a/synapse/handlers/sync.py
+++ b/synapse/handlers/sync.py
@@ -697,6 +697,15 @@ class SyncHandler:
         else:
             # no events in this room - so presumably no state
             state = {}
+
+            # (erikj) This should be rarely hit, but we've had some reports that
+            # we get more state down gappy syncs than we should, so let's add
+            # some logging.
+            logger.info(
+                "Failed to find any events in room %s at %s",
+                room_id,
+                stream_position.room_key,
+            )
         return state
 
     async def compute_summary(