Add logging to /sync for debugging #11916 (#12068)

Author: erikjohnston

changelog.d/12068.misc

@@ -0,0 +1 @@
+Add some logging to `/sync` to try and track down #11916.

synapse/handlers/sync.py

@@ -697,6 +697,15 @@ async def get_state_at(
         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(