Reduce DB hits for replication

Some streams will occaisonally advance their positions without actually
having any new rows to send over federation. Currently this means that
the token will not advance on the workers, leading to them repeatedly
sending a slightly out of date token. This in turns requires the master
to hit the DB to check if there are any new rows, rather than hitting
the no op logic where we check if the given token matches the current
token.

This commit changes the API to always return an entry if the position
for a stream has changed, allowing workers to advance their tokens
correctly.

1 files changed, 2 insertions, 2 deletions

diff --git a/synapse/app/pusher.py b/synapse/app/pusher.py
index d59f4a571c..1a6f5507a9 100644
--- a/synapse/app/pusher.py
+++ b/synapse/app/pusher.py
@@ -197,7 +197,7 @@ class PusherServer(HomeServer):
                     yield start_pusher(user_id, app_id, pushkey)
 
             stream = results.get("events")
-            if stream:
+            if stream and stream["rows"]:
                 min_stream_id = stream["rows"][0][0]
                 max_stream_id = stream["position"]
                 preserve_fn(pusher_pool.on_new_notifications)(
@@ -205,7 +205,7 @@ class PusherServer(HomeServer):
                 )
 
             stream = results.get("receipts")
-            if stream:
+            if stream and stream["rows"]:
                 rows = stream["rows"]
                 affected_room_ids = set(row[1] for row in rows)
                 min_stream_id = rows[0][0]