Delete delivered to-device messages earlier in /sync

changelog.d/10124.misc

@@ -0,0 +1 @@
+Work to improve the responsiveness of `/sync` requests.

synapse/handlers/sync.py

@@ -315,6 +315,17 @@ async def _wait_for_sync_for_user(
  if context:
  context.tag = sync_type
 
+ # if we have a since token, delete any to-device messages before that token
+ # (since we now know that the device has received them)
+ if since_token is not None:
+ since_stream_id = since_token.to_device_key
+ deleted = await self.store.delete_messages_for_device(
+ sync_config.user.to_string(), sync_config.device_id, since_stream_id
+ )
+ logger.debug(
+ "Deleted %d to-device messages up to %d", deleted, since_stream_id
+ )
+
  if timeout == 0 or since_token is None or full_state:
  # we are going to return immediately, so don't bother calling
  # notifier.wait_for_events.
@@ -1230,16 +1241,6 @@ async def _generate_sync_entry_for_to_device(
  since_stream_id = int(sync_result_builder.since_token.to_device_key)
 
  if since_stream_id != int(now_token.to_device_key):
- # We only delete messages when a new message comes in, but that's
- # fine so long as we delete them at some point.
-
- deleted = await self.store.delete_messages_for_device(
- user_id, device_id, since_stream_id
- )
- logger.debug(
- "Deleted %d to-device messages up to %d", deleted, since_stream_id
- )
-
  messages, stream_id = await self.store.get_new_messages_for_device(
  user_id, device_id, since_stream_id, now_token.to_device_key
  )