Fixup pusher pool notifications (#8287)

`pusher_pool.on_new_notifications` expected a min and max stream ID, however that was not what we were passing in. Instead, let's just pass it the current max stream ID and have it track the last stream ID it got passed.

I believe that it mostly worked as we called the function for every event. However, it would break for events that got persisted out of order, i.e, that were persisted but the max stream ID wasn't incremented as not all preceding events had finished persisting, and push for that event would be delayed until another event got pushed to the effected users.

synapse/push/pusherpool.py

@@ -64,6 +64,12 @@ class PusherPool:
         self._pusher_shard_config = hs.config.push.pusher_shard_config
         self._instance_name = hs.get_instance_name()
 
+        # Record the last stream ID that we were poked about so we can get
+        # changes since then. We set this to the current max stream ID on
+        # startup as every individual pusher will have checked for changes on
+        # startup.
+        self._last_room_stream_id_seen = self.store.get_room_max_stream_ordering()
+
         # map from user id to app_id:pushkey to pusher
         self.pushers = {}  # type: Dict[str, Dict[str, Union[HttpPusher, EmailPusher]]]
 
@@ -178,20 +184,27 @@ class PusherPool:
                 )
                 await self.remove_pusher(p["app_id"], p["pushkey"], p["user_name"])
 
-    async def on_new_notifications(self, min_stream_id, max_stream_id):
+    async def on_new_notifications(self, max_stream_id):
         if not self.pushers:
             # nothing to do here.
             return
 
+        if max_stream_id < self._last_room_stream_id_seen:
+            # Nothing to do
+            return
+
+        prev_stream_id = self._last_room_stream_id_seen
+        self._last_room_stream_id_seen = max_stream_id
+
         try:
             users_affected = await self.store.get_push_action_users_in_range(
-                min_stream_id, max_stream_id
+                prev_stream_id, max_stream_id
             )
 
             for u in users_affected:
                 if u in self.pushers:
                     for p in self.pushers[u].values():
-                        p.on_new_notifications(min_stream_id, max_stream_id)
+                        p.on_new_notifications(max_stream_id)
 
         except Exception:
             logger.exception("Exception in pusher on_new_notifications")