explicitly drain events when gossip/heartbeat will not - fix for #1847

kudos for @sabw8217 who's fix i copied verbatim.

celery/tests/worker/test_loops.py

@@ -119,6 +119,10 @@ def add(x, y):
             return x + y
         self.add = add
 
+    def test_drain_after_consume(self):
+        x, _ = get_task_callback(self.app)
+        x.connection.drain_events.assert_called_with()
+
     def test_setup_heartbeat(self):
         x = X(self.app, heartbeat=10)
         x.hub.call_repeatedly = Mock(name='x.hub.call_repeatedly()')

celery/worker/loops.py

@@ -47,6 +47,11 @@ def asynloop(obj, connection, consumer, blueprint, hub, qos,
     if not obj.restart_count and not obj.pool.did_start_ok():
         raise WorkerLostError('Could not start worker processes')
 
+    # consumer.consume() may have prefetched up to our
+    # limit - drain an event so we are in a clean state
+    # prior to starting our event loop.
+    connection.drain_events()
+
     # FIXME: Use loop.run_forever
     # Tried and works, but no time to test properly before release.
     hub.propagate_errors = errors