title: Celery throwing BacklogLimitExceeded
body: <p>I start a task that updates it's state periodically and watch the result however after the second cycle on the calling side the program throws a BacklogLimitExceeded exception (the task itself finishes successfully after a while)</p>
  
  <p>calling side:</p>
  
  <pre><code>      task = signature("worker.taskname", args=(url, ), queue="worker")
        g = group(task).apply_async() 
        while not g.ready():
            print(g[0].result)
            time.sleep(5)
  </code></pre>
  
  <p>task side:</p>
  
  <pre><code> with open(filename, "wb") as w:
       fd = stream.open()
       while True:
           data = fd.read(2048)
           if data:
               w.write(data)
               size = w.tell()
               # taskname.update_state(meta={'size': size})
           else:
               break
  </code></pre>
  
  <p>(if I comment that line out everything works fine)</p>
  
  <p>I'm on Ubuntu 14.04 and using RabbitMQ as a broker and as a backend too. Any idea how to fix this?</p>
  
  <p>Here is the exact stracktrace</p>
  
  <pre><code>Traceback (most recent call last):
    File "main.py", line 55, in &lt;module&gt;
      while not g.ready():
    File "python3.4/site-packages/celery/result.py", line 503, in ready
      return all(result.ready() for result in self.results)
    File "python3.4/site-packages/celery/result.py", line 503, in &lt;genexpr&gt;
      return all(result.ready() for result in self.results)
    File "python3.4/site-packages/celery/result.py", line 259, in ready
      return self.state in self.backend.READY_STATES
    File "python3.4/site-packages/celery/result.py", line 394, in state
      return self._get_task_meta()['status']
    File "python3.4/site-packages/celery/result.py", line 339, in _get_task_meta
      return self._maybe_set_cache(self.backend.get_task_meta(self.id))
    File "python3.4/site-packages/celery/backends/amqp.py", line 180, in get_task_meta
      raise self.BacklogLimitExceeded(task_id)
  celery.backends.amqp.BacklogLimitExceeded: 0a4fb653-0f05-48dc-ac43-fb0c8fbaba9a
  </code></pre>