title: "Handling signal: ttou" message while running DAG in airflow
body: <p>I have created sample DAG, where I had DAG config as below.</p>
  
  <pre><code>default_args = {
      'owner': 'airflow',         
      'depends_on_past': False, 
      'start_date': one_min_ago,
      'email': ['admin@airflow.com'],
      'email_on_failure': True, 
      'email_on_retry': True, 
      'retries': 5, 
      'retry_delay': timedelta(hours=30))
  </code></pre>
  
  <p>With this when I run airflow webserver I'm getting below message.</p>
  
  <pre><code>/home/af_user/anaconda/lib/python3.5/site-packages/flask/exthook.py:71: 
  ExtDeprecationWarning: Importing flask.ext.cache is deprecated, use 
  flask_cache instead.
   .format(x=modname), ExtDeprecationWarning
  [2017-12-18 12:41:27,967] [17328] {models.py:167} INFO - Filling up the 
  DagBag from /home/af_user/airflow/dags
  [2017-12-18 12:41:28 +0000] [16648] [INFO] Handling signal: ttou
  [2017-12-18 12:41:57 +0000] [16655] [INFO] Worker exiting (pid: 16655)
  </code></pre>
  
  <p>And also DAG will be there in running state only.</p>
  
  <p>Let me know if any once came across this issue and fixed it already.</p>