title: How to limit Airflow to run only one instance of a DAG run at a time?
body: <p>I want the tasks in the DAG to all finish before the 1st task of the next run gets executed.</p>
  <p>I have max_active_runs = 1, but <a href="https://i.stack.imgur.com/sxe54.png" rel="noreferrer">this</a> still happens.</p>
  <pre><code>default_args = {
      'depends_on_past': True,
      'wait_for_downstream': True,
      'max_active_runs': 1,
      'start_date': datetime(2018, 03, 04),
      'owner': 't.n',
      'email': ['t.n@example.com'],
      'email_on_failure': True,
      'email_on_retry': False,
      'retries': 3,
      'retry_delay': timedelta(minutes=4)
  }
  
  dag = DAG('example', default_args=default_args, schedule_interval = schedule_interval)
  </code></pre>
  <p>(All of my tasks are dependent on the previous task. Airflow version is 1.8.0)</p>
  <p>Thank you</p>