title: Airflow : ExternalTaskSensor doesn't trigger the task
body: <p>I have already seen <a href="https://stackoverflow.com/questions/46807297/airflow-externaltasksensor-gets-stuck">this</a> and <a href="https://stackoverflow.com/questions/52796647/airflow-external-sensor-gets-stuck-at-poking">this</a> questions on SO and made the changes accordingly. However, my dependent DAG still gets stuck in poking state. Below is my master DAG:</p>
  
  <pre><code>from airflow import DAG
  from airflow.operators.jdbc_operator import JdbcOperator
  from datetime import datetime
  from airflow.operators.bash_operator import BashOperator
  
  today = datetime.today()
  
  default_args = {
      'depends_on_past': False,
      'retries': 0,
      'start_date': datetime(today.year, today.month, today.day),
      'schedule_interval': '@once'
  }
  
  dag = DAG('call-procedure-and-bash', default_args=default_args)
  
  call_procedure = JdbcOperator(
      task_id='call_procedure',
      jdbc_conn_id='airflow_db2',
      sql='CALL AIRFLOW.TEST_INSERT (20)',
      dag=dag
  )
  
  call_procedure
  </code></pre>
  
  <p>Below is my dependent DAG:</p>
  
  <pre><code>from airflow import DAG
  from airflow.operators.jdbc_operator import JdbcOperator
  from datetime import datetime, timedelta
  from airflow.sensors.external_task_sensor import ExternalTaskSensor
  
  today = datetime.today()
  
  default_args = {
      'depends_on_past': False,
      'retries': 0,
      'start_date': datetime(today.year, today.month, today.day),
      'schedule_interval': '@once'
  }
  
  dag = DAG('external-dag-upstream', default_args=default_args)
  
  task_sensor = ExternalTaskSensor(
      task_id='link_upstream',
      external_dag_id='call-procedure-and-bash',
      external_task_id='call_procedure',
      execution_delta=timedelta(minutes=-2),
      dag=dag
  )
  
  count_rows = JdbcOperator(
      task_id='count_rows',
      jdbc_conn_id='airflow_db2',
      sql='SELECT COUNT(*) FROM AIRFLOW.TEST',
      dag=dag
  )
  
  count_rows.set_upstream(task_sensor)
  </code></pre>
  
  <p>Below are the logs of dependent DAG once the master DAG gets executed:</p>
  
  <pre><code>[2019-01-10 11:43:52,951] {{external_task_sensor.py:91}} INFO - Poking for call-procedure-and-bash.call_procedure on 2019-01-10T11:45:47.893735+00:00 ... 
  [2019-01-10 11:44:52,955] {{external_task_sensor.py:91}} INFO - Poking for call-procedure-and-bash.call_procedure on 2019-01-10T11:45:47.893735+00:00 ... 
  [2019-01-10 11:45:52,961] {{external_task_sensor.py:91}} INFO - Poking for call-procedure-and-bash.call_procedure on 2019-01-10T11:45:47.893735+00:00 ... 
  [2019-01-10 11:46:52,949] {{external_task_sensor.py:91}} INFO - Poking for call-procedure-and-bash.call_procedure on 2019-01-10T11:45:47.893735+00:00 ... 
  [2019-01-10 11:47:52,928] {{external_task_sensor.py:91}} INFO - Poking for call-procedure-and-bash.call_procedure on 2019-01-10T11:45:47.893735+00:00 ... 
  [2019-01-10 11:48:52,928] {{external_task_sensor.py:91}} INFO - Poking for call-procedure-and-bash.call_procedure on 2019-01-10T11:45:47.893735+00:00 ... 
  [2019-01-10 11:49:52,905] {{external_task_sensor.py:91}} INFO - Poking for call-procedure-and-bash.call_procedure on 2019-01-10T11:45:47.893735+00:00 ... 
  </code></pre>
  
  <p>Below are the logs of master DAG execution:</p>
  
  <pre><code>[2019-01-10 11:45:20,215] {{jdbc_operator.py:56}} INFO - Executing: CALL AIRFLOW.TEST_INSERT (20)
  [2019-01-10 11:45:21,477] {{logging_mixin.py:95}} INFO - [2019-01-10 11:45:21,476] {{dbapi_hook.py:166}} INFO - CALL AIRFLOW.TEST_INSERT (20)
  [2019-01-10 11:45:24,139] {{logging_mixin.py:95}} INFO - [2019-01-10 11:45:24,137] {{jobs.py:2627}} INFO - Task exited with return code 0
  </code></pre>
  
  <p>My assumption is, Airflow should trigger the dependent DAG if the master runs fine? I have tried playing around with <code>execution_delta</code> but that doesn't seem to work.</p>
  
  <p>Also, <code>schedule_interval</code> and <code>start_date</code> are same for both of the DAGs so don't think that should cause any trouble.</p>
  
  <p>Am I missing anything?</p>