title: Airflow DAG: my execution "terminated" after first task
body: <p>I had tried a simple DAG to run 3 tasks with sensor on each of them, to run on EMR, which looks like the following:</p>
  <pre><code>EMR_creation -&gt; T1 -&gt; S1 -&gt; T2 -&gt; S2 -&gt; T3 -&gt; S3 -&gt; EMR_termination.
  </code></pre>
  <p>The problem I ran into was that T1-S1 succeeded but T2 never started due the the error:</p>
  <blockquote>
  <p>ERROR - An error occurred (ValidationException) when calling the AddJobFlowSteps operation: A job flow that is shutting down, terminated, or finished may not be modified.</p>
  </blockquote>
  <p>The same error would repeat as I set retry the step while not record in the EMR cluster which make sense.  I had tried to put different task in T1 or T2 but not improvement.</p>