public void addSourceTaskFailureListener(FragmentInstanceFailureListener listener) {
    Map<FragmentInstanceId, Throwable> failures;
    synchronized (this) {
      sourceInstanceFailureListeners.add(listener);
      failures = ImmutableMap.copyOf(sourceInstanceFailures);
    }
    executor.execute(() -> failures.forEach(listener::onTaskFailed));
  }
Listener is always notified asynchronously using a dedicated notification thread pool so, care
should be taken to avoid leaking {@code this} when adding a listener in a constructor.
Additionally, it is possible notifications are observed out of order due to the asynchronous
execution.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/fragment/FragmentInstanceStateMachine.java