@VisibleForTesting
  List<StateChangeListener<T>> getStateChangeListeners() {
    synchronized (lock) {
      return ImmutableList.copyOf(stateChangeListeners);
    }
  }
Adds a listener to be notified when the state instance changes according to {@code .equals()}.
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. The listener is notified immediately of the current state.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/StateMachine.java