@Override
  public Throwable getFailure() {
    return iterator.getFailure();
  }
If isBlocked is null, it means the next is not fetched yet. If isBlocked.isDone() is false, it
means the next is being fetched. If isBlocked.get() is true, it means hasNext, otherwise, it
means no more info to be fetched.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/schemaengine/schemaregion/read/resp/reader/impl/TimeseriesReaderWithViewFetch.java