private WALFlushListener getResult() {
    switch (status) {
      case SUCCESS:
        return successListener;
      case FAILURE:
      default:
        return failListener;
    }
  }
This class provides fake wal node when wal is disabled or exception happens.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/wal/node/WALFakeNode.java