@TestOnly
  public Map<Long, Set<Long>> getMemTableIdsOfWal() {
    return memTableIdsOfWal;
  }
This task syncs syncingBuffer to disk. The precondition is that syncingBuffer cannot be null.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/storageengine/dataregion/wal/buffer/WALBuffer.java