SyncFuture unloadSync() {
    SyncFuture sync = this.sync;
    this.sync = null;
    this.type = Type.EMPTY;
    return sync;
  }
Unload the truck of its {@link SyncFuture} payload. The internal reference is released.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/RingBufferTruck.java