@Override
  public synchronized void notifyOnCancel(RpcCallback<Object> callback, CancellationCallback action)
    throws IOException {
    if (cancelled) {
      action.run(true);
    } else {
      cancellationCbs.add(callback);
      action.run(false);
    }
  }
Returns One-shot cell scanner (you cannot back it up and restart)
hbase-client/src/main/java/org/apache/hadoop/hbase/ipc/HBaseRpcControllerImpl.java