private Blocker(InterruptibleTask<?> task) {
      this.task = task;
    }

    @Override
    public void run() {}

    private void setOwner(Thread thread) {
      super.setExclusiveOwnerThread(thread);
    }

    @VisibleForTesting
    @Nullable Thread getOwner() {
      return super.getExclusiveOwnerThread();
    }

    @Override
    public String toString() {
      return task.toString();

    static class OwnerReentrantLock extends ReentrantLock {
        @Override
        public Thread getOwner() {
            return super.getOwner();
        }
    }

    static class OwnerReentrantReadWriteLock extends ReentrantReadWriteLock {
        @Override
        public Thread getOwner() {
            return super.getOwner();
        }
    }

    private static void warn(String msg) {

            final FileOwnerAttributeView ownerAttrView = Files.getFileAttributeView(file.toPath(), FileOwnerAttributeView.class);
            if (ownerAttrView != null) {
                final UserPrincipal owner = ownerAttrView.getOwner();
                if (owner != null) {
                    responseData.addMetaData(FS_FILE_USER, owner.getName());
                }
            }

  //
  // 3. I think "enterWhen(notEmpty)" reads better than "notEmpty.enter()".
  //
  // TODO(user): Implement ReentrantLock features:
  //    - toString() method
  //    - getOwner() method
  //    - getQueuedThreads() method
  //    - getWaitingThreads(Guard) method
  //    - implement Serializable
  //    - redo the API to be as close to identical to ReentrantLock as possible,