* identify deadlocks.
   */
  @VisibleForTesting
  static final class Blocker extends AbstractOwnableSynchronizer implements Runnable {
    private final InterruptibleTask<?> task;

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

    @Override
    public void run() {}

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

         * problems can exist with methods like FutureTask.done(), not to mention slow calls to
         * Thread.interrupt() (as discussed in InterruptibleTask). At the end of the day, it's
         * unlikely that cancel() will be slow, so we can probably get away with calling it while
         * holding a lock. Still, it would be nice to avoid somehow.
         */