// shutDown or runOneIteration) run concurrently with one another.
    // TODO(lukes): why don't we use ListenableFuture to sequence things? Then we could drop the
    // lock.
    private final ReentrantLock lock = new ReentrantLock();

    @WeakOuter
    class Task implements Runnable {
      @Override
      public void run() {
        lock.lock();
        try {
          /*