// nothing
    }

    /**
     * A list of Throwables representing failures that occurred during the operation.
     */
    protected List<Throwable> failures = Collections.synchronizedList(new ArrayList<>());

    /**
     * Adds a Throwable to the list of failures.
     *
     * @param t the Throwable to add
     */
    public void addFailure(final Throwable t) {
        failures.add(t);

  private static final LazyLogger logger = new LazyLogger(ListenerCallQueue.class);

  // TODO(chrisn): promote AppendOnlyCollection for use here.
  private final List<PerListenerQueue<L>> listeners =
      Collections.synchronizedList(new ArrayList<PerListenerQueue<L>>());

  /** Method reference-compatible listener event. */
  interface Event<L> {
    /** Call a method on the listener. */
    void call(L listener);
  }

  /**