/**
   * Register a pending Future that has a callback.
   *
   * @param future must have at least one callback
   */
  public static void addPending(Future<?> future) {
    Pending pend = pending.get();
    if (pend == null) {
      pend = new Pending();
      pending.set(pend);
    }

    pend.add(future);
  }