Listener gasListeners(AbstractFuture<?> future, Listener update) {
      return (Listener) UNSAFE.getAndSetObject(future, LISTENERS_OFFSET, update);
    }

    /** Performs a GAS operation on the {@link #waiters} field. */
    @Override
    Waiter gasWaiters(AbstractFuture<?> future, Waiter update) {
      return (Waiter) UNSAFE.getAndSetObject(future, WAITERS_OFFSET, update);
    }