return UNSAFE.compareAndSwapObject(future, LISTENERS_OFFSET, expect, update);
    }

    /** Performs a GAS operation on the {@link #listeners} field. */
    @Override
    Listener gasListeners(AbstractFuture<?> future, Listener update) {
      while (true) {
        Listener listener = future.listeners;
        if (update == listener) {

 * guaranteed to execute. There is no guarantee, however, that listeners will be executed in the
 * order that they are added.
 *
 * <p>Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown
 * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException} or an exception