public void addListener(Runnable listener, Executor executor) {
        whenCompleteAsync((__, ___) -> listener.run(), executor);
    }
A {@link CompletableFuture} where {@link #get()}/{@link #get(long, TimeUnit)} may have side-effects (e.g trigger the underlying computation).

@author bjorncs
container-core/src/main/java/com/yahoo/processing/impl/ProcessingFuture.java