private void subscribe(Subscriber<? super U> subscriber, EventExecutor executor,
                           boolean withPooledObjects, boolean notifyCancellation) {
        final FilteringSubscriber filteringSubscriber = new FilteringSubscriber(
                subscriber, withPooledObjects);
        final SubscriptionOption[] options = toSubscriptionOptions(filterSupportsPooledObjects,
                                                                   notifyCancellation);
        upstream.subscribe(filteringSubscriber, executor, options);
    }
A callback executed when this {@link StreamMessage} is canceled by the {@link Subscriber}.
core/src/main/java/com/linecorp/armeria/common/stream/FilteredStreamMessage.java