void clearSubscriber() {
            // Replace the subscriber with a placeholder so that it can be garbage-collected and
            // we conform to the Reactive Streams specification rule 3.13.
            if (!(subscriber instanceof AbortingSubscriber)) {
                subscriber = NeverInvokedSubscriber.get();
            }
        }
{@link CancelledSubscriptionException} if cancelled. {@link Throwable} if aborted.
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessageDuplicator.java