@Override
        public void cancel() {
            if (cancelledUpdater.compareAndSet(this, 0, 1)) {
                if (outerSubscriber != null) {
                    final Subscription upstream = outerSubscriber.upstream;
                    if (upstream != null) {
                        upstream.cancel();
                    }
                }
                super.cancel();
                final CancelledSubscriptionException cause = CancelledSubscriptionException.get();
                publisher.abort(cause);
                if (containsNotifyCancellation(options) && !currentPublisherCompleted && !error) {
                    downstream.onError(cause);
                }
                downstream = NoopSubscriber.get();
            }
        }
Sets the {@link OuterSubscriber}.
This should be set before subscribing to upstream.
core/src/main/java/com/linecorp/armeria/common/stream/ConcatPublisherStreamMessage.java