final void notifySubscriber() {
        final SubscriptionImpl subscription = this.subscription;
        if (subscription == null) {
            return;
        }

        if (subscription.subscriber() instanceof AbortingSubscriber) {
            // The stream is being aborted.
            return;
        }

        if (queue.isEmpty()) {
            return;
        }

        if (subscription.needsDirectInvocation()) {
            notifySubscriber0();
        } else {
            subscription.executor().execute(this::notifySubscriber0);
        }
    }
Invoked whenever a new demand is requested.
@param n Newly requested demand
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessage.java