@Override
        public void subscribe(Subscriber<? super T> writeSubscriber) {
            synchronized (this) {
                checkState(this.writeSubscriber == null, "Only one write subscriber supported");
                this.writeSubscriber = writeSubscriber;
                if (error != null || (completed && item == null)) {
                    this.writeSubscriber.onSubscribe(Operators.emptySubscription());
                    emitCachedSignals();
                } else {
                    this.writeSubscriber.onSubscribe(this);
                }
            }
        }
The actual writeSubscriber from the HTTP server adapter.
spring/boot3-webflux-autoconfigure/src/main/java/com/linecorp/armeria/spring/web/reactive/ChannelSendOperator.java