private void doSubscribe(DownstreamSubscription<T> subscription) {
            if (state == State.ABORTED) {
                subscription.failLateProcessorSubscriber();
                return;
            }

            downstreamSubscriptions.add(subscription);
            unsubscribedUpdater.decrementAndGet(duplicator);
            if (upstreamSubscription != null) {
                subscription.invokeOnSubscribe();
            }
        }
Returns the default {@link EventExecutor} which will be used when a user subscribes to a child
stream using {@link StreamMessage#subscribe(Subscriber, SubscriptionOption...)}.
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessageDuplicator.java