@Override
    public void subscribe(Subscriber<? super T> subscriber, EventExecutor executor,
                          SubscriptionOption... options) {
        requireNonNull(subscriber, "subscriber");
        requireNonNull(executor, "executor");
        if (executor.inEventLoop()) {
            subscribe0(subscriber);
        } else {
            executor.execute(() -> subscribe0(subscriber));
        }
    }
A {@link FixedStreamMessage} that publishes no objects, just a close event.
core/src/main/java/com/linecorp/armeria/internal/common/stream/EmptyFixedStreamMessage.java