private void request0(long n) {
            final long oldDemand = demandOfMultipart;
            demandOfMultipart = LongMath.saturatedAdd(oldDemand, n);
            if (oldDemand == 0) {
                // We want first body publisher
                if (currentExposedBodyPartPublisher == null) {
                    assert subscription != null;
                    //This will trigger DecodedHttpStreamMessage's upstream.
                    subscription.request(1);
                } else {
                    // Just wait bodyPart finish.
                }
            }
        }
For hiding Subscriber interface from MultipartDecoder.
Subscribing {@code DecodedHttpStreamMessage<BodyPart>} and delegating to external subscriber.
core/src/main/java/com/linecorp/armeria/common/multipart/MultipartDecoder.java