private void cancel0() {
            if (cancelled) {
                return;
            }
            cancelled = true;

            // `this` is published after subscription assigned. So cancel on `this` always has subscription.
            assert subscription != null;
            subscription.cancel();

            if (currentExposedBodyPartPublisher != null) {
                currentExposedBodyPartPublisher.abort(CancelledSubscriptionException.get());
            }
            cleanup();
        }
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