void requestUpstreamForBodyPartData() {
            if (executor.inEventLoop()) {
                requestUpstreamForBodyPartData0();
            } else {
                executor.execute(this::requestUpstreamForBodyPartData0);
            }
        }
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