@Override
        public void onComplete() {
            if (completed) {
                return;
            }

            // If 'innerSubscriber' is not complete, 'downstream' will be completed
            // when 'innerSubscriber' receives `onComplete` signal.
            completed = true;
            if (!inInnerOnSubscribe && innerSubscriber.currentPublisherCompleted) {
                innerSubscriber.onComplete();
            }
        }
Relay items in order from a {@link StreamMessage} of {@link StreamMessage}s as a single {@link StreamMessage}
source.
core/src/main/java/com/linecorp/armeria/common/stream/ConcatPublisherStreamMessage.java