@Override
    public void abort(Throwable cause) {
        sources.abort(cause);
        final OuterSubscriber<T> outerSubscriber = this.outerSubscriber;
        if (outerSubscriber != null) {
            outerSubscriber.abort(cause);
        }
    }
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