@Override
        public void onSubscribe(Subscription s) {
            upstream = s;
            try {
                beforeSubscribe(delegate, this);
            } catch (Throwable ex) {
                s.cancel();
                logger.warn("Unexpected exception from {}#beforeSubscribe()",
                            FilteredStreamMessage.this.getClass().getName(), ex);
                return;
            }

            delegate.onSubscribe(this);
        }
A callback executed when this {@link StreamMessage} is canceled by the {@link Subscriber}.
core/src/main/java/com/linecorp/armeria/common/stream/FilteredStreamMessage.java