@Override
    final void cancel() {
        if (setState(State.OPEN, State.CLEANUP) || setState(State.CLOSED, State.CLEANUP)) {
            // It the state was CLOSED, close() or close(cause) has been called before cancel() or abort()
            // is called. We just ignore the previously pushed event and deal with CANCELLED_CLOSE.
            final SubscriptionImpl subscription = this.subscription;
            assert subscription != null;
            notifySubscriberOfCloseEvent(subscription, CANCELLED_CLOSE);
        }
    }
Invoked whenever a new demand is requested.
@param n Newly requested demand
core/src/main/java/com/linecorp/armeria/common/stream/DefaultStreamMessage.java