code: @Override
      final void doRequest(SubscriptionImpl subscription, long unused) {
          if (requested() != 0) {
              // Already have demand so don't need to do anything.
              return;
          }
          setRequested(1);
          notifySubscriberOfCloseEvent(subscription, SUCCESSFUL_CLOSE);
      }
docs: No objects, so just notify of close as soon as there is demand.