roscpp: getNumSubscribers() does not return 0 when a topic is unsubscribed

[ce047]

Tested in melodic.

It seems that the method getNumSubscribers() does not return 0 when a topic is unsubscribed if a roscpp publisher is not publishing.

How to reproduce: Have a simple publisher running that does not publish anything but only checks the number of subscribers

int main(int argc, char** argv)
{
  ros::init(argc, argv, "test_get_num_subscribers");

  ros::NodeHandle nh;
  ros::Publisher pub = nh.advertise<std_msgs::String>("test_topic", 1000);

  ros::Rate r(1);
  while(ros::ok())
  {
    if (pub.getNumSubscribers() > 0)
    {
      ROS_INFO("Someone is listening :)");
    } else {
      ROS_INFO("I am alone :(");
    }
    ros::spinOnce();
    r.sleep();
  }

  return 0;
}

and then subcribe to and unsubscribe from that topic, e.g, with

rostopic echo /test_topic && date +%s
^C
1555934665

What should happen: The publisher node should recognize the new subscriber, print out Someone is listening :) as long as the subscriber is active, and more or less immediately realize that there are no subscribers any more and switch back to I am alone :(.

What happens: The publisher recognizes a new subscriber but does not notice that the subscriber unsubscribes shortly after:

...
[ INFO] [1555934661.098353493]: I am alone :(
[ INFO] [1555934662.098477395]: I am alone :(
[ INFO] [1555934663.098413898]: Someone is listening :)
[ INFO] [1555934664.098555165]: Someone is listening :)
[ INFO] [1555934665.098341645]: Someone is listening :)  (subscriber is shut off again) 
[ INFO] [1555934666.098435134]: Someone is listening :)
[ INFO] [1555934667.098370738]: Someone is listening :)
[ INFO] [1555934668.098414957]: Someone is listening :)
[ INFO] [1555934669.098372386]: Someone is listening :)
[ INFO] [1555934670.098341077]: Someone is listening :)
...

Only after approx. 120 seconds the publisher realizes that there is no subscriber any longer:

...
[ INFO] [1555934784.098598787]: Someone is listening :)
[ INFO] [1555934785.098545304]: Someone is listening :) (120 seconds later)
[ INFO] [1555934786.098460942]: Someone is listening :)
[ INFO] [1555934787.098624760]: I am alone :(
[ INFO] [1555934788.098622298]: I am alone :(
[ INFO] [1555934789.098441571]: I am alone :(
...

Notes:

• behavior is different when node actually publishes messages when someone listens: then getNumSubscribers() returns 0 immediately after subscribers are gone
• could not reproduce with rospy
• the 120 seconds is quite reproducible, so I guess there is a simple timeout set somewhere to update the subscription info

[dirk-thomas]

Addressed by #1704.