Bonds cannot guarantee a termination notification to the other side

[stonier]

At least for the rospy bonds. I haven't looked at the cpp bonds yet.

Relevant Code: the shutdown() method which publishes a message to the sister bond and then promptly unregisters the publisher.

Is there a way that can reliably guarantee that a terminating node can properly shutdown a bond without having to rely on the sister inferring it from a heartbeat that has gone missing?

Seems as though there are two difficulties with this:

• publish(), followed immediately by unregister()...can this disrupt the sending?
• If the bond is shutting down because it's node is shutting down (e.g. ctrl-c), then the call to shutdown() from __del__ may, or may not happen before the ros communications have shut down.

You could put the bond shutdown function in a rospy shutdown hook, but that is only guaranteed for services, not publishers. So the only way to do this properly is if sister bonds had a service that could accept a one-time request from the bond.

If this is actually not possible to guarantee with the current code, ideas? I wouldn't mind putting some time into a PR for this.