How can a dbus user get notified when the dbus service it is using exits/crashes/restarts?
The tutorial suggests there is a way to do this, but in the specification I only found a signal intended for the name owner.
The dbus tutorial says:
Names have a second important use, other than routing messages. They are used to track lifecycle. When an application exits (or crashes), its connection to the message bus will be closed by the operating system kernel. The message bus then sends out notification messages telling remaining applications that the application's names have lost their owner. By tracking these notifications, your application can reliably monitor the lifetime of other applications.
The dbus specification has a section about the NameLost signal:
org.freedesktop.DBus.NameLost
This signal is sent to a specific application when it loses ownership of a name.
One way to find this out is to listen for the org.freedesktop.DBus.NameOwnerChanged signal, as specified in the D-Bus specification
Your client needs to have some logic implemented to analyse the arguments of the signal to figure out when a name has been claimed, when a service has been restarted, when it is gone etc. But the above signal can be used to receive the relevant information at least.
In your handler function you can check if the name argument matches the service name you want to know about. If the old_owner argument is empty, then the service has just claimed the name on the bus. If new_owner is empty then the service has gone away from the bus (for whatever reason).