Expose the url of a location as service properties

[laeubi]

Currently SystemBundleActivator registers all Locations with a specific type, but there is one important property that is missing the URL of the location.

As an example the instance location currently could be acquired by (&(objectClass=org.eclipse.osgi.service.datalocation.Location)(type=osgi.instance.area)) but then the user is quite lost if the location is currently not set. As the URL might later change, a component wanted to access the location as soon as it is initialized is out of luck...

My proposal would be to simply expose the URL as a service property, that means when the URL is initial not set there is no url property and when it is set the service properties are updated.

That way one might filter for (&(objectClass=org.eclipse.osgi.service.datalocation.Location)(type=osgi.instance.area)(url=*))

this is related to eclipse-platform/eclipse.platform.resources#52 where the resources plugin need the instance area to be initialized. At the moment this leads to a NPE when accessing the location for example here: eclipse-platform/eclipse.platform.runtime#23

[laeubi]

Thanks for the review and merges :-)