fix interactions between missing/deleted params and param subscription (get_param_cached)

[jobafr]

Fixes:

• subscribing to non-existent params (maybe they've not yet been set?) now notifies the subscriber
• deleting a param also notifies the subscriber

Because ROS seems to use the empty struct as the placeholder value for non-existing values in the param tree, for this to work we need a version of dxr that supports creating that value. I have opened a PR there as well.

• allow other crates to create Value of empty Struct ironthree/dxr#25

That would need to be merged first (and a new release made on crates.io) for us to be able to use it in ros-core-rs.

Edit: failing CI is expected at the moment, due to the required change in dxr's API.

Edit 2: @decathorpe pointed out a way to create an empty struct without changing dxr's API, so I changed the code accordingly.

The remaining build failure is unrelated and seems to be caused by some version of xml-rs having been yanked. This affects rosrust as well, and is unrelated to this change. You can still build with an old Cargo.lock file in place.

Edit 3: The build with rust nightly is broken as well (for the same reason), I don't know why this shows up as green though.

[PatWie]

Thanks a ton for the pull request and for all your effort in fixing these bugs—it’s really appreciated! 🙌 I’m still on vacation for another week, so I can’t dig into the build errors just yet. Once I’m back, I’ll give it a proper review and also bump the Cargo crate version.

[jobafr]

ok have fun :)

if you want to build it, it works fine for me using this Cargo.lock file:
https://gist.github.com/jobafr/505aeed796b8785b8f9a27bfb1afa69a