Make os-specific overrides for source packages

[chhtz]

Currently only os-packages are easy to define depending on the current OS version, but sometimes it would be convenient to also allow pinning certain source packages to a specific version on some OS versions (e.g., if the main branch gets incompatible with old OS versions).

We have a mechanism for that in our RIC package set (here it is mostly used to activate qt5-branches for some packages when building with Ubuntu after 20.04):
https://github.com/dfki-ric/dfki-ric-package_set/blob/master/lib/os_management.rb

This still needs to be actively activated using Rock::OSManagement.activate_distribution_overrides (e.g. in a package set's init.rb) and then one can create an overrides folder with OS-specific (sub-) sub-folders:
https://github.com/dfki-ric/dfki-ric-package_set/tree/master/overrides/ubuntu

@rock-core/core Any opinions on this?

[planthaber]

Just a follow-up:

The code creates and manages a symlink in the buildconf/overrides.d folder to keep tracability of the overrides.

We'd like to merge the code into autoproj, to make it available to others, but we also know that this is quite a corner case, as normally all the overrides should be placed in the buildconf and not into package_sets (and buildconfs are normally os-dependent).

We use it for a kurated "proven to work" package_set, that lets us apply the overrides across multiple workspaces.

It cloud also be handy to be able to have os-specific overrides in a buildconf, at least when the code should be migrated to a newer OS, otherwise a branch has to be used (with the risk of diverging).

[g-arjones]

I don't understand the rationale for the symlink mechanism. Why not just use Autoproj::PackageSet.add_source_file ?

[planthaber]

When searching for overrides, you'll normally search for them on the overides.d folder. The idea was to have all the overrides applied to the worksapce in one single place, so you can easily find what is going on.

You are also able to create overrides for the package_set overrides because of the alphanumerical order in which the files are applied in the overrides.d folder. Just add another file after the links.