Properly resolve platforms given a cname

[nkraetzschmar]

What would you like to be added:

Correctly resolve the platform for a given input cname in src/gardenlinux/features/cname.py#L139 by resolving the cname into the feature set it represents and then filtering this feature set by those marked as platform (see gardenlinux/builder: builder/parse_features#L89 for an example how to correctly resolve platforms given features).

In src/gardenlinux/features/main.py#L157 this is already done correctly, so cname.py just needs to be adjusted to work the same.

Why is this needed:

The current implementation makes the assumption that there is always one - and only one - platform and that this will always be the first component in the cname. While this is going to be correct in the vast majority of cases, there technically is nothing stopping a cname to contain more or less then 1 platform.

Note

This issue is analogous to gardenlinux/gardenlinux#3243 as both appear to be a result of the same misunderstanding of what makes a feature a platform and how cnames are encoded

[NotTheEvilOne]

The canonical name representation of the python-gardenlinux-lib can and it's command line tool gl-cname can currently handle executions without an features directory being present. That being said a special exception is currently the feature_set property for obvious reasons.

We could either depend the platform (or platforms) property to be dependent on the features directory as well or add a fallback and a logging warning to still return the current element of a cname as the platform. In the later case that would mean we could have different behaviour based on if a features directory is present/configured or not.

Please be aware that the builder currently prints a warning about multiple platforms as this may result in issues similar: print("warning: number of platforms != 1", file=sys.stderr)

We would need to reconsider if multiple platforms are indeed a common and expected use case (extend the tooling and remove the warning) or prevent that case and error out.

@gehoern, @pnpavlov

[nkraetzschmar]

for running without the features directory we might wanna consider something like gardenlinux/gardenlinux#3244 rather than assuming the first feature is always the one and only platform