reporegistry: introduce new distro.aliases file

[mvo5]

This commit introduces a new distro.aliases file that replaces the
role of the existing symlinks to provide an alias name for a distro.
The reason is that when using go:embed symlinks are not supported
and we have currently centos-9.json -> centos-stream-9.json in
our osbuild-comopser distro definitions.

By moving them from symlinks to alias files we can start using
go:embed which is nice for the daemonless work, i.e. the
image-builder binary will be self-contained and one can do:

$ go run github.com/osbuild/image-builder/cmd/image-builder --list-images

directly without any further configuration. It also solves how to
keep the default repository configuration between osbuild-composer and
image-builder-cli in sync.

---

reporegistry: remove duplication in LoadRepositories()

This commit removes some directory traversal duplication from
LoadRepositories() on the expense of more "expensive" repo loading.
The new code will just always load all repos and return the one
matching the distro. The advantage of this approach is that when
aliases are introduced there is only a single code path that needs
changing.

[mvo5]

I'm not too happy with the names, ideas for better naming welcome here :)

[thozza]

Just a quick comment: I am not sure if distro.aliases is the best name since we already have distro aliases, and it is a different thing:

• images/pkg/distrofactory/distrofactory.go

  Lines 68 to 99 in 70fd73a

  	// RegisterAliases configures the factory with aliases for distro names.
  	// The provided aliases map has the following constraints:
  	// - An alias must not mask an existing distro.
  	// - An alias target must map to an existing distro.
  	func (f *Factory) RegisterAliases(aliases map[string]string) error {
  	var errors []string
  	for alias, target := range aliases {
  	var targetExists bool
  	for _, factory := range f.factories {
  	if factory(alias) != nil {
  	errors = append(errors, fmt.Sprintf("alias '%s' masks an existing distro", alias))
  	}
  	if factory(target) != nil {
  	targetExists = true
  	}
  	}
  	if !targetExists {
  	errors = append(errors, fmt.Sprintf("alias '%s' targets a non-existing distro '%s'", alias, target))
  	}
  	}
  	// NB: iterating over a map of aliases is not deterministic, so sort the
  	// errors to make the output deterministic
  	sort.Strings(errors)
  	if len(errors) > 0 {
  	return fmt.Errorf("invalid aliases: %q", errors)
  	}
  	f.aliases = aliases
  	return nil
  	}

• https://github.com/osbuild/osbuild-composer/blob/2a5d25d9c02e29a8898166f1e1cdc9c58313e78c/cmd/osbuild-composer/config.go#L18

From my PoV, the ability to add a new repo config file or a symlink is a feature. I have yet to look at the code, but I would keep the option to redefine the alias for the user (ideally, not compiled into the binary).

[mvo5]

Just a quick comment: I am not sure if distro.aliases is the best name since we already have distro aliases, and it is a different thing:
[..]

Oh, this super interesting - the only reason I am adding this is to avoid the symlink centos-9.json -> centos-stream-9.json in the default repo configuration. If a distro_alias in the composer config fixes this equally nicely and we can drop the symlink none of the distro.alises support is needed.

From my PoV, the ability to add a new repo config file or a symlink is a feature. I have yet to look at the code, but I would keep the option to redefine the alias for the user (ideally, not compiled into the binary).

Both the ability to add new repo config, symlinks and to mask existing repos would be preserved. The goal of what I'm doing is strictly about making it easier to share the default/canonical repos from composer across multiple projects (image-builder-cli in my specific case).

[mvo5]

Moving back to "draft" as it seems people do not really like this idea very much so far. So I will think a bit more what else we can do.

[mvo5]

Closing in favor of #1038 that hopefully addresses the concerns here.