Override releasever_{major,minor} with system-release provides

[evan-goode]

The releasever_major and releasever_minor substitution variables are usually derived by splitting releasever on the first .. However, to support EPEL 10 [1], we would like a way for distributions to override these values. Specifically, we would like RHEL 10 to have a releasever of 10 with a releasever_major of 10 and a releasever_minor of 0 (later incrementing to 1, 2, to correspond with the RHEL minor version).

This commit adds a new API function, detect_releasevers, which derives releasever, releasever_major, and releasever_minor from virtual provides on the system-release package (any of DISTROVERPKG). The detection of releasever is unchanged. releasever_major and releasever_minor are specified by the versions of the system-release-major and system-release-minor provides, respectively.

If the user specifies a --releasever=X.Y on the command line, the distribution settings for releasever, releasever_major, and releasever_minor will all be overridden: releasever will be set to X.Y, releasever_major will be set to X, and releasever_minor will be set to Y, same as before.

If a user wants to specify a custom releasever_{major,minor}, they have to set all three with --setopt=releasever=X --setopt=releasever_major=Y --setopt=releasever_minor=z, taking care to put releasever_major and releasever_minor after releasever so they are not overridden. This is admittedly not ideal, but I can't think of another solution to this problem that preserves the following properties:

1. If they are not overridden, releasever_{major,minor} are derived by splitting releasever. This behavior was added in Split $releasever to $releasever_major and $releasever_minor #1989.
2. If --releasever is specified on the command line, releasever_{major,minor} should be set accordingly, even if releasever{major_minor} are specified by provides in the distribution system-release package.
3. The user should somehow be able to specify their own custom releasever, releasever_major, and releasever_minor.

Maybe we could add --releasever_major= and --releasever_minor options that take priority over --releasever to improve point (3).

Another caveat to allowing overriding releasever_{major,minor} is existing API users using detect_releasever. Take this snippet from our doc/examples/install_extension.py:

    with dnf.Base() as base:
        # Substitutions are needed for correct interpretation of repo files.
        RELEASEVER = dnf.rpm.detect_releasever(base.conf.installroot)
        base.conf.substitutions['releasever'] = RELEASEVER

Now, in order to detect the correct releasever_{major,minor}, the correct code would be

    with dnf.Base() as base:
        # Substitutions are needed for correct interpretation of repo files.
        RELEASEVER, MAJOR, MINOR = dnf.rpm.detect_releasevers(base.conf.installroot)
        base.conf.substitutions['releasever'] = RELEASEVER
        if MAJOR is not None:
            base.conf.substitutions['releasever_major']
        if MINOR is not None:
            base.conf.substitutions['releasever_minor']

Requires rpm-software-management/libdnf#1689.

[1] https://issues.redhat.com/browse/RHEL-68034

[jan-kolarik]

Hi Evan and thanks for figuring this out! For me it sounds fine having new --releasever_major and --releasever_minor options that will take priority over --releasever + adding new detect_releasevers method. Just make sure to clearly document that existing use cases are preserved and how the new API should be used and why we are introducing it.

[evan-goode]

ci-dnf-stack PR: rpm-software-management/ci-dnf-stack#1621

[evan-goode]

Hi Evan and thanks for figuring this out! For me it sounds fine having new --releasever_major and --releasever_minor options that will take priority over --releasever + adding new detect_releasevers method. Just make sure to clearly document that existing use cases are preserved and how the new API should be used and why we are introducing it.

Cool, I've added the --releasever-major and --releasever-minor command-line options for overriding the major,minor variables, and I documented detect_releasevers in api_rpm.rst.