Handling of system release version with other sources (e.g. subscription management)

[ptoscano]

dnf currently manages the version of the system (represented by the $releasever variable) on its own, using the version of the *-release package (e.g. redhat-release, centos-release, etc) as default, with the possibility to override it via variables.

subscription-manager is the tool currently in use in RHEL to let users registers their systems. One of the functions it has is managing the version of the system from the "subscription" point of view. This is functionality is exposed to the user as separate subcommand:

# subscription-manager release --help
usage: subscription-manager release [OPTIONS]

Configure which operating system release to use

optional arguments:
  -h, --help            show this help message and exit
  --proxy PROXY_URL     proxy URL in the form of hostname:port
  --proxyuser PROXY_USER
                        user for HTTP proxy with basic authentication
  --proxypassword PROXY_PASSWORD
                        password for HTTP proxy with basic authentication
  --noproxy NO_PROXY    host suffixes that should bypass HTTP proxy
  --no-progress-messages
                        Do not display progress messages
  --show                shows current release setting; default command
  --list                list available releases
  --set RELEASE         set the release for this system
  --unset               unset the release for this system

Determining the list of available releases for a certain system requires at least contacting the registration server (there is more to it, I'm keeping this simple in this context), and so also setting/unsetting the release (which is saved on the registration server).

While this approach "works", in practice it has a lot of drawbacks:

• the concept of "release" that dnf and subscription-manager have are totally separate and alien to each other
• subscription-manager is basically managing the base version of the OS... which is what dnf does already managing the installed software
• when the release is changed via subscription-manager release --set/--unset, subscription-manager rewrites its own redhat.repo file replacing the value of $releasever (the issues with the repository management of subscription-manager are a separate discussion, i.e. Better handling of repositories coming from external sources #422); this means that other repositories are not following the release set for the system

Hence, it would be nice if dnf had a clean way for external sources to help determine the release of the system using the plugin infrastructure. The request here is not to implement the logic of subscription-manager in dnf, of course, but rather something along the lines of:

• a plugin would be able to provide a source of the release of the system
• dnf would either use the source of that plugin as default, e.g. when configured to do so
• changing the the release to use (e.g. via --releasever) might be constrained to the releases available
• possibly a new subcommand in dnf to manage the release cleanly? this also would need to notify the plugin about it, so the plugin could be able to accept/reject the release change and do additional work