nixos/systemd-boot: fix systemd-boot-builder dowgrade to fail

[waldheinz]

Since, 4ddc788 systemd-boot-builder
is broken in two ways:

• if no systemd-boot is currently installed and the NIXOS_INSTALL_BOOTLOADER
  env variable is not set, it will try to run "bootctl update", which will fail
• if the currently installed systemd-boot version is newer than the version
  we're about to install, it will also try to run "bootctl update", which will fail

This patch changes the behaviour,

• for the first case to still fail, but not even bother to try running
  "bootctl update" and instead throwing erroring out with an exception
• for the second case to leave the newer version in place, restoring
  the pre - 4ddc788 behaviour

To do the proper version check a new "should_update" helper function was introduced,
mimicing the compare_product C function from bootctl.

This change allows to again switch to a different NixOS configuration which contains
an older systemd-boot.

[sorki]

LGTM! Thank you!

[waldheinz]

Could you please have a look, @dasJ ?

[waldheinz]

See also systemd/systemd#23450

[waldheinz]

Ping @jrobsonchase

[waldheinz]

I improved the commit with what @mweinelt suggestend and additionally renamed the variables to have available_ (what we'd like to update to) and installed_ (what's currently installed) prefixes, making the code easier to follow.

[dasJ]

I don't feel comfortable reviewing that Python code.
If others think it's good, I have no objections to the change in general.
I added the backport label for 22.05 so we can get it on the release branch as well since we have already branched off.

[mweinelt]

I think this will work.

One more nit, regarding the regex parsing. It didn't originate in this pull request, but since you're touching it we might as well.

[waldheinz]

@mweinelt I wanted to add that I just tested this again and it still works. Thanks for merging!

[github-actions]

Successfully created backport PR #175115 for release-22.05.

[danielfullmer]

Shouldn't this and #175611 both get backported to NixOS 21.11 as well? I thought we had an additional month of support for 21.11.

I was unable to rollback from 22.05 back to 21.11, since the 21.11 branch doesn't include these changes.

(This is potentially a good example of why tying the bootloader entry generation to the NixOS generation is problematic, and that whatever program generates bootloader entries from NixOS bootspec definitions should be independently upgradable.) CC @grahamc

[RuRo]

I'm a bit late to the party, but I just wanted to point out that the

leaving systemd-boot X in place (Y is not newer)

message is a bit poorly worded, IMHO.

Maybe, I am just paranoid, but to me this message reads like something went wrong when trying to upgrade systemd-boot: the installed version of systemd-boot is newer than the one in nixpkgs. This message gets printed on every nixos-rebuild switch. Perhaps, systemd-boot-builder shouldn't print anything when installed_version == available_version?