Add --rev to update-source-version, add a generic unstable updater

[fgaz]

Motivation for this change

Unstable packages are tedious to keep up to date. You have to manually check the repository for new commits, update the version to HEAD's commit date, and update the revision to HEAD's sha.

This patch adds a --rev parameter to update-source-version (necessary since in unstable packages version and rev are independent), and a generic unstable updater to handle the case described above.

Finally, in the last commit, the updater is applied to a package, qbe, that benefits from it, and on which you can test the updater.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[jtojnar]

I like the u-s-v change.

The update script itself looks fine. But we might want to implement it using genericUpdater, cc @romildo. Though it might need changes before it is usable (e.g. make it more flexible, do not create fileForGitCommands now that update.nix supports auto-comitting).

[jtojnar]

Relevant: #100833

[fgaz]

Not sure whether to leave it as is until #100833 is in, or to make it configurable by taking a makeVersionString function as an argument (with default date → "unstable-$date")

[fgaz]

I think I'll leave it as is for now

[jtojnar]

We probably should make the name signal that it is git specific.

[fgaz]

I based the name on genericUpdater. Should I rename that one too or is it too late?

[jtojnar]

genericUpdater is quite fresh an undocumented so I do not see an issue with renaming it. We can leave it for a separate PR, though, if we decide to not use it now.

[fgaz]

Done

[fgaz]

I named it "unstableGitUpdater", not sure if it's clear enough

[fgaz]

@jtojnar

we might want to implement it using genericUpdater

I considered that, but

• it would require at least a versionSorter, a versionFilter, maybe a makeVersion parameters
• and/or a way to trigger the --rev behavior
  • which is pretty much specific to unstable versions
• unstableUpdater can make shallow clones, potentially using much less resources (edit: nevermind, we probably get a full clone with the later commands anyway)
• unstableUpdater is quite simple on its own

[fgaz]

Ping

[jtojnar]

Could you autosquash the fixup commits and fix the evaluation?

[jtojnar]

Looks good, thanks.