xen: nuke, move to by-name.

[SigmaSquadron]

Description of changes

• Migrate Xen to by-name, since it can't be migrated automatically.
  • This has the side effect of simplifying some Xen stuff.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review pr 345192". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
• Fits CONTRIBUTING.md.
  • Intermediate commits will be squashed.

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[CertainLach]

Do package sets work with by-name?

[SigmaSquadron]

Do package sets work with by-name?

I don't think so. This also sends all xenPackages to the top-level attribute. (xenPackages.xen_4_19 -> xen_4_19).

[SigmaSquadron]

I have reached the conclusion I have no idea what I'm doing.

[emilazy]

Hate to say it, but this is a violation of the by-name contract. cc @infinisil for advice on the correct way to handle it, but the one being used for the Darwin SDK rework that seemed okay was to have the package take e.g. a xenVersion argument that the all-packages.nix calls override appropriately.

It’s also fine to just not be in by-name. It’s not expected that every package can currently fit into the schema.

[CertainLach]

Ah, yes:

For each package directory, pkgs.lib.isDerivation pkgs.${name} must be true.

[emilazy]

The problem is that by-name packages have to look like callPackage ../by-name/ab/abcd args and have to be derivations. With the current pkgs/by-name/xe/xen structure, it’s impossible to satisfy both.

With the xenVersion thing, you can even get fancy and make e.g. pkgs/by-name/xe/xen_4_18-slim/package.nix be { xen }: xen.override { xenVersion = "4.18"; xenSlim = true; } if you really don’t want any all-packages.nix entries.

[SigmaSquadron]

Ah, yes:

For each package directory, pkgs.lib.isDerivation pkgs.${name} must be true.

I still don't see what exactly is wrong here?

[emilazy]

But you only satisfy that condition because you violate this one:

• For each package directory, the pkgs.${name} attribute must be defined as callPackage pkgs/by-name/${shard}/${name}/package.nix args for some args.