lib/systems: elaborate properly with non-matching system / config / parsed args

[wolfgangwalther]

This is broken out of #303849, because even if that PR has stalled for now, imho it's still useful to fix lib.systems.elaborate.

---

When elaborating a system with both config and system arguments given, they might not match the parsed results. Example:

elaborate {
  config = "i686-unknown-linux-gnu";
  system = "x86_64-linux";
}

This would result in a parsed system for i686, because the config argument is preferred. But since // args // comes after system has been inferred from parsed, it is overwritten again. This results in config and parsed all pointing to i686, while system still tells the story of x86_64.

Inconsistent arguments can also be given when passing parsed directly. This happened in stage.nix for the various package sets.

The solution is simple: One of the three arguments needs to be treated as the ultimate source of truth. system can already be losslessly extracted from parsed. However, config currently can not, for example for various -mingw32 cases. Thus everything must be derived from config.

To do so, system and parsed arguments are made non-overrideable for lib.systems.elaborate. This means, that system will be used to parse when config is not given - and parsed will be ignored entirely (why elaborate an already elaborated system anyway?).

The systemToAttrs helper is exposed on lib.systems, because it's useful to deal with top-level localSystem / crossSystem arguments elsewhere (not in this PR).

@NixOS/stdenv

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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 rev HEAD". 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
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[wolfgangwalther]

Note to myself: I should probably add a test in lib/tests/systems.nix.

[wolfgangwalther]

Added some tests to show the bad and fixed behavior. Before these changes, the tests fail like this:

% nix-instantiate --eval --strict lib/tests/systems.nix
[
  { expected = "i686"; name = "test_elaborate_config_over_parsed"; result = "x86-64"; }
  { expected = "i686-linux"; name = "test_elaborate_config_over_system"; result = "x86_64-linux"; }
  { expected = "i686"; name = "test_elaborate_system_over_parsed"; result = "x86-64"; }
]

The result is self-conflicting elaborated systems, which leads to problems downstream.

After, the tests pass.

[wolfgangwalther]

@nix-owners nix-owners bot requested a review from infinisil

@infinisil Looking at the ci/OWNERS file, I don't understand why the bot pinged you. Or why it did now, but not before. On the latest force push, I only added tests to /lib/tests/systems.nix. I guess this triggers because you are set up as code owner for /lib overall. But you were not pinged before, when I already made changes to /lib/systems. Is this expected, i.e. will a later rule for a sub directory "take away" your code owner for the top-level? And if so, it's an odd combination that you will be pinged for tests for systems, but not the code itself ;)