cudaPackages.cuda-library-samples: Only enable on Linux, fix channel

[infinisil]

This very weirdly broke the channel evaluation: https://hydra.nixos.org/build/245871962

It appears that this attribute is only evaluated by Hydra, not by ofborg. So this wouldn't have been detected by CI anyways in the PR that introduced the problem: #276800.

However, due to #271123 (comment), the channel only broke once that was fixed with #278777

Whether the fix is good, I don't know, but the failing-on-darwin attribute doesn't exist anymore with this commit, making the tarball build succeed again:

nix-build pkgs/top-level/release.nix -A tarball

Ping @vcunat @NixOS/cuda-maintainers @amjoseph-nixpkgs @Mic92

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[SomeoneSerge]

The more cursed that the linked PR didn't even touch cuda-library-samples directly (must've relaxes some filters at the scope level?)

The "package" is in a pretty horrible shape, almost unmaintained, doesn't set neither platforms nor badPlatforms. It's also somehwat un-packageable: the upstream uses recursive-gnumake which eventually calls cmake.

I also think that optionalAttrs is a good way to create more eval erros in the future, but if this fixes Hydra right now we should just merge

Thanks for the ping and for the fix

[infinisil]

The more cursed that the linked PR didn't even touch cuda-library-samples directly (must've relaxes some filters at the scope level?)

Yeah this was a pain to figure out. Apparently the files pkgs/test/cuda/*/extension.nix are being used to extend the cudaPackages set:

nixpkgs/pkgs/top-level/cuda-packages.nix

Lines 116 to 117 in 2c9008a

	(callPackage ../test/cuda/cuda-samples/extension.nix {inherit cudaVersion;})
	(callPackage ../test/cuda/cuda-library-samples/extension.nix {})

This is not great, it violates implicit Nixpkgs boundary: pkgs/test should only contain files to test the entire package set, the package set should not depend on this.

[infinisil]

And only adding packages to a package set if they are supported on the platform is also very much not conventional. meta.platforms should be used instead..

[SomeoneSerge]

And only adding packages to a package set if they are supported on the platform is also very much not conventional. meta.platforms should be used instead..

Tracking in #266475 (even if the title doesn't suggest)

pkgs/test/cuda/*/extension.nix are being used to extend the cudaPackages

Opened #279703. Not even sure how long they've been in test/