nix flake show should be filterable by the current system

[NobbZ]

Is your feature request related to a problem? Please describe.

Often when one runs nix flake show nix tries to evaluate every well known output, though some of them depend on the system, and even worse, some of them use IFD, so to nix being able to show the information it needs to build for a foreign system.

This leads to errors like this:

$ nix flake show github:nix-community/nix-doom-emacs
github:nix-community/nix-doom-emacs/ae22b4a3fe31ae31b3e8b415889f8c2c5a77d8dc
├───checks
│   ├───aarch64-darwin
error: a 'aarch64-darwin' with features {} is required to build '/nix/store/a5dhhm5nhn936q5177nlv3fn3b4jvqly-source.drv', but I am a 'x86_64-linux' with features {benchmark, big-parallel, kvm, nixos-test}
(use '--show-trace' to show detailed location information)

Describe the solution you'd like

nix flake show becomes system aware and does not try to evaluate foreign systems, but instead just shows an ellipsis as child for foreign systems. A fictive example could look like this:

$ nix flake show github:nobbz/nixos-config
github:nobbz/nixos-config/e460e5272935ea1e902c04e9c705e8dc938d6383
└───checks
    ├───aarch64-darwin
    │   └───... omitted (use '--foreign-systems' to show)
    └───x86_64-linux
        ├───alejandra: derivation 'alejandra-run-e460e5272935ea1e902c04e9c705e8dc938d6383'
        └───statix: derivation 'statix-run-e460e5272935ea1e902c04e9c705e8dc938d6383'

An extra flag could be introduced to "try anway" similar to how we use --legacy to get the full list of packages in legacyPackages, perhaps --foreign-systems as in the example above.

Describe alternatives you've considered

Fully forbid any kind of IFD in flake contexts.

Additional context

Actually I would totally prefer the alternative, though I see that the community isn't there yet and instead we need to raise awareness about the issue.

[Kha]

Actually I would totally prefer the alternative, though I see that the community isn't there yet and instead we need to raise awareness about the issue.

I think your suggestion is the correct UX default irrespective of IFD. For any nontrivial flake with a nontrivial amount of supported systems, the output way is just way too redundant to answer simple questions about platform support like a) what is the total set of derivations available on any platform, and b) what platforms does this derivation support. If those are use cases flake show wants to support, I think it would have to learn to deduplicate the output as in e.g.

└───checks
    └───aarch64-darwin, x86_64-linux, ...
        ├───alejandra: derivation 'alejandra-run-e460e5272935ea1e902c04e9c705e8dc938d6383'  # assuming that hash is not platform-dependent
        └───statix: derivation 'statix-run-e460e5272935ea1e902c04e9c705e8dc938d6383'

[peterbecich]

Is this still an issue?

% nix --version
nix (Nix) 2.14.1


% nix flake show                      
...
error: a 'aarch64-darwin' with features {} is required to build 
'/nix/store/2gir7h9fgl7rkdxfs1nc2q84s9szxbgx-cabal2nix-example.drv', 
but I am a 'x86_64-linux' with features {benchmark, big-parallel, kvm, nixos-test, uid-range}

#6988 notes that the fix does not apply to Hydra jobs. Is this cabal2nix-example a Hydra job? Thanks

[peterbecich]

Also with Nix 2.15:

%  nix --version
nix (Nix) 2.15.0


% nix flake show

error:
....
features {} is required to build 
'/nix/store/2gir7h9fgl7rkdxfs1nc2q84s9szxbgx-cabal2nix-example.drv', 
but I am a 'x86_64-linux' with features {benchmark, big-parallel, kvm, nixos-test, uid-range}

[peterbecich]

The prior error must have been because the build was a Hydra job.

In this example show works correctly:

└───packages
    ├───aarch64-darwin
    │   ├───default omitted (use '--all-systems' to show)
    │   └───main-hackage-server omitted (use '--all-systems' to show)
    ├───x86_64-darwin
    │   ├───default omitted (use '--all-systems' to show)
    │   └───main-hackage-server omitted (use '--all-systems' to show)
    └───x86_64-linux
        ├───default: package 'hackage-server-0.5.1'
        └───main-hackage-server: package 'hackage-server-0.5.1'

https://github.com/haskell/hackage-server/blob/master/flake.nix