[package outputs] generate buildInputs for package-outputs in flake

[savil]

Summary

This PR reads the outputs field from the Config for a package.

For packages with non-default outputs in the config, we specify:

1. They are not to be taken from the binary cache
2. They are exempt from the flakeInput.buildInputs() that are specified in the generated flake's devshell.buildInputs

Instead, we generate a symlinkJoin derivation of the form:

            (pkgs.symlinkJoin {
              name = "prometheus-combined";
              paths = [
                # each output is printed here
                nixpkgs-fd04be-pkgs.prometheus.cli
                nixpkgs-fd04be-pkgs.prometheus.out
              ];
            })

This prometheus-combined derivation is useful so that when we derive the nix-profile from the flake's buildInputs as in #1692, it is treated as a single package. Without this, we could inline each output as a distinct buildInput in the flake's devshell, but then the nix profile would treat each output as its own package.

How was it tested?

Added testscript unit-test. Also, ran the same steps inside the testscript manually.

setup:

# add package with multiple outputs
devbox add prometheus --outputs cli,out

# add package for a specific nixpkgs commit hash (as a control)
devbox add github:nixos/nixpkgs/fd04bea4cbf76f86f244b9e2549fca066db8ddff#hello

then the generated flake is:

❯ cat .devbox/gen/flake/flake.nix
{
   description = "A devbox shell";

   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/fd04bea4cbf76f86f244b9e2549fca066db8ddff";
     nixpkgs-fd04be.url = "github:NixOS/nixpkgs/fd04bea4cbf76f86f244b9e2549fca066db8ddff";
     gh-nixos-nixpkgs-fd04bea4cbf76f86f244b9e2549fca066db8ddff.url = "github:NixOS/nixpkgs/fd04bea4cbf76f86f244b9e2549fca066db8ddff";
   };

   outputs = {
     self,
     nixpkgs,
     nixpkgs-fd04be,
     gh-nixos-nixpkgs-fd04bea4cbf76f86f244b9e2549fca066db8ddff,
   }:
      let
        pkgs = nixpkgs.legacyPackages.x86_64-darwin;
        nixpkgs-fd04be-pkgs = (import nixpkgs-fd04be {
          system = "x86_64-darwin";
          config.allowUnfree = true;
          config.permittedInsecurePackages = [
          ];
        });
        gh-nixos-nixpkgs-fd04bea4cbf76f86f244b9e2549fca066db8ddff-pkgs = (import gh-nixos-nixpkgs-fd04bea4cbf76f86f244b9e2549fca066db8ddff {
          system = "x86_64-darwin";
          config.allowUnfree = true;
          config.permittedInsecurePackages = [
          ];
        });
      in
      {
        devShells.x86_64-darwin.default = pkgs.mkShell {
          buildInputs = [
            (builtins.fetchClosure {
              fromStore = "https://cache.nixos.org";
              fromPath = "/nix/store/0djljz0g4s6f55xcnw7fpzcy7af7rxid-go-1.21.4";
              inputAddressed = true;
            })




            (pkgs.symlinkJoin {
              name = "prometheus-combined";
              paths = [
                nixpkgs-fd04be-pkgs.prometheus.cli
                nixpkgs-fd04be-pkgs.prometheus.out
              ];
            })
            gh-nixos-nixpkgs-fd04bea4cbf76f86f244b9e2549fca066db8ddff-pkgs.hello
          ];
        };
      };
 }

this leads to $buildInputs in a devbox shell to be:

❯ echo $buildInputs
/nix/store/0djljz0g4s6f55xcnw7fpzcy7af7rxid-go-1.21.4 /nix/store/9dn3rzv04x63n0kz2jwpgz82rdlsa56h-prometheus-combined /nix/store/4xy6iv0ph2v6w7n06cw5ra7cmyvignkm-hello-2.12.1

where the prometheus-combined derivation has the cli and out outputs combined:

❯ ls -al /nix/store/9dn3rzv04x63n0kz2jwpgz82rdlsa56h-prometheus-combined/bin/
total 0
dr-xr-xr-x 4 root wheel 128 Dec 31  1969 .
dr-xr-xr-x 4 root wheel 128 Dec 31  1969 ..
lrwxr-xr-x 1 root wheel  76 Dec 31  1969 prometheus -> /nix/store/hizpsf2f5gc7l810328382xicjb9gc73-prometheus-2.48.1/bin/prometheus
lrwxr-xr-x 1 root wheel  78 Dec 31  1969 promtool -> /nix/store/8x6psdqn3945pbs0ww5wanmfhvvz2iyl-prometheus-2.48.1-cli/bin/promtool

[savil]

Current dependencies on/for this PR:

• main
  • PR [Refactor ensureStateIsUpToDate] split into two modes for when to recompute state #1723
    • PR Derive nix profile from flake: attempt 2 #1724
      • PR [package outputs] generate buildInputs for package-outputs in flake #1721 👈

This stack of pull requests is managed by Graphite.

[mikeland73]

LGTM, will let @gcurtis take a look as well.