lib: add lib.getExe' function

[gytis-ivaskevicius]

Description of changes

We need this function to encourage the efficiency of build artifacts. Issues with alternatives:

• Current implementation of lib.getExe: Throws a warning on the majority of packages due to lack of meta.mainProgram. I consider it useless at this point.
• "${lib.getBin pkgs.something}/bin/something: Verbose, nobody is going to bother checking if package has bin output (View comment below)
• "${pkgs.something}/bin/something: In case of derivations with multiple outputs is less space efficient (View comment below)

Ideal implementation

While I do realize issues that may come up with old implementation of lib.getExe, I think we gain more from having it than from removing it. I think ideally we should revert the patch which adds builtins.trace on lib.getExe and encourage usage of this function

I am okay with leaving lib.getExe as it is right now, but for that, I'd like for meta.mainProgram to be a required attribute on almost all derivations

Alternative implementation

Someone needs to go over the packages and implement `meta.mainProgram where it is appropriate

Things done

Copied old implementation of lib.getExe and named it lib.getExe'

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• 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/)
• 23.11 Release Notes (or backporting 23.05 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.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/lib-getexe-best-practices-discussion-thread/31290/1

[gytis-ivaskevicius]

Alternative name:

Suggested change

	getExe' = x:
	guessExe = x:

[gytis-ivaskevicius]

Not ready to be merged, needs to be exposed in lib/default.nix. Will do that once we figure out the name for the function. It might be unnecessary if we end up reverting e6e16bc11843

[gytis-ivaskevicius]

After not sure how many years I may only now find out about this feature:

nix-repl> toString oxygen
"/nix/store/hwiqax93mgl5fv0w0jycb87h2xvgz2f5-oxygen-5.27.6-bin"

nix-repl> :b oxygen

This derivation produced the following outputs:
  bin -> /nix/store/hwiqax93mgl5fv0w0jycb87h2xvgz2f5-oxygen-5.27.6-bin
  dev -> /nix/store/cl4dha602ljs6vry2ccw73mpr1kxmncx-oxygen-5.27.6-dev
  out -> /nix/store/54g6b8b44388kxqzm3w3qh345q9v2hzp-oxygen-5.27.6

nix-repl> toString acl
"/nix/store/7djfrlhxlfmdk2cdafhg6gx2ykqwf99n-acl-2.3.1-bin"

nix-repl> :b acl

This derivation produced the following outputs:
  bin -> /nix/store/7djfrlhxlfmdk2cdafhg6gx2ykqwf99n-acl-2.3.1-bin
  dev -> /nix/store/d2b85ipf9jz0mdphzkzipls7cgnsfj59-acl-2.3.1-dev
  doc -> /nix/store/lq027yz89vc2c28c3678kvm4d6d4mq8i-acl-2.3.1-doc
  man -> /nix/store/dq5hv904jlkc4dwfcjmpg0c88r644qf3-acl-2.3.1-man
  out -> /nix/store/k64mgy0pwn8vh0zdfafplbxpllavd1jm-acl-2.3.1

Whenever stringifying derivation it results in bin output if it is available. Where is this implemented? I assume its nix feature, is it documented anywhere?

I would like some confirmation on this but I don't think that I have much of a use for the current implementation of lib.getExe if that's the case. It makes me wonder what is the usecase for lib.getBin since one may just stringify the derivation

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/lib-getexe-best-practices-discussion-thread/31290/3

[figsoda]

Whenever stringifying derivation it results in bin output if it is available.

I believe this is because bin is the first entry in outputs:

nixpkgs/pkgs/desktops/plasma-5/oxygen.nix

Line 38 in 66aedfd

outputs = [ "bin" "dev" "out" ];

nix-repl> toString tzdata
"/nix/store/hwdckdgh3y73qravhmx7dmkv825zbipi-tzdata-2023c"

nix-repl> tzdata ? bin
true

[infinisil]

See also #248895