lib.systems.extensions.sharedLibrary: do not throw

[ghost]

Description of changes

Because downstream code expects to use == on platform attrsets, we are unfortunately not able to throw a useful error message when the sharedLibrary attribute is accessed.

When users do a comparison like:

stdenv.hostPlatform == pkgsStatic.stdenv.hostPlatform

... in a situation where stdenv.hostPlatform.hasSharedLibraries, they expect this to return false. Unfortunately Nix does a deep equality comparison here, and ends up forcing the pkgsStatic.stdenv.hostPlatform.extensions.sharedLibrary attribute, which throws the error.

Rather than returning null, this commit instead simply omits the extensions.sharedLibrary attribute. This provides the user with a more-useful error message: instead of waiting until the null is used (and hoping that produces an error), the user will get an error about the extensions.sharedLibrary attribute being missing, at the position where it was referenced.

Big thanks to @trofi for his PR to add NIX_VALIDATE_EVAL_NONDETERMINISM to Nix, which I am now using. It made tracking this down really easy!

Fixes #244045

Things done

• 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.

[trofi]

Another option is not to provide the attribute at all with a fancy computed assignment:

nix-repl> { ${if false then "attr" else null} = 42; }
{ }

nix-repl> { ${if true then "attr" else null} = 42; }
{ attr = 42; }

That way error messages attempting to use irrelevant attribute might be slightly clearer. But I did not test it. Might break more than it fixes.

[ghost]

Updated to implement @trofi's suggestion, but using lib.optionalAttrs instead of dynamic attributes.

No drv-hash change (or eval failure) with NIX_VALIDATE_EVAL_NONDETERMINISM=1 for all my usual builds (mips, powerpc, aarch64, x86_64) as well as tests.cross.sanity.

[vkryachko]

Any chance this could be reviewed? thanks