gem-config: add ruby-vips

[smaximov]

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

---

[alyssais]

Suggested change

	--replace "vips_libname = 'vips'" "vips_libname = '${vips.out}/lib/libvips${stdenv.hostPlatform.extensions.sharedLibrary}'"
	--replace "vips_libname = 'vips'" "vips_libname = '${vips}/lib/libvips${stdenv.targetPlatform.extensions.sharedLibrary}'"

[smaximov]

@alyssais can you explain why you think hostPlatform should be replaced with targetPlatform? I've read the manual on platform parameters, and it suggests that targetPlatform should only be used for a limited class of packages. The rest of overrides in defaultGemConfig uses hostPlatform, too.

As for the "${glib.out}" → "${glib}" change, it won't work because glib is a multiple-output package. "${glib}" produces a store path which differs from the store path produced by "${glib.out}" and contains GLib binaries only, so using it will break ruby-vips (it actually makes my app's test suite fail with LoadError).

Since vips is a single-output package, "${vips.out}" and "${vips}" produce the same store path. I've seen both conventions used in the gem config, can I assume that the latter is preferred?

[alyssais]

@alyssais can you explain why you think `hostPlatform` should be replaced with `targetPlatform`? I've read [the manual](https://nixos.org/nixpkgs/manual/#sec-cross-platform-parameters) on platform parameters, and it suggests that `targetPlatform` should only be used for a limited class of packages. The rest of overrides in `defaultGemConfig` uses `hostPlatform`, too. hostPlatform The "host platform" is the platform on which a package will be run. This is the simplest platform to understand, but also the one with the worst name.

Sorry, I got caught out by the poor naming.

As for the `"${glib.out}"` → `"${glib}"` change, it won't work because [`glib`](https://github.com/NixOS/nixpkgs/blob/1056100039733ed9496a9d779207c1c8cb371e1a/pkgs/development/libraries/glib/default.nix#L77) is a [multiple-output package](https://nixos.org/nixpkgs/manual/#chap-multiple-output). `"${glib}"` produces a store path which differs from the store path produced by `"${glib.out}"` and contains GLib binaries only, so using it will break `ruby-vips` (it actually makes my app's test suite fail with `LoadError`).

Also my mistake. Apologies again.

Since `vips` is a single-output package, `"${vips.out}"` and `"${vips}"` produce the same store path. I've seen both conventions used in the gem config, can I assume that the latter is preferred?

Yeah, the latter is preferred where possible. Sorry for the errors in my initial review.

[smaximov]

Sorry for the errors in my initial review.

No worries! It made me re-read the relevant parts of the manual and refresh some points that I had missed before, so I'd call it a win 😉

[smaximov]

@alyssais, I've replaced "${vips.out}" with "${vips}".

[alyssais]

Thanks @smaximov!