gtk-doc + json-glib: fix cross-compilation

[Mindavi]

Motivation for this change

Fixes #117181.

I first thought that the patch for gtk-doc didn't make sense, but I think it does. Still have to disable building docs for json-glib, since it also uses introspection, which doesn't support cross-compilation (well).

Important for cross-compilation, since glib is used in a lot of (GUI) applications.

Please let me know if this is the right approach. I think we should disable introspection on cross-compiled packages until we have proper support for it, so we can at least get cross-compilation working for most of the system. However, I'm not 100% sure if that's the right approach. If there are objections, I'd like an explanation of why we need introspection / what it's useful for.

I'm planning on doing more fixes as I get further with cross-compiling. I've quite a lot done, but it needs tidying up and a good look before I want to send them here. Also, I'd like some feedback on this approach before pushing all of that into nixpkgs.

Cc @Ericson2314 @samueldr

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[Mindavi]

Does it help if I split this up? Only fixing gtk-doc and submitting the json-glib change later?

[Mindavi]

I tried building some packages with these changes (native x86-64):

• xwayland
• gtk3
• flatpak

I guess that gives pretty good confidence this at least builds.

Also built:

• pkgsCross.aarch64-multiplatform.gtk-doc
• pkgsCross.aarch64-multiplatform.json-glib

Note that disabling gobject-introspection has some disadvantages, e.g. python apps using pygobject won't work / compile. However, I think it's a reasonable tradeoff considering some application are able to be cross-compiled when you disable introspection, and something is more than nothing.

In the future we could consider re-enabling introspection whenever we figure out how to properly cross-compile apps using introspection (see also #88222).

[jtojnar]

I agree that disabling introspection for cross is good idea for now since it makes greater range of packages build.

I think including the gtk-doc changes here is fine, as it was done in order to make json-glib cross-compile. It would be nice to add assert !isCross to gobject-introspection / python3.pkgs.pygobject3 so that the apps relying on introspection could fail fast instead of at runtime but that is probably too out of scope.

My understanding of cross-compilation is still somewhat incomplete so I am not really qualified to review that.

[Mindavi]

Thanks for the comments. I'll have a good look at this and will try some things to make cross-compilation happy. Indeed soms of the depsBuildBuild feel wrong, it's good to have that confirmed.

[Mindavi]

I've updated the PR so depsBuildBuild are not used anymore in json-glib. It appears that that was not needed / I couldn't make sense of it. Maybe it's something to look at in the future.

I did have to disable strictDeps again though. It would work when I put pkg-config in depsBuildBuild, but I didn't really understand why that was needed and I couldn't really figure it out together with @Ericson2314. It doesn't seem logical to put pkg-config there, and I can't find that pattern anywhere else in nixpkgs. Somehow when I enabled strictDeps, I got an error that gobject-introspection couldn't be found anymore (even though pkg-config and gobject-introspection are both in nativeBuildInputs).

[Ericson2314]

It is logical to put pkg-config there too when it is needed for build time packages' pc files, and I do recommend that.

[Mindavi]

This is ready IMO. Let me know if there's anything still to be addressed.