qtwebengine: replace targetPlatform with hostPlatform

[ghost]

Description of changes

stdenv.targetPlatform really shouldn't be used by software that doesn't generate or manipulate binaries.

I did a mass-replacement in #267229 and it affected eval for only two packages; one of them is qt6-qtwebengine, but it is affected only on Darwin. So I am breaking this out into a separate PR.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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.

[puetzk]

I bet the difference on darwin is coming from

nixpkgs/pkgs/os-specific/darwin/apple-sdk-11.0/default.nix

Lines 66 to 69 in 7206143

	targetPlatform = stdenv.targetPlatform // {
	darwinMinVersion = "10.12";
	darwinSdkVersion = "11.0";
	};

which overrides darwinSdkVersion for targetPlatform but not hostPlatform. It seems like Qt was definitely aiming to get the higher CMAKE_OSX_DEPLOYMENT_TARGET = "11.0" set there; they went out of their way to override stdenv on darwin accordingly:

nixpkgs/pkgs/development/libraries/qt-6/default.nix

Line 30 in 91ac57c

stdenv = if stdenv.isDarwin then darwin.apple_sdk_11_0.stdenv else stdenv;

And this is presumably per the upstream platform support being macOS 11, 12, 13 but nothing older.

So, while this change seems logically correct (qtWebEngine isn't a cross-compiler and should use hostPlatform, I think darwin.apple_sdk_11_0 would need to be fixed to set hostPlatform as well to make that all work out.

Either that or qtwebengine should just be literally coding CMAKE_OSX_DEPLOYMENT_TARGET=11.0 (the minimum per upstream's requirements), and not trying to make different binaries per host revision. greping nixpkgs for CMAKE_OSX_DEPLOYMENT_TARGET seems to find that more often, and it would make for fewer distinct builds...

[puetzk]

Also seems like

nixpkgs/pkgs/development/libraries/qt-6/default.nix

Lines 28 to 31 in 91ac57c

	callPackage = self.newScope ({
	inherit qtModule srcs python3;
	stdenv = if stdenv.isDarwin then darwin.apple_sdk_11_0.stdenv else stdenv;
	});

is perhaps not using apple_sdk_11_0 as intended (or at least not as documented).

https://nixos.org/manual/nixpkgs/unstable/#sec-darwin says

x86_64-darwin uses the 10.12 SDK by default, but some software is not compatible with that version of the SDK. In that case, the 11.0 SDK used by aarch64-darwin is available for use on x86_64-darwin. To use it, reference apple_sdk_11_0 instead of apple_sdk in your derivation and use pkgs.darwin.apple_sdk_11_0.callPackage instead of pkgs.callPackage. On Linux, this will have the same effect as pkgs.callPackage, so you can use pkgs.darwin.apple_sdk_11_0.callPackage regardless of platform.

So it seems like this perhaps should be using the whole apple_sdk_11_0.callPackage instead of its own callPackage with only the stdenv changes from 11.0, but all the other SDK packages left at 10.12?

I don't have any x86_64-darwin machine, though.

[puetzk]

Tagging @wegank, since they're the one who put this all into qt6 during #226377 and probably know what they were trying to do (or at least have this platform...)

[wegank]

So, while this change seems logically correct (qtWebEngine isn't a cross-compiler and should use hostPlatform, I think darwin.apple_sdk_11_0 would need to be fixed to set hostPlatform as well to make that all work out.

Yes, and this has actually been reported in #238993 (comment). So for now CMAKE_OSX_DEPLOYMENT_TARGET should probably just be hardcoded to 11.0 .

And this is presumably per the upstream platform support being macOS 11, 12, 13 but nothing older.

Yes, once again. A modern approach would be stdenv = if stdenv.isDarwin then overrideSDK stdenv "11.0" else stdenv, but this was introduced very recently. As a result, all Qt 6 applications were built with, for example, frameworks from darwin.apple_sdk_11_0.frameworks explicitly.