CMake: Set CMAKE_OSX_ only on Darwin?

[primeos]

In 68513e4 (part of #77632) CMAKE_OSX_ARCHITECTURES=x86_64 was moved from pkgs/stdenv/darwin/default.nix to pkgs/development/tools/build-managers/cmake/setup-hook.sh. As a result -DCMAKE_OSX_ARCHITECTURES=x86_64 is since then set on Linux as well.
Until now this wasn't a problem as https://cmake.org/cmake/help/v3.18/variable/CMAKE_OSX_ARCHITECTURES.html states that:

This variable is ignored on platforms other than Apple.

Unfortunately that behaviour changed in CMake 3.18:

• Nixpkgs issue: Some builds fail with invalid gcc flag "-Xarch_x86_64" (Linux+CMake+PCH) #94905
• CMake issue: https://gitlab.kitware.com/cmake/cmake/-/issues/21072

To avoid such regressions in the future it might be a good idea to set the following two CMake flags (source) only on Darwin:

    # on macOS i686 was only relevant for 10.5 or earlier.
    cmakeFlags="-DCMAKE_OSX_ARCHITECTURES=x86_64 $cmakeFlags"

    # we never want to use the global macOS SDK
    cmakeFlags="-DCMAKE_OSX_SYSROOT= $cmakeFlags"

cc CMake maintainers: @ttuegel @LnL7
cc @orivej (found this regression)

[ttuegel]

I don't think setting it as an environment variable will have any effect, so we will need to put a guard around it in the setup hook.

[matthewbauer]

I think just adding if [ "$(uname)" = Darwin ] on the CMake setup-hook is probably the easiest fix.

[doronbehar]

Why not use a Nix conditional? It should spare at least the Darwin rebuilds if done right.

[matthewbauer]

Why not use a Nix conditional? It should spare at least the Darwin rebuilds if done right.

That will probably work, but splitting the setup hook could be a little tricky. We'd need two setup hooks - one for all systems, and one for darwin (and maybe even one for linux for consistency). It would definitely be better for issues like these in the future, but I'm always a little hesitant to rework some of the Bash logic given its brittleness.

[primeos]

I think just adding if [ "$(uname)" = Darwin ] on the CMake setup-hook is probably the easiest fix.

I kinda like that idea as it's simple and minimal. Though that might cause problems for cross-builds (if that even works between e.g. Darwin and Linux)? Would be nice if there would be an environment variable available in the Nix build for this (both with and without sandboxing).

That will probably work, but splitting the setup hook could be a little tricky.

Yeah, I guess merging the common parts is the most tricky part. One option could be to add scripts for Linux and Darwin that set and export an environment variable and then source setup-hook.sh? Or I guess we could use Nix conditionals if we change setup-hook.sh; to a Nix expression like writeText (but that would likely be less optimal as it won't be recognized as a shell script if we cannot somehow read the content of setup-hook.sh and interpret it as text with Nix expressions).

[stale]

I marked this as stale due to inactivity. → More info

[kvtb]

Another problem: since libjpeg-turbo/libjpeg-turbo@399aa37 libjpeg-turbo is broken on i686-linux.

cmakeFlags = [ "-DCMAKE_OSX_ARCHITECTURES=" does fix the build, but it would be much better if OSX featues won't leak into Linux at all.

[kvtb]

I hope it is fixed in #114817

[primeos]

Nice! d16a875 (#114817) fixes/avoids the issues came from CMAKE_OSX_ARCHITECTURES so I guess we can close this now.

CMAKE_OSX_SYSROOT is still set on Linux but at least that didn't seem to cause any issues so far. If someone has time and motivation it might be nice to drop it on Linux as well (like in d16a875).