windows: Thread model mismatch between Guix builds and CI #36314

The contrib/devtools/shell-win64-cross.nix script used in CI builds a GCC cross-compiler configured with --enable-threads=mcf:

$ NIX_BUILD_SHELL=bash HOST=x86_64-w64-mingw32ucrt nix-shell contrib/devtools/shell-win64-cross.nix
$ $CXX -v
Using built-in specs.
COLLECT_GCC=/nix/store/i67bfgf5xy32wzh6cck9jpxcjkvnq9k5-x86_64-w64-mingw32-gcc-14.3.0/bin/x86_64-w64-mingw32-g++
COLLECT_LTO_WRAPPER=/nix/store/i67bfgf5xy32wzh6cck9jpxcjkvnq9k5-x86_64-w64-mingw32-gcc-14.3.0/libexec/gcc/x86_64-w64-mingw32/14.3.0/lto-wrapper
Target: x86_64-w64-mingw32
Configured with: ../gcc-14.3.0/configure --prefix=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-x86_64-w64-mingw32-gcc-14.3.0 --with-gmp-include=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gmp-with-cxx-6.3.0-dev/include --with-gmp-lib=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-gmp-with-cxx-6.3.0/lib --with-mpfr-include=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-mpfr-4.2.2-dev/include --with-mpfr-lib=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-mpfr-4.2.2/lib --with-mpc=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-libmpc-1.4.0 --with-native-system-header-dir=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-mingw-w64-x86_64-w64-mingw32-13.0.0-dev/include --with-build-sysroot=/ --with-gxx-include-dir=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-x86_64-w64-mingw32-gcc-14.3.0/include/c++/14.3.0/ --program-prefix=x86_64-w64-mingw32- --enable-lto --disable-libstdcxx-pch --without-included-gettext --with-system-zlib --enable-static --enable-languages=c,c++ --disable-multilib --enable-plugin --with-isl=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-isl-0.20 --with-as=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-x86_64-w64-mingw32-binutils-wrapper-2.46/bin/x86_64-w64-mingw32-as --with-headers=/nix/store/eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee-mingw-w64-x86_64-w64-mingw32-13.0.0-dev/include --enable-__cxa_atexit --enable-long-long --enable-threads=mcf --enable-nls --disable-bootstrap --enable-default-pie --build=x86_64-unknown-linux-gnu --host=x86_64-unknown-linux-gnu --target=x86_64-w64-mingw32
Thread model: mcf
Supported LTO compression algorithms: zlib
gcc version 14.3.0 (GCC) 

The Guix release builds, however, use a GCC configured with the posix thread model.

Therefore, the CI jobs exercise a toolchain whose threading differs from the one used for release binaries. This reduces the value of the Windows cross-compilation CI jobs.

Somewhat related: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1054148.

GCC is being built from source anyway, IIRC, according to #35877 (review)

So if this is important, it may be possible to configure GCC with the thread model that is configured by the Guix release builds.

GCC is being built from source

Correct, we can change the thread model here

Hi @hebasto @willcl-ark , I would like to work on fixing this mismatch.

To align the environments, would the preferred approach be to override the Nix derivation in contrib/devtools/shell-win64-cross.nix to enforce the posix thread model so it matches our current Guix releases? Or is there an active plan to migrate the Guix builds to use the newer mcf thread model instead?

Let me know which direction is preferred and I can start researching the implementation for the fix!

So if this is important...

I believe it is.

Ok, I was thinking that there is no relevant end-user visible behavior change, and this is mostly a style/consistency cleanup.

I'm pretty sure we can ~ trivially override with:

diff --git a/contrib/devtools/shell-win64-cross.nix b/contrib/devtools/shell-win64-cross.nix
index 4fd83b0da7a..2c81e7e7555 100644
--- a/contrib/devtools/shell-win64-cross.nix
+++ b/contrib/devtools/shell-win64-cross.nix
@@ -16,11 +16,20 @@ let
       else throw "Unsupported HOST: ${host}";
   toolchain = crossPkgs.stdenv.cc.targetPrefix;
   pthreads = crossPkgs.windows.pthreads;
+  gcc = crossPkgs.wrapCCWith {
+    cc = crossPkgs.gcc14.cc.override {
+      threadsCross = {
+        model = "posix"; # match Guix release binaries
+        package = null; # don't add mcfgthread when using posix thread model
+      };
+    };
+    extraPackages = [ ];
+  };
 in
 
 pkgs.mkShellNoCC {
   packages = [
-    crossPkgs.gcc14
+    gcc
     pkgs.nsis
   ];

But waiting for a build to finish to confirm.