ci: use POSIX threads for Nix Windows builds #36316

Guix release binaries use GCC’s POSIX thread model, while the pinned Nixpkgs toolchain defaults to mcf. This makes POSIX the default for Nix Windows cross builds. Nightly builds can select mcf or win32 with the threadModel argument.

The Nixpkgs threads setting selects the model before GCC is built. Only the mcf model pulls in mcfgthreads. For POSIX, GCC’s target libraries need winpthreads headers and libraries during the compiler build. The EXTRA_*_FOR_TARGET flags provide those paths without exposing target headers to GCC’s native build tools, which caused the earlier build failure.

The compiler wrapper includes winpthreads through extraPackages, so the shell no longer needs to export pthread include and library flags manually. CI also prints GCC’s configured thread model to make mismatches visible.

Fixes #36314

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36316.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #36271 (Revert #36057 & document mingw-w64 requirement by fanquake)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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Added a second commit we may want to drop to print/verify first in the CI run.

Concept ACK.

3edcc510370c7fd804060960a0a5e16bbe356c96

Could we make the thread model configurable? This would help with testing MCF in nightly builds.

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🚧 At least one of the CI tasks failed. _{Task lint: https://github.com/bitcoin/bitcoin/actions/runs/35753251918/job/106832510905} _{LLM reason (✨ experimental): CI failed because the shell lint step (shellcheck) reported SC2016 in ci/test/01_base_install.sh line 55 (expressions inside single quotes).}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

</details>

Could we make the thread model configurable? This would help with testing MCF in nightly builds.

I’ve made the thread model configurable through a Nix argument. e.g. to test mcf:

HOST=x86_64-w64-mingw32ucrt nix-shell contrib/devtools/shell-win64-cross.nix --argstr threadModel mcf

Tested c1052aa56ed5b851e9e388fc2b86c0d766acd12c on Fedora 44.

Could we make the thread model configurable? This would help with testing MCF in nightly builds.

I’ve made the thread model configurable through a Nix argument. e.g. to test mcf:

HOST=x86_64-w64-mingw32ucrt nix-shell contrib/devtools/shell-win64-cross.nix --argstr threadModel mcf

$ HOST=x86_64-w64-mingw32ucrt nix-shell contrib/devtools/shell-win64-cross.nix --argstr threadModel mcf
$ $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) 

$ gmake -C depends -j 16
<snip>
/home/hebasto/dev/bitcoin/depends/work/build/x86_64-w64-mingw32ucrt/zeromq/4.3.5-cbb97251921/src/thread.cpp:13:10: fatal error: pthread.h: No such file or directory
   13 | #include "pthread.h"
      |          ^~~~~~~~~~~
compilation terminated.
<snip>

Hmmm, we are starting to veer into quite a lot of extra code needed to support this on-the-fly switching:

commit 78331095c68b8df4cf34f3b7b761506af6239915
Author: will <will@256k1.dev>
Date:   Wed Sep 23 13:06:56 2026 +0100

    ci: expose winpthreads for all Windows thread models
    
    ZeroMQ includes pthread.h even when GCC uses the MCF thread model.
    The POSIX-only wrapper exposed winpthreads, so MCF builds failed
    despite GCC itself selecting mcf successfully.
    
    Add winpthreads to every wrapper while retaining mcfgthreads for MCF.
    Keep the target-only GCC build flags specific to POSIX, where GCC needs
    them while building its runtime libraries.

diff --git a/contrib/devtools/shell-win64-cross.nix b/contrib/devtools/shell-win64-cross.nix
index f61d3e93497..8b22d55ebbf 100644
--- a/contrib/devtools/shell-win64-cross.nix
+++ b/contrib/devtools/shell-win64-cross.nix
@@ -25,20 +25,20 @@ let
   toolchain = crossPkgs.stdenv.cc.targetPrefix;
   pthreads = crossPkgs.windows.pthreads;
   crossGcc = crossPkgs.buildPackages.gcc14;
-  gcc = if threadModel == "posix" then
-    crossGcc.override {
-      cc = crossGcc.cc.overrideAttrs (old: {
-        # Keep target pthread headers out of GCC's native build tools.
-        env = old.env // {
-          EXTRA_FLAGS_FOR_TARGET =
-            "${old.env.EXTRA_FLAGS_FOR_TARGET} -idirafter ${pthreads}/include -B${pthreads}/lib";
-          EXTRA_LDFLAGS_FOR_TARGET =
-            "${old.env.EXTRA_LDFLAGS_FOR_TARGET} -Wl,-L${pthreads}/lib";
-        };
-      });
-      extraPackages = [ pthreads ];
-    }
-  else crossGcc;
+  gcc = crossGcc.override {
+    cc = if threadModel == "posix" then crossGcc.cc.overrideAttrs (old: {
+      # Keep target pthread headers out of GCC's native build tools.
+      env = old.env // {
+        EXTRA_FLAGS_FOR_TARGET =
+          "${old.env.EXTRA_FLAGS_FOR_TARGET} -idirafter ${pthreads}/include -B${pthreads}/lib";
+        EXTRA_LDFLAGS_FOR_TARGET =
+          "${old.env.EXTRA_LDFLAGS_FOR_TARGET} -Wl,-L${pthreads}/lib";
+      };
+    }) else crossGcc.cc;
+    # ZeroMQ needs pthread.h even when GCC uses a different thread model.
+    extraPackages =
+      [ pthreads ] ++ pkgs.lib.optional (threadModel == "mcf") crossPkgs.threads.package;
+  };
 in
 
 pkgs.mkShellNoCC {

Are we sure we want' need this?

Are we sure we want' need this?

We can drop non-posix thread model support altogether. I can use customized scripts in the nightly builds.

We can drop non-posix thread model support altogether. I can use customized scripts in the nightly builds.

Dropped in the latest push.

ACK 9fd18aedb088b5ca6a7c424aab2a4a1ec3ef4eeb.

Need to backport this to 32.x as well.

Backported to 32.x in #36300.