ci: add Windows job

.github/workflows/ci.yml

@@ -133,10 +133,29 @@ jobs:
           export PATH="$(brew --prefix llvm)/bin:$PATH"
           CI_CONFIG="ci/configs/macos.bash" bash ci/scripts/ci.sh
 
+  test-windows-cross:
+    runs-on: windows-latest
+    name: test • windows runner
+    needs: build-windows-cross
+    timeout-minutes: 30
+
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Download Windows artifacts
+        uses: actions/download-artifact@v8
+        with:
+          name: windows-cross-build
+          path: windows-cross-artifact
+
+      - name: Run Windows tests
+        shell: pwsh
+        run: ci/scripts/windows_native_test.ps1 -ArtifactRoot windows-cross-artifact
+
   build:
     runs-on: ubuntu-latest
 
-    env:
+    env: &nix-build-env
       CI_NIXPKGS_CHANNEL: nixos-26.05
       CI_NIX_CONFIG: |
         keep-env-derivations = true
@@ -153,15 +172,18 @@ jobs:
     name: build • ${{ matrix.config }}
 
     steps:
-      - uses: actions/checkout@v5
+      - &nix-checkout
+        uses: actions/checkout@v5
 
-      - name: Determine CI configuration
+      - &nix-config
+        name: Determine CI configuration
         id: config
         env:
           CI_CONFIG: ci/configs/${{ matrix.config }}.bash
         run: ci/scripts/config.sh
 
-      - name: Install Nix
+      - &nix-install
+        name: Install Nix
         uses: cachix/install-nix-action@v31       # 2025-05-27, from https://github.com/cachix/install-nix-action/tags
         with:
           # Act executes inside an unprivileged container (Docker or Podman),
@@ -171,7 +193,8 @@ jobs:
             ${{ env.CI_NIX_CONFIG }}
             ${{ github.actor == 'nektos/act' && env.CI_NIX_CONFIG_ACT || '' }}
 
-      - name: Cache Nix store
+      - &nix-cache
+        name: Cache Nix store
         if: steps.config.outputs.cache_nix_store == 'true'
         uses: nix-community/cache-nix-action@v7
         with:
@@ -182,7 +205,44 @@ jobs:
             nix-${{ runner.os }}-
           gc-max-store-size-linux: 10G
 
-      - name: Run CI script
+      - &nix-run
+        name: Run CI script
         env:
           CI_CONFIG: ci/configs/${{ matrix.config }}.bash
         run: ci/scripts/run.sh
+
+  build-windows-cross:
+    runs-on: ubuntu-latest
+    env: *nix-build-env
+
+    # Keep the shared steps and cache keys using the same configuration context.
+    strategy:
+      fail-fast: false
+      matrix:
+        config: [windows]
+
+    name: build • ${{ matrix.config }}
+
+    steps:
+      - *nix-checkout
+      - *nix-config
+      - *nix-install
+      - *nix-cache
+      - *nix-run
+
+      - name: Package Windows artifacts
+        env:
+          CI_CONFIG: ci/configs/${{ matrix.config }}.bash
+        run: |
+          source "$CI_CONFIG"
+          nix_args=("${NIX_ARGS[@]+"${NIX_ARGS[@]}"}")
+          if [[ -n "${CI_NIXPKGS_CHANNEL:-}" ]]; then
+            nix_args+=(-I "nixpkgs=channel:${CI_NIXPKGS_CHANNEL}")
+          fi
+          nix develop --ignore-environment --keep CI_CONFIG "${nix_args[@]}" -f shell.nix --command bash ci/scripts/windows_package.sh
+
+      - name: Upload Windows artifacts
+        uses: actions/upload-artifact@v7
+        with:
+          name: windows-cross-build
+          path: windows-cross-artifact

CMakeLists.txt

@@ -198,6 +198,7 @@ target_link_libraries(multiprocess PUBLIC CapnProto::capnp)
 target_link_libraries(multiprocess PUBLIC CapnProto::capnp-rpc)
 target_link_libraries(multiprocess PUBLIC CapnProto::kj)
 target_link_libraries(multiprocess PUBLIC CapnProto::kj-async)
+target_link_libraries(multiprocess PUBLIC Threads::Threads)
 set_target_properties(multiprocess PROPERTIES
     PUBLIC_HEADER "${MP_PUBLIC_HEADERS}")
 install(TARGETS multiprocess EXPORT LibTargets

ci/README.md

@@ -22,6 +22,7 @@ CI_CONFIG=ci/configs/gnu32.bash    ci/scripts/run.sh
 CI_CONFIG=ci/configs/sanitize.bash ci/scripts/run.sh
 CI_CONFIG=ci/configs/olddeps.bash  ci/scripts/run.sh
 CI_CONFIG=ci/configs/newdeps.bash  ci/scripts/run.sh
+CI_CONFIG=ci/configs/windows.bash  ci/scripts/run.sh
 ```
 
 By default CI jobs will reuse their build directories. `CI_CLEAN=1` can be specified to delete them before running instead.

ci/configs/windows.bash

@@ -0,0 +1,21 @@
+CI_DESC="CI job cross-compiling to Windows with MinGW, tested with Wine"
+CI_DIR=build-windows
+CI_CACHE_NIX_STORE=true
+NIX_ARGS=(
+  --arg windows true
+  --arg minimal true
+)
+CMAKE_ARGS=(
+  -G Ninja
+  -DCMAKE_SYSTEM_NAME=Windows
+  -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER
+  -DCMAKE_CROSSCOMPILING_EMULATOR=wine
+  # -Wa,-mbig-obj: template-heavy C++ generates >32K COFF sections per .obj;
+  # BigCOFF format raises the limit.  Must be passed to the assembler via -Wa.
+  "-DCMAKE_CXX_FLAGS=-Wa,-mbig-obj"
+)
+# CXX is set by the nix cross shell to the mingw g++ wrapper; cmake picks
+# it up from the environment, so no CMAKE_CXX_COMPILER override needed.
+BUILD_ARGS=(-k 0)
+# Build native mpgen first (as a code generator for cross-compiled test/example targets).
+MPGEN_PRE_BUILD=1

ci/scripts/ci.sh

@@ -38,6 +38,49 @@ if [ -n "${CAPNP_CHECKOUT-}" ]; then
 fi
 
 src_dir=$PWD
+
+# If cross-compiling, build native mpgen first so it can be used as a code
+# generator when cmake invokes it from add_custom_command (which does not go
+# through CMAKE_CROSSCOMPILING_EMULATOR, unlike add_test executables).
+if [ -n "${MPGEN_PRE_BUILD-}" ]; then
+  native_dir="${src_dir}/${CI_DIR}-native"
+  [ -n "${CI_CLEAN-}" ] && rm -rf "$native_dir"
+  mkdir -p "$native_dir"
+  # Unset cross-compilation env vars so cmake uses the native compiler and
+  # does a native (not cross) build.  Key vars to clear:
+  #   CXX/CC/AR/RANLIB/LD - set to cross-compiler by the cross shell
+  #   cmakeFlags - nix cross shell injects -DCMAKE_SYSTEM_NAME=Windows etc.
+  # Pass NATIVE_CAPNPROTO_PREFIX as CMAKE_PREFIX_PATH so cmake finds the
+  # native Cap'n Proto rather than the cross-compiled one.
+  native_cmake_args=()
+  if [ -n "${NATIVE_CAPNPROTO_PREFIX-}" ]; then
+    # Build a cmake prefix path with native capnproto and its dependencies
+    # (openssl, zlib) so find_package and find_dependency succeed.
+    native_prefix="${NATIVE_CAPNPROTO_PREFIX}"
+    [ -n "${NATIVE_OPENSSL_DEV-}" ] && native_prefix="${native_prefix};${NATIVE_OPENSSL_DEV}"
+    [ -n "${NATIVE_OPENSSL_LIB-}" ] && native_prefix="${native_prefix};${NATIVE_OPENSSL_LIB}"
+    [ -n "${NATIVE_ZLIB_DEV-}" ]    && native_prefix="${native_prefix};${NATIVE_ZLIB_DEV}"
+    [ -n "${NATIVE_ZLIB_LIB-}" ]    && native_prefix="${native_prefix};${NATIVE_ZLIB_LIB}"
+    native_cmake_args+=(
+      "-DCMAKE_PREFIX_PATH=${native_prefix}"
+      "-DCapnProto_DIR=${NATIVE_CAPNPROTO_PREFIX}/lib/cmake/CapnProto"
+    )
+  fi
+  # -static-libstdc++ / -static-libgcc: the native mpgen must run inside the
+  # cross nix shell where the native libstdc++.so may not be in LD_LIBRARY_PATH.
+  native_cmake_args+=("-DCMAKE_EXE_LINKER_FLAGS=-static-libstdc++ -static-libgcc")
+  (cd "$native_dir" && env -u CXX -u CC -u AR -u RANLIB -u LD -u cmakeFlags cmake "$src_dir" "${native_cmake_args[@]+${native_cmake_args[@]}}" && cmake --build . -t mpgen)
+  CMAKE_ARGS+=("-DMPGEN_EXECUTABLE=${native_dir}/mpgen")
+
+  # Override capnp tool executables: the cross capnproto cmake config sets
+  # CAPNP_EXECUTABLE to capnp.exe (Windows binary), which can't run on Linux.
+  # Use the native capnp/capnpc-c++ binaries from pkgs.capnproto in nativeBuildInputs.
+  _capnp=$(command -v capnp 2>/dev/null || true)
+  _capnpc=$(command -v capnpc-c++ 2>/dev/null || true)
+  [ -n "$_capnp" ]   && CMAKE_ARGS+=("-DCAPNP_EXECUTABLE=$_capnp")
+  [ -n "$_capnpc" ]  && CMAKE_ARGS+=("-DCAPNPC_CXX_EXECUTABLE=$_capnpc")
+fi
+
 mkdir -p "$CI_DIR"
 cd "$CI_DIR"
 export CMAKE_BUILD_PARALLEL_LEVEL="$(nproc)"
@@ -67,4 +110,13 @@ else
     cmake --build . --target "$t" -- "${BUILD_ARGS[@]+"${BUILD_ARGS[@]}"}"
   done
 fi
+# When cross-compiling for Windows, copy GCC and MCF thread runtime DLLs
+# alongside the test executables so wine can find them (wine DLL loading
+# checks the executable's directory first, before any search-path logic).
+if [ -n "${WIN_RUNTIME_DLLS-}" ]; then
+  IFS=: read -ra _dll_dirs <<< "$WIN_RUNTIME_DLLS"
+  for _dir in "${_dll_dirs[@]}"; do
+    find "$_dir" -maxdepth 1 -name "*.dll" -exec cp -n {} test/ \;
+  done
+fi
 ctest --output-on-failure

ci/scripts/windows_native_test.ps1

@@ -0,0 +1,39 @@
+param(
+    [Parameter(Mandatory = $true)]
+    [string]$ArtifactRoot
+)
+
+# Copyright (c) The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or https://opensource.org/license/mit/.
+
+$ErrorActionPreference = "Stop"
+# Prevent PowerShell 7 from turning native-command stderr into a terminating
+# error before we get a chance to see it.
+$PSNativeCommandUseErrorActionPreference = $false
+
+$artifactPath = (Resolve-Path $ArtifactRoot).Path
+$env:PATH = "$artifactPath;$env:PATH"
+
+$mptest = Join-Path $artifactPath "test\mptest.exe"
+& $mptest 2>&1 | ForEach-Object { "$_" }
+$code = $LASTEXITCODE
+Write-Host ("mptest exit code: {0} (0x{0:X8})" -f $code)
+if ($code -ne 0) {
+    exit $code
+}
+
+$exampleDir = Join-Path $artifactPath "example"
+Push-Location $exampleDir
+try {
+    $output = "2+2`nexit`n" | & ".\mpexample.exe" 2>&1 | Out-String
+    Write-Host $output
+    if ($LASTEXITCODE -ne 0) {
+        exit $LASTEXITCODE
+    }
+    if ($output -notmatch "mpprinter:" -or $output -notmatch "Bye!") {
+        throw "Unexpected mpexample output."
+    }
+} finally {
+    Pop-Location
+}

ci/scripts/windows_package.sh

@@ -0,0 +1,104 @@
+#!/usr/bin/env bash
+#
+# Copyright (c) The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or https://opensource.org/license/mit/.
+#
+# Package cross-compiled Windows binaries + runtime DLLs into a directory
+# suitable for uploading as a workflow artifact. Must be run inside the
+# nix shell produced by `ci/configs/windows.bash` so the mingw toolchain
+# (objdump, g++) is on PATH.
+
+export LC_ALL=C.UTF-8
+
+set -o errexit -o nounset -o pipefail -o xtrace
+
+readonly SCRIPT_DIR="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)"
+readonly REPO_DIR="$(cd -- "${SCRIPT_DIR}/../.." && pwd)"
+
+[ "${CI_CONFIG+x}" ] && source "$CI_CONFIG"
+: "${CI_DIR:=build-windows}"
+
+readonly BUILD_DIR="${REPO_DIR}/${CI_DIR}"
+readonly ARTIFACT_DIR="${REPO_DIR}/windows-cross-artifact"
+
+# Nix's cross stdenv exports these; fall back to the canonical names if not.
+: "${CXX:=x86_64-w64-mingw32-g++}"
+: "${OBJDUMP:=x86_64-w64-mingw32-objdump}"
+
+copy_artifact_file() {
+  install -D -m 0755 "$1" "${ARTIFACT_DIR}/$2"
+}
+
+# Runtime DLLs for cross-compiled dependencies live in the sibling bin/
+# directory of each -L path the cross stdenv adds to NIX_LDFLAGS (e.g.
+# capnproto, mcfgthreads). Collect those bin/ dirs once for dll lookup.
+EXTRA_DLL_DIRS=("${BUILD_DIR}/test")
+if [[ -n "${WIN_RUNTIME_DLLS:-}" ]]; then
+  IFS=: read -r -a runtime_dirs <<< "$WIN_RUNTIME_DLLS"
+  EXTRA_DLL_DIRS+=("${runtime_dirs[@]}")
+fi
+for flag in ${NIX_LDFLAGS:-} ${NIX_LDFLAGS_x86_64_w64_mingw32:-}; do
+  case "${flag}" in
+    -L*)
+      candidate="${flag#-L}/../bin"
+      [[ -d "${candidate}" ]] && EXTRA_DLL_DIRS+=("${candidate}")
+      ;;
+  esac
+done
+
+copy_runtime_dlls() {
+  local exe="$1"
+  local dll dll_path dir
+  while read -r dll; do
+    [[ -n "${dll}" ]] || continue
+    # Skip DLLs that ship with Windows / Wine.
+    case "${dll}" in
+      ADVAPI32.dll|COMBASE.dll|COMCTL32.dll|GDI32.dll|KERNEL32.dll|OLE32.dll|OLEAUT32.dll|RPCRT4.dll|SHELL32.dll|UCRTBASE.dll|USER32.dll|WS2_32.dll)
+        continue
+        ;;
+    esac
+    # Already copied dependencies also break cycles between DLLs.
+    [[ -f "${ARTIFACT_DIR}/${dll}" ]] && continue
+    dll_path="$(${CXX} -print-file-name="${dll}")"
+    if [[ "${dll_path}" == "${dll}" || ! -f "${dll_path}" ]]; then
+      dll_path=""
+      for dir in "${EXTRA_DLL_DIRS[@]+"${EXTRA_DLL_DIRS[@]}"}"; do
+        if [[ -f "${dir}/${dll}" ]]; then
+          dll_path="${dir}/${dll}"
+          break
+        fi
+      done
+    fi
+    if [[ -z "${dll_path}" || ! -f "${dll_path}" ]]; then
+      case "${dll}" in
+        lib*.dll)
+          echo "Could not locate runtime DLL ${dll}." >&2
+          exit 1
+          ;;
+        *)
+          continue
+          ;;
+      esac
+    fi
+    install -D -m 0755 "${dll_path}" "${ARTIFACT_DIR}/${dll}"
+    copy_runtime_dlls "${dll_path}"
+  done < <(${OBJDUMP} -p "${exe}" | awk '/DLL Name: / {print $3}' | sort -u)
+}
+
+rm -rf "${ARTIFACT_DIR}"
+
+readonly EXES=(
+  test/mptest.exe
+  example/mpexample.exe
+  example/mpcalculator.exe
+  example/mpprinter.exe
+)
+
+for exe in "${EXES[@]}"; do
+  copy_artifact_file "${BUILD_DIR}/${exe}" "${exe}"
+done
+
+for exe in "${EXES[@]}"; do
+  copy_runtime_dlls "${BUILD_DIR}/${exe}"
+done

example/example.cpp

@@ -8,6 +8,7 @@
 #include <cstring> // IWYU pragma: keep
 #include <filesystem>
 #include <fstream>
+#include <functional>
 #include <future>
 #include <iostream>
 #include <kj/common.h>
@@ -26,11 +27,11 @@ namespace fs = std::filesystem;
 
 static auto Spawn(mp::EventLoop& loop, const std::string& process_argv0, const std::string& new_exe_name)
 {
-    const auto [pid, socket] = mp::SpawnProcess([&](mp::SpawnConnectInfo info) -> std::vector<std::string> {
+    const auto [pid, socket] = mp::SpawnProcess([&](std::string connect_info) -> std::vector<std::string> {
         fs::path path = process_argv0;
         path.remove_filename();
         path.append(new_exe_name);
-        return {path.string(), std::move(info)};
+        return {path.string(), std::move(connect_info)};
     });
     return std::make_tuple(mp::ConnectStream<InitInterface>(loop, mp::MakeStream(loop, socket)), pid);
 }