set VSCMD_ARG_TGT_ARCH based on targetted architecture

cibuildwheel/windows.py

@@ -152,6 +152,16 @@ def setup_setuptools_cross_compile(
     # set the platform name
     map_plat = {"32": "win32", "64": "win-amd64", "ARM64": "win-arm64"}
     plat_name = map_plat[python_configuration.arch]
+
+    # Set environment variable so that setuptools._distutils.get_platform()
+    # identifies the target, not the host
+    vscmd_arg_tgt_arch = {"32": "x86", "64": "x64", "ARM64": "arm64"}
+    current_tgt_arch = vscmd_arg_tgt_arch[python_configuration.arch]
+    if (env.get("VSCMD_ARG_TGT_ARCH") or current_tgt_arch) != current_tgt_arch:
+        msg = f"VSCMD_ARG_TGT_ARCH must be set to {current_tgt_arch!r}, got {env['VSCMD_ARG_TGT_ARCH']!r}. If you're setting up MSVC yourself (e.g. using vcvarsall.bat or msvc-dev-cmd), make sure to target the right architecture. Alternatively, run cibuildwheel without configuring MSVC, and let the build backend handle it."
+        raise errors.FatalError(msg)
+    env["VSCMD_ARG_TGT_ARCH"] = current_tgt_arch
+
     # (This file must be default/locale encoding, so we can't pass 'encoding')
     distutils_cfg.write_text(
         textwrap.dedent(

pyproject.toml

@@ -75,6 +75,7 @@ test = [
     "pytest-timeout",
     "pytest-xdist",
     "pytest>=6",
+    "setuptools",
     "tomli_w",
     "validate-pyproject",
 ]
@@ -140,6 +141,8 @@ disallow_untyped_decorators = true
 [[tool.mypy.overrides]]
 module = [
     "setuptools",
+    "setuptools._distutils", # needed even if only directly import setuptools._distutils.util
+    "setuptools._distutils.util",
     "pytest", # ignored in pre-commit to speed up check
     "bashlex",
     "bashlex.*",

unit_test/get_platform_test.py

@@ -0,0 +1,92 @@
+import contextlib
+import sys
+from pathlib import Path
+from typing import Dict
+
+import pytest
+import setuptools._distutils.util
+
+from cibuildwheel.errors import FatalError
+from cibuildwheel.util import CIProvider, detect_ci_provider
+from cibuildwheel.windows import PythonConfiguration, setup_setuptools_cross_compile
+
+# monkeypatching os.name is too flaky. E.g. It works on my machine, but fails in pipeline
+if not sys.platform.startswith("win"):
+    pytest.skip("Windows-only tests", allow_module_level=True)
+
+
+@contextlib.contextmanager
+def patched_environment(monkeypatch: pytest.MonkeyPatch, environment: Dict[str, str]):
+    with monkeypatch.context() as mp:
+        for envvar, val in environment.items():
+            mp.setenv(name=envvar, value=val)
+        yield
+
+
+def test_x86(tmp_path: Path, monkeypatch: pytest.MonkeyPatch):
+    arch = "32"
+    environment: Dict[str, str] = {}
+
+    configuration = PythonConfiguration("irrelevant", arch, "irrelevant", None)
+
+    setup_setuptools_cross_compile(tmp_path, configuration, tmp_path, environment)
+    with patched_environment(monkeypatch, environment):
+        target_platform = setuptools._distutils.util.get_platform()
+
+    assert environment["VSCMD_ARG_TGT_ARCH"] == "x86"
+    assert target_platform == "win32"
+
+
+def test_x64(tmp_path: Path, monkeypatch: pytest.MonkeyPatch):
+    arch = "64"
+    environment: Dict[str, str] = {}
+
+    configuration = PythonConfiguration("irrelevant", arch, "irrelevant", None)
+
+    setup_setuptools_cross_compile(tmp_path, configuration, tmp_path, environment)
+    with patched_environment(monkeypatch, environment):
+        target_platform = setuptools._distutils.util.get_platform()
+
+    assert environment["VSCMD_ARG_TGT_ARCH"] == "x64"
+    assert target_platform == "win-amd64"
+
+
+@pytest.mark.skipif(
+    detect_ci_provider() == CIProvider.azure_pipelines, reason="arm64 not recognised on azure"
+)
+def test_arm(tmp_path: Path, monkeypatch: pytest.MonkeyPatch):
+    arch = "ARM64"
+    environment: Dict[str, str] = {}
+
+    configuration = PythonConfiguration("irrelevant", arch, "irrelevant", None)
+
+    setup_setuptools_cross_compile(tmp_path, configuration, tmp_path, environment)
+    with patched_environment(monkeypatch, environment):
+        target_platform = setuptools._distutils.util.get_platform()
+
+    assert environment["VSCMD_ARG_TGT_ARCH"] == "arm64"
+    assert target_platform == "win-arm64"
+
+
+def test_env_set(tmp_path: Path):
+    arch = "32"
+    environment = {"VSCMD_ARG_TGT_ARCH": "x64"}
+
+    configuration = PythonConfiguration("irrelevant", arch, "irrelevant", None)
+
+    with pytest.raises(FatalError, match="VSCMD_ARG_TGT_ARCH"):
+        setup_setuptools_cross_compile(tmp_path, configuration, tmp_path, environment)
+
+
+def test_env_blank(tmp_path: Path, monkeypatch: pytest.MonkeyPatch):
+    arch = "32"
+    environment = {"VSCMD_ARG_TGT_ARCH": ""}
+
+    configuration = PythonConfiguration("irrelevant", arch, "irrelevant", None)
+
+    setup_setuptools_cross_compile(tmp_path, configuration, tmp_path, environment)
+    with patched_environment(monkeypatch, environment):
+        target_platform = setuptools._distutils.util.get_platform()
+
+    assert environment["VSCMD_ARG_TGT_ARCH"] == "x86"
+    assert target_platform == "win32"