CI: add a job for running tests under MSVC CPython with GCC as the de…

…fault compiler

The tests currently assume everywhere that there is only one compiler per platform,
and while it would be possible to parametrize all the tests it would make things more
complex and we'd also have to decide which compiler is required for running the tests and
which one is optional etc.

To avoid all this introduce a DISTUTILS_TEST_DEFAULT_COMPILER env var which can be used
to override the default compiler type for the whole test run. This keeps the tests as is
and makes sure all tests run against the alternative compiler. Also add it to pass_env
for tox, so it gets passed to pytest, if set.

The added CI job installs an ucrt targeting GCC via MSYS2, and forces the MSVC CPython
to use it via DISTUTILS_TEST_DEFAULT_COMPILER=mingw32.

.github/workflows/main.yml

@@ -163,6 +163,28 @@ jobs:
           source /tmp/venv/bin/activate
           pytest
 
+  test_msvc_python_mingw:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Setup Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: 3.12
+      - name: Install tox
+        run: python -m pip install tox
+      - name: Install GCC
+        uses: msys2/setup-msys2@v2
+        with:
+          msystem: ucrt64
+          install: mingw-w64-ucrt-x86_64-cc
+      - name: Run
+        run: |
+          $env:MSYS2_ROOT = msys2 -c 'cygpath -m /'
+          $env:PATH = "$env:MSYS2_ROOT/ucrt64/bin;$env:PATH"
+          $env:DISTUTILS_TEST_DEFAULT_COMPILER = "mingw32"
+          tox
+
   ci_setuptools:
     # Integration testing with setuptools
     strategy:

conftest.py

@@ -162,3 +162,26 @@ def disable_macos_customization(monkeypatch):
     from distutils import sysconfig
 
     monkeypatch.setattr(sysconfig, '_customize_macos', lambda: None)
+
+
+@pytest.fixture(autouse=True, scope="session")
+def monkey_patch_get_default_compiler():
+    """
+    Monkey patch distutils get_default_compiler to allow overriding the
+    default compiler. Mainly to test mingw32 with a MSVC Python.
+    """
+    from distutils import ccompiler
+
+    default_compiler = os.environ.get("DISTUTILS_TEST_DEFAULT_COMPILER")
+
+    if default_compiler is not None:
+
+        def patched_get_default_compiler(*args, **kwargs):
+            return default_compiler
+
+        original = ccompiler.get_default_compiler
+        ccompiler.get_default_compiler = patched_get_default_compiler
+        yield
+        ccompiler.get_default_compiler = original
+    else:
+        yield

tox.ini

@@ -5,6 +5,8 @@ setenv =
 	PYTHONWARNDEFAULTENCODING = 1
 	# pypa/distutils#99
 	VIRTUALENV_NO_SETUPTOOLS = 1
+pass_env =
+	DISTUTILS_TEST_DEFAULT_COMPILER
 commands =
 	pytest {posargs}
 usedevelop = True