Test suite is brittle around global sysconfig._config_vars state.

[jaraco]

I applied that diff, but unfortunately, that led to more failures in test_cc_overrides_ldshared_for_cxx_correctly... and while debugging the issue, I found that test is itself fragile - it fails differently depending on whether it's run in isolation or not (even if before the patch).

 distutils 765bbbd7 @ tox -e py311 -- -p no:cov -k test_cc_overrides_ldshared_for_cxx_correctly
.pkg-cpython311: _optional_hooks> python '/Users/jaraco/Library/Application Support/pipx/venvs/tox/lib/python3.12/site-packages/pyproject_api/_backend.py' True setuptools.build_meta
.pkg-cpython311: get_requires_for_build_editable> python '/Users/jaraco/Library/Application Support/pipx/venvs/tox/lib/python3.12/site-packages/pyproject_api/_backend.py' True setuptools.build_meta
.pkg-cpython311: build_editable> python '/Users/jaraco/Library/Application Support/pipx/venvs/tox/lib/python3.12/site-packages/pyproject_api/_backend.py' True setuptools.build_meta
py311: install_package> python -I -m pip install --force-reinstall --no-deps /Users/jaraco/code/pypa/distutils/.tox/.tmp/package/20/distutils-0.1.dev3944+g765bbbd-0.editable-py3-none-any.whl
py311: commands[0]> pytest -p no:cov -k test_cc_overrides_ldshared_for_cxx_correctly
============================================================== test session starts ===============================================================
platform darwin -- Python 3.11.8, pytest-8.0.2, pluggy-1.4.0
cachedir: .tox/py311/.pytest_cache
rootdir: /Users/jaraco/code/pypa/distutils
configfile: pytest.ini
plugins: enabler-3.0.0, checkdocs-2.10.1, pyfakefs-5.3.5, ruff-0.2.1
collected 478 items / 477 deselected / 1 selected                                                                                                

distutils/tests/test_unixccompiler.py F                                                                                                    [100%]

==================================================================== FAILURES ====================================================================
_________________________________________ TestUnixCCompiler.test_cc_overrides_ldshared_for_cxx_correctly _________________________________________

self = <distutils.tests.test_unixccompiler.TestUnixCCompiler object at 0x1031f8b50>

    @pytest.mark.skipif('platform.system == "Windows"')
    def test_cc_overrides_ldshared_for_cxx_correctly(self):
        """
        Ensure that setting CC env variable also changes default linker
        correctly when building C++ extensions.
    
        pypa/distutils#126
        """
    
        def gcv(v):
            if v == 'LDSHARED':
                return 'gcc-4.2 -bundle -undefined dynamic_lookup '
            elif v == 'LDCXXSHARED':
                return 'g++-4.2 -bundle -undefined dynamic_lookup '
            elif v == 'CXX':
                return 'g++-4.2'
            elif v == 'CC':
                return 'gcc-4.2'
            return ''
    
        def gcvs(*args, _orig=sysconfig.get_config_vars):
            if args:
                return list(map(sysconfig.get_config_var, args))
            return _orig()
    
        sysconfig.get_config_var = gcv
        sysconfig.get_config_vars = gcvs
        with mock.patch.object(
            self.cc, 'spawn', return_value=None
        ) as mock_spawn, mock.patch.object(
            self.cc, '_need_link', return_value=True
        ), mock.patch.object(
            self.cc, 'mkpath', return_value=None
        ), EnvironmentVarGuard() as env:
            env['CC'] = 'ccache my_cc'
            env['CXX'] = 'my_cxx'
            del env['LDSHARED']
>           sysconfig.customize_compiler(self.cc)

distutils/tests/test_unixccompiler.py:275: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

compiler = <distutils.tests.test_unixccompiler.compiler_wrapper.<locals>.CompilerWrapper object at 0x1031f8f50>

    def customize_compiler(compiler):  # noqa: C901
        """Do any platform-specific customization of a CCompiler instance.
    
        Mainly needed on Unix, so we can plug in the information that
        varies across Unices and is stored in Python's Makefile.
        """
        if compiler.compiler_type == "unix":
            if sys.platform == "darwin":
                # Perform first-time customization of compiler-related
                # config vars on OS X now that we know we need a compiler.
                # This is primarily to support Pythons from binary
                # installers.  The kind and paths to build tools on
                # the user system may vary significantly from the system
                # that Python itself was built on.  Also the user OS
                # version and build tools may not support the same set
                # of CPU architectures for universal builds.
                global _config_vars
                # Use get_config_var() to ensure _config_vars is initialized.
                if not get_config_var('CUSTOMIZED_OSX_COMPILER'):
                    import _osx_support
    
                    _osx_support.customize_compiler(_config_vars)
>                   _config_vars['CUSTOMIZED_OSX_COMPILER'] = 'True'
E                   TypeError: 'NoneType' object does not support item assignment

distutils/sysconfig.py:291: TypeError
============================================================ short test summary info =============================================================
FAILED distutils/tests/test_unixccompiler.py::TestUnixCCompiler::test_cc_overrides_ldshared_for_cxx_correctly - TypeError: 'NoneType' object does not support item assignment
======================================================= 1 failed, 477 deselected in 0.16s ========================================================
py311: exit 1 (0.30 seconds) /Users/jaraco/code/pypa/distutils> pytest -p no:cov -k test_cc_overrides_ldshared_for_cxx_correctly pid=70507
.pkg-cpython311: _exit> python '/Users/jaraco/Library/Application Support/pipx/venvs/tox/lib/python3.12/site-packages/pyproject_api/_backend.py' True setuptools.build_meta
  py311: FAIL code 1 (0.86=setup[0.56]+cmd[0.30] seconds)
  evaluation failed :( (0.90 seconds)

Probably we should figure out why that test is reliant on other tests and fix that too.

Originally posted by @jaraco in #228 (comment)

The issue seems to be surfaced by this change, where the fallback value is now '', which allows the darwin-specific behavior to be reached:

distutils/distutils/sysconfig.py

Lines 285 to 291 in e651e53

	global _config_vars
	# Use get_config_var() to ensure _config_vars is initialized.
	if not get_config_var('CUSTOMIZED_OSX_COMPILER'):
	import _osx_support
	_osx_support.customize_compiler(_config_vars)
	_config_vars['CUSTOMIZED_OSX_COMPILER'] = 'True'

That code attempts to mutate a global variable, which, if it wasn't initialized by another test, will lead to the error.

I believe the proposed diff is safer and more correct. We'll want to fix that undesirable entanglement.

Originally posted by @jaraco in #228 (comment)

[jaraco]

Here's another case where the problem could have been encountered except that the test ensures that the global variable is initialized.