tests: add workarounds for python brokenness on Windows

msys2 is broken only on clang, due to -Werror issues in the python
headers as patched by msys2.

MSVC is simply weird... due to the use of an unversioned platlib/purelib
directory, the python2 and python3 components overlap.

unittests/baseplatformtests.py

@@ -484,3 +484,6 @@ class BasePlatformTests(TestCase):
     def assertPathDoesNotExist(self, path):
         m = f'Path {path!r} should not exist'
         self.assertFalse(os.path.exists(path), msg=m)
+
+    def assertLength(self, val, length):
+        assert len(val) == length, f'{val} is not length {length}'

unittests/helpers.py

@@ -204,3 +204,11 @@ def get_path_without_cmd(cmd: str, path: str) -> str:
         paths.discard(dirname)
         path = pathsep.join([str(p) for p in paths])
     return path
+
+def xfail_if_jobname(name: str):
+    if os.environ.get('MESON_CI_JOBNAME') == name:
+        return unittest.expectedFailure
+
+    def wrapper(func):
+        return func
+    return wrapper

unittests/pythontests.py

@@ -20,8 +20,9 @@ from run_tests import (
 
 from .allplatformstests import git_init
 from .baseplatformtests import BasePlatformTests
+from .helpers import *
 
-from mesonbuild.mesonlib import TemporaryDirectoryWinProof
+from mesonbuild.mesonlib import MachineChoice, TemporaryDirectoryWinProof
 
 class PythonTests(BasePlatformTests):
     '''
@@ -61,6 +62,9 @@ python = pymod.find_installation('python3', required: true)
     def _test_bytecompile(self, py2=False):
         testdir = os.path.join(self.src_root, 'test cases', 'python', '2 extmodule')
 
+        env = get_fake_env(testdir, self.builddir, self.prefix)
+        cc = detect_c_compiler(env, MachineChoice.HOST)
+
         self.init(testdir, extra_args=['-Dpython2=auto', '-Dpython.bytecompile=1'])
         self.build()
         self.install()
@@ -71,7 +75,11 @@ python = pymod.find_installation('python3', required: true)
                 realfile = os.path.join(root, file)
                 if file.endswith('.py'):
                     cached = glob.glob(realfile+'?') + glob.glob(os.path.join(root, '__pycache__', os.path.splitext(file)[0] + '*.pyc'))
-                    self.assertEqual(len(cached), 2)
+                    if cc.get_id() == 'msvc':
+                        # MSVC python installs python2/python3 into the same directory
+                        self.assertLength(cached, 4)
+                    else:
+                        self.assertLength(cached, 2)
                     count += 1
         # there are 5 files x 2 installations
         if py2:
@@ -79,6 +87,7 @@ python = pymod.find_installation('python3', required: true)
         else:
             self.assertEqual(count, 5)
 
+    @xfail_if_jobname('msys2-clangx64ninja')
     def test_bytecompile_multi(self):
         if not shutil.which('python2'):
             raise self.skipTest('python2 not installed')