MAINT: detect free-threaded CPython ("nogil") and handle limited API

The free-threaded CPython build does not support the limited API yet,
and won't in the near future. To avoid either cryptic build failures
or a successfull build yielding an `abi3` tag which would not be
correct, raise a clear error and don't attempt to build.

mesonpy/__init__.py

@@ -758,6 +758,12 @@ def __init__(
             if not value:
                 self._limited_api = False
 
+            if self._limited_api:
+                if bool(sysconfig.get_config_var('Py_GIL_DISABLED')):
+                    raise BuildError('Package targets Python\'s Limited API, which is not supported by free-threaded '
+                                     'CPython. The `python.allow_limited_api` Meson build option may be used to '
+                                     'override the package default.')
+
     def _run(self, cmd: Sequence[str]) -> None:
         """Invoke a subprocess."""
         # Flush the line to ensure that the log line with the executed

tests/test_wheel.py

@@ -39,6 +39,8 @@
     'win32': '.dll',
 }.get(sys.platform, '.so')
 
+NOGIL_BUILD = bool(sysconfig.get_config_var("Py_GIL_DISABLED"))
+
 # Test against the wheel tag generated by packaging module.
 tag = next(packaging.tags.sys_tags())
 ABI = tag.abi
@@ -287,6 +289,7 @@ def test_skip_subprojects(package_subproject, tmp_path, arg):
 
 # Requires Meson 1.3.0, see https://github.com/mesonbuild/meson/pull/11745.
 @pytest.mark.skipif(MESON_VERSION < (1, 2, 99), reason='Meson version too old')
+@pytest.mark.skipif(NOGIL_BUILD, reason='Free-threaded CPython does not support the limited API')
 def test_limited_api(wheel_limited_api):
     artifact = wheel.wheelfile.WheelFile(wheel_limited_api)
     name = artifact.parsed_filename
@@ -297,6 +300,7 @@ def test_limited_api(wheel_limited_api):
 
 # Requires Meson 1.3.0, see https://github.com/mesonbuild/meson/pull/11745.
 @pytest.mark.skipif(MESON_VERSION < (1, 2, 99), reason='Meson version too old')
+@pytest.mark.skipif(NOGIL_BUILD, reason='Free-threaded CPython does not support the limited API')
 @pytest.mark.xfail('__pypy__' in sys.builtin_module_names, reason='PyPy does not use special modules suffix for stable ABI')
 def test_limited_api_bad(package_limited_api, tmp_path):
     with pytest.raises(mesonpy.BuildError, match='The package declares compatibility with Python limited API but '):