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

@@ -747,7 +747,7 @@ def __init__(
             self._metadata.requires_python.prereleases = True
             if platform.python_version().rstrip('+') not in self._metadata.requires_python:
                 raise MesonBuilderError(
-                    f'Package requires Python version {self._metadata.requires_python}, '
+                    f'The package requires Python version {self._metadata.requires_python}, '
                     f'running on {platform.python_version()}')
 
         # limited API
@@ -759,6 +759,11 @@ def __init__(
             if not value:
                 self._limited_api = False
 
+        if self._limited_api and bool(sysconfig.get_config_var('Py_GIL_DISABLED')):
+            raise BuildError(
+                'The 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_project.py

@@ -23,7 +23,7 @@
 
 
 def test_unsupported_python_version(package_unsupported_python_version):
-    with pytest.raises(mesonpy.MesonBuilderError, match='Package requires Python version ==1.0.0'):
+    with pytest.raises(mesonpy.MesonBuilderError, match='The package requires Python version ==1.0.0'):
         with mesonpy._project():
             pass
 

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 '):