ENH: add support for PEP 803 abi3t with Python 3.15.0b2+

mesonpy/__init__.py

@@ -426,6 +426,13 @@ def _stable_abi(self) -> Optional[str]:
         # not use the stable ABI filename suffix and wheels should not
         # be tagged with the abi3 tag.
         if self._limited_api and '__pypy__' not in sys.builtin_module_names:
+            # On free-threaded Python 3.15.0b2+, we expect to be
+            # building 'abi3t' wheels for the time being. In the future
+            # we will want an option to target 'abi3t' from GIL-enabled
+            # Python too.
+            abi3t = bool(sysconfig.get_config_var('Py_GIL_DISABLED')) and sys.version_info >= (3, 15)
+            expected_abi = 'abi3t' if abi3t else 'abi3'
+
             # Verify stable ABI compatibility: examine files installed
             # in {platlib} that look like extension modules, and raise
             # an exception if any of them has a Python version
@@ -434,11 +441,11 @@ def _stable_abi(self) -> Optional[str]:
                 match = _EXTENSION_SUFFIX_REGEX.match(entry.dst.name)
                 if match:
                     abi = match.group('abi')
-                    if abi is not None and abi != 'abi3':
+                    if abi is not None and abi != expected_abi:
                         raise BuildError(
                             f'The package declares compatibility with Python limited API but extension '
                             f'module {os.fspath(entry.dst)!r} is tagged for a specific Python version.')
-            return 'abi3'
+            return 'abi3.abi3t' if abi3t else 'abi3'
         return None
 
     def _install_path(self, wheel_file: mesonpy._wheelfile.WheelFile, origin: Path, destination: pathlib.Path) -> None:
@@ -874,10 +881,10 @@ def __init__(
             if not allow_limited_api:
                 self._limited_api = False
 
-        if self._limited_api and bool(sysconfig.get_config_var('Py_GIL_DISABLED')):
+        if self._limited_api and bool(sysconfig.get_config_var('Py_GIL_DISABLED')) and sys.version_info < (3, 15):
             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.')
+                'The package targets Python\'s Limited API, which is not supported by free-threaded CPython before version '
+                '3.15. The "python.allow_limited_api" Meson build option may be used to override the package default.')
 
         # Shared library support on Windows requires collaboration
         # from the package, make sure the developers acknowledge this.

tests/packages/limited-api-ft/meson.build

@@ -0,0 +1,14 @@
+# SPDX-FileCopyrightText: 2023-2026 The meson-python developers
+#
+# SPDX-License-Identifier: MIT
+
+project('limited-api-ft', 'c', version: '1.0.0', meson_version: '>= 1.3')
+
+py = import('python').find_installation(pure: false)
+
+py.extension_module(
+  'module',
+  'module.c',
+  limited_api: '3.15',
+  install: true,
+)

tests/packages/limited-api-ft/module.c

@@ -0,0 +1,33 @@
+// SPDX-FileCopyrightText: 2023-2026 The meson-python developers
+//
+// SPDX-License-Identifier: MIT
+
+#include <Python.h>
+
+static PyObject* add(PyObject *self, PyObject *args) {
+    int a, b;
+
+    if (!PyArg_ParseTuple(args, "ii", &a, &b))
+        return NULL;
+
+    return PyLong_FromLong(a + b);
+}
+
+static PyMethodDef methods[] = {
+    {"add", add, METH_VARARGS, NULL},
+    {NULL, NULL, 0, NULL},
+};
+
+PyABIInfo_VAR(abi_info);
+
+static PySlot module_slots[] = {
+    PySlot_STATIC_DATA(Py_mod_name, "module"),
+    PySlot_STATIC_DATA(Py_mod_methods, methods),
+    PySlot_DATA(Py_mod_gil, Py_MOD_GIL_NOT_USED),
+    PySlot_DATA(Py_mod_abi, &abi_info),
+    PySlot_END,
+};
+
+PyMODEXPORT_FUNC PyModExport_module(void) {
+    return module_slots;
+}

tests/packages/limited-api-ft/pyproject.toml

@@ -0,0 +1,10 @@
+# SPDX-FileCopyrightText: 2023-2026 The meson-python developers
+#
+# SPDX-License-Identifier: MIT
+
+[build-system]
+build-backend = 'mesonpy'
+requires = ['meson-python']
+
+[tool.meson-python]
+limited-api = true

tests/packages/limited-api/module.c

@@ -20,7 +20,7 @@ static PyMethodDef methods[] = {
 
 static struct PyModuleDef module = {
     PyModuleDef_HEAD_INIT,
-    "plat",
+    "module",
     NULL,
     -1,
     methods,

tests/test_tags.py

@@ -19,6 +19,7 @@
 import mesonpy._tags
 
 from .conftest import adjust_packaging_platform_tag
+from .test_wheel import NOGIL_BUILD
 
 
 # Test against the wheel tag generated by packaging module.
@@ -29,6 +30,9 @@
 
 
 def get_abi3_suffix():
+    # EXTENSION_SUFFIXES are ordered in preference order, and more specific ABI is preferred.
+    # On free-threaded 3.15+, this will match ".abi3t" as the only supported stable ABI.
+    # On GIL-enabled 3.15+, it will match plain ".abi3" first, since that ABI is more specific.
     for suffix in importlib.machinery.EXTENSION_SUFFIXES:
         if '.abi3' in suffix:  # Unix
             return suffix
@@ -130,7 +134,12 @@ def test_tag_stable_abi():
         'platlib': [f'extension{ABI3SUFFIX}'],
     }, limited_api=True)
     # PyPy does not support the stable ABI.
-    abi = 'abi3' if '__pypy__' not in sys.builtin_module_names else ABI
+    if '__pypy__' in sys.builtin_module_names:
+        abi = ABI
+    elif NOGIL_BUILD and sys.version_info >= (3, 15):
+        abi = 'abi3.abi3t'
+    else:
+        abi = 'abi3'
     assert str(builder.tag) == f'{INTERPRETER}-{abi}-{PLATFORM}'
 
 

tests/test_wheel.py

@@ -412,3 +412,14 @@ def test_cmake_subproject(wheel_cmake_subproject):
         f'.cmake_subproject.mesonpy.libs/libcmaketest{LIB_SUFFIX}',
         f'cmakesubproject{EXT_SUFFIX}',
     }
+
+
+# Requires Meson 1.3.0, see https://github.com/mesonbuild/meson/pull/11745.
+@pytest.mark.skipif(MESON_VERSION < (1, 2, 99), reason='meson too old')
+@pytest.mark.skipif(sys.version_info < (3, 15), reason='Requires PEP 820 API, present since Python 3.15')
+def test_limited_api_ft(wheel_limited_api_ft):
+    artifact = wheel.wheelfile.WheelFile(wheel_limited_api_ft)
+    name = artifact.parsed_filename
+    assert name.group('pyver') == INTERPRETER
+    assert name.group('abi') == 'abi3.abi3t' if NOGIL_BUILD else 'abi3'
+    assert name.group('plat') == PLATFORM