BUG: adapt to platform-specific abi3 and abi3t filename suffixes

.github/workflows/tests.yml

@@ -105,6 +105,14 @@ jobs:
           - os: windows-latest
             python: '3.14'
             meson: '@git+https://github.com/mesonbuild/meson.git'
+          # Test free-threaded builds to verify the correct handling of the
+          # 'abi3t' stable ABI, see PEP 803. Test on Linux and Windows to cover
+          # the case where stable ABI extension modules use a dedicated
+          # filename suffix and the case where they do not.
+          - os: ubuntu-latest
+            python: '3.15t'
+          - os: windows-latest
+            python: '3.15t'
 
     steps:
       - name: Checkout

mesonpy/__init__.py

@@ -99,7 +99,7 @@ class InvalidLicenseExpression(Exception):  # type: ignore[no-redef]
 _MESON_ARGS_KEYS = ['dist', 'setup', 'compile', 'install']
 
 _SUFFIXES = importlib.machinery.all_suffixes()
-_EXTENSION_SUFFIX_REGEX = re.compile(r'^[^.]+\.(?:(?P<abi>[^.]+)\.)?(?:so|pyd|dll)$')
+_EXTENSION_SUFFIX_REGEX = re.compile(r'^[^.]+\.(?:(?P<abi>[^.-]+)(?:-[^.]+)?\.)?(?:so|pyd|dll)$')
 assert all(re.match(_EXTENSION_SUFFIX_REGEX, f'foo{x}') for x in importlib.machinery.EXTENSION_SUFFIXES)
 
 # Map Meson installation path placeholders to wheel installation paths.
@@ -429,7 +429,7 @@ 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
+            # On free-threaded Python 3.15 or later, 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.

tests/test_tags.py

@@ -29,13 +29,22 @@
 
 
 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.
+    """Get the filename suffix identifying stable ABI extension modules."""
+
+    # EXTENSION_SUFFIXES is ordered from the more to the less specific ABI.
+    # On Python 3.15 or later, this resolves to ``.abi3t-${arch}`` for
+    # free-threaded builds and to ``.abi3-${arch}`` on regular builds.
+    #
+    # Older Python versions do not support a stable ABI for free-threaded
+    # builds, and do not support a platform-specific filename suffix, and
+    # this resolves to simply ``.abi3``.
+    #
+    # On Windows, Python does not use a dedicated filename suffix for stable
+    # ABI extension modules, and this resolves to ``.pyd``.
     for suffix in importlib.machinery.EXTENSION_SUFFIXES:
-        if '.abi3' in suffix:  # Unix
+        if suffix.startswith('.abi3'):
             return suffix
-        elif suffix == '.pyd':  # Windows
+        if suffix == '.pyd':
             return suffix