gh-102304: Debug mode no longer supports Limited API 3.9

Doc/whatsnew/3.12.rst

@@ -1536,13 +1536,10 @@ Build Changes
   :file:`!configure`.
   (Contributed by Christian Heimes in :gh:`89886`.)
 
-* C extensions built with the :ref:`limited C API <limited-c-api>`
-  on :ref:`Python build in debug mode <debug-build>` no longer support Python
-  3.9 and older. In this configuration, :c:func:`Py_INCREF` and
-  :c:func:`Py_DECREF` are now always implemented as opaque function calls,
-  but the called functions were added to Python 3.10. Build C extensions
-  with a release build of Python or with Python 3.12 and older, to keep support
-  for Python 3.9 and older.
+* Python built in debug mode can no longer build C extensions using the
+  :ref:`limited C API <limited-c-api>` version 3.9 and older. Build C
+  extensions with a release build of Python (recommended), or with Python 3.12
+  and older.
   (Contributed by Victor Stinner in :gh:`102304`.)
 
 

Include/object.h

@@ -62,6 +62,12 @@ whose size is determined when the object is allocated.
 #  error Py_LIMITED_API is incompatible with Py_TRACE_REFS
 #endif
 
+// gh-102304: In debug mode, the limited API implements Py_INCREF() by calling
+// _Py_IncRef() which was added to Python 3.10.
+#if defined(Py_REF_DEBUG) && defined(Py_LIMITED_API) && Py_LIMITED_API+0 < 0x030a0000
+#  error "Python built in debug mode is incompatible with limited C API older than 3.10"
+#endif
+
 #ifdef Py_TRACE_REFS
 /* Define pointers to support a doubly-linked list of all live heap objects. */
 #define _PyObject_HEAD_EXTRA            \

Lib/test/test_xxlimited.py

@@ -3,7 +3,10 @@
 import types
 
 xxlimited = import_helper.import_module('xxlimited')
-xxlimited_35 = import_helper.import_module('xxlimited_35')
+try:
+    import xxlimited_35
+except ImportError:
+    xxlimited_35 = None
 
 
 class CommonTests:
@@ -70,6 +73,7 @@ def test_buffer(self):
         self.assertEqual(b2[0], 1)
 
 
+@unittest.skipIf(xxlimited_35 is None, 'need xxlimited_35 extension')
 class TestXXLimited35(CommonTests, unittest.TestCase):
     module = xxlimited_35
 

Misc/NEWS.d/next/C API/2023-06-06-17-15-55.gh-issue-102304.RPP7AB.rst

@@ -0,0 +1,4 @@
+Python built in debug mode can no longer build C extensions using the
+:ref:`limited C API <limited-c-api>` version 3.9 and older. Build C
+extensions with a release build of Python (recommended), or with Python 3.12
+and older. Patch by Victor Stinner.

configure.ac

@@ -7390,7 +7390,9 @@ dnl Limited API template modules.
 dnl The limited C API is not compatible with the Py_TRACE_REFS macro.
 dnl Emscripten does not support shared libraries yet.
 PY_STDLIB_MOD([xxlimited], [test "$with_trace_refs" = "no"], [test "$ac_cv_func_dlopen" = yes])
-PY_STDLIB_MOD([xxlimited_35], [test "$with_trace_refs" = "no"], [test "$ac_cv_func_dlopen" = yes])
+dnl xxlimited_35 cannot be built with Python built in debug mode
+dnl (if the Py_REF_DEBUG macro is defined)
+PY_STDLIB_MOD([xxlimited_35], [test "$with_trace_refs" = "no" -a "$Py_DEBUG" != "true"], [test "$ac_cv_func_dlopen" = yes])
 
 # substitute multiline block, must come after last PY_STDLIB_MOD()
 AC_SUBST([MODULE_BLOCK])