Select the stable ABI free-threaded builds

Author: encukou

Include/Python.h

@@ -47,10 +47,6 @@
 #endif
 
 #if defined(Py_GIL_DISABLED)
-#  if defined(Py_LIMITED_API) && !defined(_Py_OPAQUE_PYOBJECT)
-#    error "Py_LIMITED_API is not currently supported in the free-threaded build"
-#  endif
-
 #  if defined(_MSC_VER)
 #    include <intrin.h>             // __readgsqword()
 #  endif

Include/patchlevel.h

@@ -61,4 +61,32 @@
 #define PYTHON_ABI_VERSION 3
 #define PYTHON_ABI_STRING "3"
 
+
+/* Stable ABI for free-threaded builds (introduced in PEP 803)
+   is enabled by one of:
+     - Py_TARGET_ABI3T, or
+     - Py_LIMITED_API and Py_GIL_DISABLED.
+   "Output" macros to be used internally:
+     - Py_LIMITED_API (defines the subset of API we expose)
+     - _Py_OPAQUE_PYOBJECT (additionally hides what's ABI-incompatible between
+       free-threaded & GIL)
+   (Don't use Py_TARGET_ABI3T directly; we merge into the above two here.
+    We do set it for users' convenience though.)
+ */
+#if defined(Py_LIMITED_API) && defined(Py_GIL_DISABLED) \
+    && !defined(Py_TARGET_ABI3T)
+#  define Py_TARGET_ABI3T Py_LIMITED_API
+#endif
+#if defined(Py_TARGET_ABI3T)
+#  define _Py_OPAQUE_PYOBJECT
+#  if !defined(Py_LIMITED_API)
+#    define Py_LIMITED_API Py_TARGET_ABI3T
+#  elif Py_LIMITED_API > Py_TARGET_ABI3T
+     // if both are defined, use the *lower* version,
+     // i.e. maximum compatibility
+#    undef Py_LIMITED_API
+#    define Py_LIMITED_API Py_TARGET_ABI3T
+#  endif
+#endif
+
 #endif //_Py_PATCHLEVEL_H