pythongh-115754: Export Py_None, Py_False, Py_True as symbols

In the limited C API and the stable ABI, implement Py_None, Py_False
and Py_True constants are symbols.

Include/boolobject.h

@@ -17,9 +17,15 @@ extern "C" {
 PyAPI_DATA(PyLongObject) _Py_FalseStruct;
 PyAPI_DATA(PyLongObject) _Py_TrueStruct;
 
-/* Use these macros */
+// Export symbols in the stable ABI
+PyAPI_DATA(PyLongObject*) Py_False;
+PyAPI_DATA(PyLongObject*) Py_True;
+
+#ifndef Py_LIMITED_API
+// Implement Py_False and Py_True as macros in the non-limited C API
 #define Py_False _PyObject_CAST(&_Py_FalseStruct)
 #define Py_True _PyObject_CAST(&_Py_TrueStruct)
+#endif
 
 // Test if an object is the True singleton, the same as "x is True" in Python.
 PyAPI_FUNC(int) Py_IsTrue(PyObject *x);

Include/object.h

@@ -1070,7 +1070,14 @@ _Py_NoneStruct is an object of undefined type which can be used in contexts
 where NULL (nil) is not suitable (since NULL often means 'error').
 */
 PyAPI_DATA(PyObject) _Py_NoneStruct; /* Don't use this directly */
+
+// Export the symbol in the stable ABI
+PyAPI_DATA(PyObject*) Py_None;
+
+#ifndef Py_LIMITED_API
+// Implement Py_None as a macro in the non-limited C API
 #define Py_None (&_Py_NoneStruct)
+#endif
 
 // Test if an object is the None singleton, the same as "x is None" in Python.
 PyAPI_FUNC(int) Py_IsNone(PyObject *x);

Misc/NEWS.d/next/C API/2024-02-21-10-54-27.gh-issue-115754.RSL-q0.rst

@@ -0,0 +1,6 @@
+In the limited C API and the stable ABI, implement ``Py_None``, ``Py_False``
+and ``Py_True`` constants are symbols, rather than implementing them as
+macros. So they can be loaded by dlopen/dlsym in an embedded in Python,
+rather than having to reimplement these macros manually. In the non-limited
+C API, these constants are still implemented as macros. Patch by Victor
+Stinner.

Objects/boolobject.c

@@ -225,3 +225,10 @@ struct _longobject _Py_TrueStruct = {
         { 1 }
     }
 };
+
+// Stable ABI: export symbols
+
+#undef Py_False
+#undef Py_True
+PyLongObject *Py_False = _Py_CAST(PyLongObject*, &_Py_FalseStruct);
+PyLongObject *Py_True = _Py_CAST(PyLongObject*, &_Py_TrueStruct);

Objects/object.c

@@ -2970,3 +2970,8 @@ _Py_SetRefcnt(PyObject *ob, Py_ssize_t refcnt)
 {
     Py_SET_REFCNT(ob, refcnt);
 }
+
+#undef Py_None
+
+// Export the symbol in the stable ABI
+PyObject *Py_None = &_Py_NoneStruct;