[WIP] bpo-39511: Add Py_GetNone() and Py_GetNoneRef() functions

Include/internal/pycore_interp.h

@@ -158,6 +158,9 @@ struct _is {
     struct _ceval_state ceval;
     struct _gc_runtime_state gc;
 
+    /* per interpreter singletons (strong reference) */
+    PyObject *none;
+
     PyObject *modules;
     PyObject *modules_by_index;
     PyObject *sysdict;

Include/internal/pycore_pylifecycle.h

@@ -54,7 +54,7 @@ extern PyStatus _PyTypes_Init(void);
 extern PyStatus _PyTypes_InitSlotDefs(void);
 extern PyStatus _PyImportZip_Init(PyThreadState *tstate);
 extern PyStatus _PyGC_Init(PyThreadState *tstate);
-
+extern PyStatus _Py_InitSingletons(PyThreadState *tstate);
 
 /* Various internal finalizers */
 
@@ -74,6 +74,7 @@ extern void _PyExc_Fini(void);
 extern void _PyImport_Fini(void);
 extern void _PyImport_Fini2(void);
 extern void _PyGC_Fini(PyThreadState *tstate);
+extern void _Py_FiniSingletons(PyThreadState *tstate);
 extern void _PyType_Fini(void);
 extern void _Py_HashRandomization_Fini(void);
 extern void _PyUnicode_Fini(PyThreadState *tstate);

Include/object.h

@@ -526,17 +526,17 @@ they can have object code that is not dependent on Python compilation flags.
 PyAPI_FUNC(void) Py_IncRef(PyObject *);
 PyAPI_FUNC(void) Py_DecRef(PyObject *);
 
-/*
-_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').
+// Get a borrowed reference to the None singleton.
+PyAPI_DATA(PyObject*) Py_GetNone(void);
 
-Don't forget to apply Py_INCREF() when returning this value!!!
-*/
-PyAPI_DATA(PyObject) _Py_NoneStruct; /* Don't use this directly */
-#define Py_None (&_Py_NoneStruct)
+// Get a strong reference to the None singleton.
+PyAPI_DATA(PyObject*) Py_GetNoneRef(void);
+
+// Macro for backward compatibility.
+#define Py_None Py_GetNone()
 
-/* Macro for returning Py_None from a function */
-#define Py_RETURN_NONE return Py_INCREF(Py_None), Py_None
+/* Macro for returning a new strong reference to None from a function */
+#define Py_RETURN_NONE return Py_GetNoneRef()
 
 /*
 Py_NotImplemented is a singleton used to signal that an operation is

Objects/object.c

@@ -1628,6 +1628,30 @@ PyObject _Py_NoneStruct = {
   1, &_PyNone_Type
 };
 
+
+PyObject*
+Py_GetNone(void)
+{
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    /* The GIL must be held to call Py_GetNone() */
+    assert(interp != NULL);
+    PyObject *none = interp->none;
+    /* Py_GetNone() must not be called before _Py_InitSingletons(),
+       nor after _Py_FiniSingletons() */
+    assert(none != NULL);
+    return none;
+}
+
+
+PyObject*
+Py_GetNoneRef(void)
+{
+    PyObject *none = Py_GetNone();
+    Py_INCREF(none);
+    return none;
+}
+
+
 /* NotImplemented is an object that can be used to signal that an
    operation is not implemented for the given type combination. */
 
@@ -2220,3 +2244,20 @@ PyObject_GET_WEAKREFS_LISTPTR(PyObject *op)
 #ifdef __cplusplus
 }
 #endif
+
+
+PyStatus
+_Py_InitSingletons(PyThreadState *tstate)
+{
+    PyObject *none = &_Py_NoneStruct;
+    Py_INCREF(none);
+    tstate->interp->none = none;
+    return _PyStatus_OK();
+}
+
+
+void
+_Py_FiniSingletons(PyThreadState *tstate)
+{
+    Py_CLEAR(tstate->interp->none);
+}

Python/pylifecycle.c

@@ -580,6 +580,11 @@ pycore_init_types(PyThreadState *tstate)
     PyStatus status;
     int is_main_interp = _Py_IsMainInterpreter(tstate);
 
+    status = _Py_InitSingletons(tstate);
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
+
     status = _PyGC_Init(tstate);
     if (_PyStatus_EXCEPTION(status)) {
         return status;
@@ -1323,6 +1328,8 @@ finalize_interp_delete(PyThreadState *tstate)
        created GIL, which ensures that Py_Initialize / Py_FinalizeEx can be
        called multiple times. */
 
+    _Py_FiniSingletons(tstate);
+
     PyInterpreterState_Delete(tstate->interp);
 }