gh-121617: Fix Py_CLEAR() in C++: replace NULL with _Py_NULL (GH-157067)

* Enhance Py_CLEAR() test in test_cext and test_cppext.
  Check that Py_CLEAR(obj) sets obj to NULL.
* Add also tests on Py_SETREF() and Py_BEGIN_CRITICAL_SECTION().
(cherry picked from commit 36c7440)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: vstinner

Include/refcount.h

@@ -484,7 +484,7 @@ static inline Py_ALWAYS_INLINE void Py_DECREF(PyObject *op)
     do { \
         _Py_TYPEOF(op)* _tmp_op_ptr = &(op); \
         _Py_TYPEOF(op) _tmp_old_op = (*_tmp_op_ptr); \
-        if (_tmp_old_op != NULL) { \
+        if (_tmp_old_op != _Py_NULL) { \
             *_tmp_op_ptr = _Py_NULL; \
             Py_DECREF(_tmp_old_op); \
         } \

Lib/test/test_cext/extension.c

@@ -95,9 +95,17 @@ _testcext_exec(PyObject *module)
     Py_BUILD_ASSERT(sizeof(int) == sizeof(unsigned int));
     assert(Py_BUILD_ASSERT_EXPR(sizeof(int) == sizeof(unsigned int)) == 0);
 
-    // Test Py_CLEAR()
-    obj = NULL;
+    // Test Py_CLEAR(): use typeof()/__typeof__() if available, or memcpy()
+    obj = Py_None;
     Py_CLEAR(obj);
+    assert(obj == NULL);
+
+#ifndef Py_LIMITED_API
+    // Test Py_SETREF(): use typeof()/__typeof__() if available, or memcpy()
+    obj = Py_None;
+    Py_SETREF(obj, NULL);
+    assert(obj == NULL);
+#endif
 
     // Test that Py_BEGIN_CRITICAL_SECTION is available
     Py_BEGIN_CRITICAL_SECTION(module);

Lib/test/test_cppext/extension.cpp

@@ -294,6 +294,22 @@ _testcppext_exec(PyObject *module)
     Py_BUILD_ASSERT(sizeof(int) == sizeof(unsigned int));
     assert(Py_BUILD_ASSERT_EXPR(sizeof(int) == sizeof(unsigned int)) == 0);
 
+    // Test Py_CLEAR(): use typeof()/__typeof__() if available, or memcpy()
+    PyObject *obj = Py_None;
+    Py_CLEAR(obj);
+    assert(obj == _Py_NULL);
+
+#ifndef Py_LIMITED_API
+    // Test Py_SETREF(): use typeof()/__typeof__() if available, or memcpy()
+    obj = Py_None;
+    Py_SETREF(obj, _Py_NULL);
+    assert(obj == _Py_NULL);
+#endif
+
+    // Test that Py_BEGIN_CRITICAL_SECTION is available
+    Py_BEGIN_CRITICAL_SECTION(module);
+    Py_END_CRITICAL_SECTION();
+
     return 0;
 }