bpo-40170: Hide impl detail of Py_TRASHCAN_BEGIN macro (GH-23235)

The Py_TRASHCAN_BEGIN macro no longer accesses PyTypeObject attributes,
but now can get the condition by calling the new private
_PyTrash_cond() function which hides implementation details.

Author: shihai1991

Include/cpython/object.h

@@ -516,6 +516,8 @@ struct _ts;
 /* Python 3.9 private API, invoked by the macros below. */
 PyAPI_FUNC(int) _PyTrash_begin(struct _ts *tstate, PyObject *op);
 PyAPI_FUNC(void) _PyTrash_end(struct _ts *tstate);
+/* Python 3.10 private API, invoked by the Py_TRASHCAN_BEGIN(). */
+PyAPI_FUNC(int) _PyTrash_cond(PyObject *op, destructor dealloc);
 
 #define PyTrash_UNWIND_LEVEL 50
 
@@ -539,7 +541,7 @@ PyAPI_FUNC(void) _PyTrash_end(struct _ts *tstate);
 
 #define Py_TRASHCAN_BEGIN(op, dealloc) \
     Py_TRASHCAN_BEGIN_CONDITION(op, \
-        Py_TYPE(op)->tp_dealloc == (destructor)(dealloc))
+        _PyTrash_cond(_PyObject_CAST(op), (destructor)dealloc))
 
 /* For backwards compatibility, these macros enable the trashcan
  * unconditionally */

Misc/NEWS.d/next/C API/2020-11-13-01-40-28.bpo-40170.uh8lEf.rst

@@ -0,0 +1,3 @@
+The ``Py_TRASHCAN_BEGIN`` macro no longer accesses PyTypeObject attributes,
+but now can get the condition by calling the new private
+:c:func:`_PyTrash_cond()` function which hides implementation details.

Objects/object.c

@@ -2134,6 +2134,15 @@ _PyTrash_end(PyThreadState *tstate)
 }
 
 
+/* bpo-40170: It's only be used in Py_TRASHCAN_BEGIN macro to hide
+   implementation details. */
+int
+_PyTrash_cond(PyObject *op, destructor dealloc)
+{
+    return Py_TYPE(op)->tp_dealloc == dealloc;
+}
+
+
 void _Py_NO_RETURN
 _PyObject_AssertFailed(PyObject *obj, const char *expr, const char *msg,
                        const char *file, int line, const char *function)