don't expand the operand to Py_XINCREF/XDECREF/CLEAR/DECREF multiple times (closes #17206)

A patch from Illia Polosukhin.

Include/object.h

@@ -680,12 +680,6 @@ is not considered to be a reference to the type object, to save
 complications in the deallocation function.  (This is actually a
 decision that's up to the implementer of each new type so if you want,
 you can count such references to the type object.)
-
-*** WARNING*** The Py_DECREF macro must have a side-effect-free argument
-since it may evaluate its argument multiple times.  (The alternative
-would be to mace it a proper function or assign it to a global temporary
-variable first, both of which are slower; and in a multi-threaded
-environment the global variable trick is not safe.)
 */
 
 /* First define a pile of simple helper macros, one set per special
@@ -764,15 +758,16 @@ PyAPI_FUNC(void) _Py_Dealloc(PyObject *);
 
 #define Py_INCREF(op) (                         \
     _Py_INC_REFTOTAL  _Py_REF_DEBUG_COMMA       \
-    ((PyObject*)(op))->ob_refcnt++)
+    ((PyObject *)(op))->ob_refcnt++)
 
 #define Py_DECREF(op)                                   \
     do {                                                \
+        PyObject *_py_decref_tmp = (PyObject *)(op);    \
         if (_Py_DEC_REFTOTAL  _Py_REF_DEBUG_COMMA       \
-        --((PyObject*)(op))->ob_refcnt != 0)            \
+        --(_py_decref_tmp)->ob_refcnt != 0)             \
-            _Py_CHECK_REFCNT(op)                        \
+            _Py_CHECK_REFCNT(_py_decref_tmp)            \
         else                                            \
-        _Py_Dealloc((PyObject *)(op));                  \
+        _Py_Dealloc(_py_decref_tmp);                    \
     } while (0)
 
 /* Safely decref `op` and set `op` to NULL, especially useful in tp_clear
@@ -811,16 +806,27 @@ PyAPI_FUNC(void) _Py_Dealloc(PyObject *);
  */
 #define Py_CLEAR(op)                            \
     do {                                        \
-        if (op) {                               \
+        PyObject *_py_tmp = (PyObject *)(op);   \
-            PyObject *_py_tmp = (PyObject *)(op);               \
+        if (_py_tmp != NULL) {                  \
             (op) = NULL;                        \
             Py_DECREF(_py_tmp);                 \
         }                                       \
     } while (0)
 
 /* Macros to use in case the object pointer may be NULL: */
-#define Py_XINCREF(op) do { if ((op) == NULL) ; else Py_INCREF(op); } while (0)
+#define Py_XINCREF(op)                                \
-#define Py_XDECREF(op) do { if ((op) == NULL) ; else Py_DECREF(op); } while (0)
+    do {                                              \
+        PyObject *_py_xincref_tmp = (PyObject *)(op); \
+        if (_py_xincref_tmp != NULL)                  \
+            Py_INCREF(_py_xincref_tmp);               \
+    } while (0)                                    
+
+#define Py_XDECREF(op)                                \
+    do {                                              \
+        PyObject *_py_xdecref_tmp = (PyObject *)(op); \
+        if (_py_xdecref_tmp != NULL)                  \
+            Py_DECREF(_py_xdecref_tmp);               \
+    } while (0)
 
 /*
 These are provided as conveniences to Python runtime embedders, so that

Misc/ACKS

@@ -983,6 +983,7 @@ Oleg Plakhotnyuk
 Remi Pointel
 Ariel Poliak
 Guilherme Polo
+Illia Polosukhin
 Michael Pomraning
 Iustin Pop
 Claudiu Popa

Misc/NEWS

@@ -10,6 +10,10 @@ What's New in Python 3.4.0 Alpha 1?
 Core and Builtins
 -----------------
 
+- Issue #17206: Py_CLEAR(), Py_DECREF(), Py_XINCREF() and Py_XDECREF() now 
+  expands their arguments once instead of multiple times. 
+  Patch written by Illia Polosukhin.
+
 - Issue #17937: Try harder to collect cyclic garbage at shutdown.
 
 - Issue #12370: Prevent class bodies from interfering with the __class__

Modules/_testcapimodule.c

@@ -2468,6 +2468,48 @@ test_pytime_object_to_timespec(PyObject *self, PyObject *args)
     return Py_BuildValue("Nl", _PyLong_FromTime_t(sec), nsec);
 }
 
+static PyObject *
+_test_incref(PyObject *ob)
+{
+    Py_INCREF(ob);
+    return ob;
+}
+
+static PyObject *
+test_xincref_doesnt_leak(PyObject *ob)
+{
+    PyObject *obj = PyLong_FromLong(0);
+    Py_XINCREF(_test_incref(obj));
+    Py_DECREF(obj);
+    Py_DECREF(obj);
+    Py_DECREF(obj);
+    Py_RETURN_NONE;
+}
+
+static PyObject *
+test_incref_doesnt_leak(PyObject *ob)
+{
+    PyObject *obj = PyLong_FromLong(0);
+    Py_INCREF(_test_incref(obj));
+    Py_DECREF(obj);
+    Py_DECREF(obj);
+    Py_DECREF(obj);
+    Py_RETURN_NONE;
+}
+
+static PyObject *
+test_xdecref_doesnt_leak(PyObject *ob)
+{
+    Py_XDECREF(PyLong_FromLong(0));
+    Py_RETURN_NONE;
+}
+
+static PyObject *
+test_decref_doesnt_leak(PyObject *ob)
+{
+    Py_DECREF(PyLong_FromLong(0));
+    Py_RETURN_NONE;
+}
 
 static PyMethodDef TestMethods[] = {
     {"raise_exception",         raise_exception,                 METH_VARARGS},
@@ -2478,6 +2520,10 @@ static PyMethodDef TestMethods[] = {
     {"test_dict_iteration",     (PyCFunction)test_dict_iteration,METH_NOARGS},
     {"test_lazy_hash_inheritance",      (PyCFunction)test_lazy_hash_inheritance,METH_NOARGS},
     {"test_long_api",           (PyCFunction)test_long_api,      METH_NOARGS},
+    {"test_xincref_doesnt_leak",(PyCFunction)test_xincref_doesnt_leak,      METH_NOARGS},
+    {"test_incref_doesnt_leak", (PyCFunction)test_incref_doesnt_leak,      METH_NOARGS},
+    {"test_xdecref_doesnt_leak",(PyCFunction)test_xdecref_doesnt_leak,      METH_NOARGS},
+    {"test_decref_doesnt_leak", (PyCFunction)test_decref_doesnt_leak,      METH_NOARGS},
     {"test_long_and_overflow", (PyCFunction)test_long_and_overflow,
      METH_NOARGS},
     {"test_long_as_double",     (PyCFunction)test_long_as_double,METH_NOARGS},