Remove explicit empty tuple reuse in cpickle.

PyTuple_New(0) always returns the same empty tuple from its free list anyway,
so we are not saving much here. Plus, the code where this was used is on
uncommon run paths.

Modules/_pickle.c

@@ -166,9 +166,6 @@ typedef struct {
 
     /* codecs.encode, used for saving bytes in older protocols */
     PyObject *codecs_encode;
-
-    /* As the name says, an empty tuple. */
-    PyObject *empty_tuple;
 } PickleState;
 
 /* Forward declaration of the _pickle module definition. */
@@ -205,7 +202,6 @@ _Pickle_ClearState(PickleState *st)
     Py_CLEAR(st->name_mapping_3to2);
     Py_CLEAR(st->import_mapping_3to2);
     Py_CLEAR(st->codecs_encode);
-    Py_CLEAR(st->empty_tuple);
 }
 
 /* Initialize the given pickle module state. */
@@ -321,10 +317,6 @@ _Pickle_InitState(PickleState *st)
     }
     Py_CLEAR(codecs);
 
-    st->empty_tuple = PyTuple_New(0);
-    if (st->empty_tuple == NULL)
-        goto error;
-
     return 0;
 
   error:
@@ -1137,8 +1129,9 @@ _Unpickler_ReadFromFile(UnpicklerObject *self, Py_ssize_t n)
         return -1;
 
     if (n == READ_WHOLE_LINE) {
-        PickleState *st = _Pickle_GetGlobalState();
-        data = PyObject_Call(self->readline, st->empty_tuple, NULL);
+        PyObject *empty_tuple = PyTuple_New(0);
+        data = PyObject_Call(self->readline, empty_tuple, NULL);
+        Py_DECREF(empty_tuple);
     }
     else {
         PyObject *len = PyLong_FromSsize_t(n);
@@ -3774,8 +3767,10 @@ save(PicklerObject *self, PyObject *obj, int pers_save)
             /* Check for a __reduce__ method. */
             reduce_func = _PyObject_GetAttrId(obj, &PyId___reduce__);
             if (reduce_func != NULL) {
-                reduce_value = PyObject_Call(reduce_func, st->empty_tuple,
+                PyObject *empty_tuple = PyTuple_New(0);
+                reduce_value = PyObject_Call(reduce_func, empty_tuple,
                                              NULL);
+                Py_DECREF(empty_tuple);
             }
             else {
                 PyErr_Format(st->PicklingError,
@@ -7412,7 +7407,6 @@ pickle_traverse(PyObject *m, visitproc visit, void *arg)
     Py_VISIT(st->name_mapping_3to2);
     Py_VISIT(st->import_mapping_3to2);
     Py_VISIT(st->codecs_encode);
-    Py_VISIT(st->empty_tuple);
     return 0;
 }