"Fix" for SF bug #520644: __slots__ are not pickled.

As promised in my response to the bug report, I'm not really fixing
it; in fact, one could argule over what the proper fix should do.
Instead, I'm adding a little magic that raises TypeError if you try to
pickle an instance of a class that has __slots__ but doesn't define or
override __getstate__.  This is done by adding a bozo __getstate__
that always raises TypeError.

Objects/typeobject.c

@@ -850,6 +850,21 @@ static PyGetSetDef subtype_getsets[] = {
 	{0},
 };
 
+/* bozo: __getstate__ that raises TypeError */
+
+static PyObject *
+bozo_func(PyObject *self, PyObject *args)
+{
+	PyErr_SetString(PyExc_TypeError,
+			"a class that defines __slots__ without "
+			"defining __getstate__ cannot be pickled");
+	return NULL;
+}
+
+static PyMethodDef bozo_ml = {"__getstate__", bozo_func};
+
+static PyObject *bozo_obj = NULL;
+
 static PyObject *
 type_new(PyTypeObject *metatype, PyObject *args, PyObject *kwds)
 {
@@ -973,6 +988,27 @@ type_new(PyTypeObject *metatype, PyObject *args, PyObject *kwds)
 			/* XXX Check against null bytes in name */
 		}
 	}
+	if (slots != NULL) {
+		/* See if *this* class defines __getstate__ */
+		PyObject *getstate = PyDict_GetItemString(dict,
+							  "__getstate__");
+		if (getstate == NULL) {
+			/* If not, provide a bozo that raises TypeError */
+			if (bozo_obj == NULL) {
+				bozo_obj = PyCFunction_New(&bozo_ml, NULL);
+				if (bozo_obj == NULL) {
+					/* XXX decref various things */
+					return NULL;
+				}
+			}
+			if (PyDict_SetItemString(dict,
+						 "__getstate__",
+						 bozo_obj) < 0) {
+				/* XXX decref various things */
+				return NULL;
+			}
+		}
+	}
 	if (slots == NULL && base->tp_dictoffset == 0 &&
 	    (base->tp_setattro == PyObject_GenericSetAttr ||
 	     base->tp_setattro == NULL)) {