Jim Fulton reported a segfault in dir(). A heavily proxied object

gvanrossum · gvanrossum · commit 4402241450fe · 2002-05-13T18:29:46.000Z
returned a proxy for __class__ whose __bases__ was also a proxy.  The
merge_class_dict() helper for dir() assumed incorrectly that __bases__
would always be a tuple and used the in-line tuple API on the proxy.

I will backport this to 2.2 as well.
diff --git a/Lib/test/test_descr.py b/Lib/test/test_descr.py
@@ -365,6 +365,26 @@ def getdict(self):
     # object.
     vereq(dir(None), dir(Ellipsis))
 
+    # Nasty test case for proxied objects
+    class Wrapper(object):
+        def __init__(self, obj):
+            self.__obj = obj
+        def __repr__(self):
+            return "Wrapper(%s)" % repr(self.__obj)
+        def __getitem__(self, key):
+            return Wrapper(self.__obj[key])
+        def __len__(self):
+            return len(self.__obj)
+        def __getattr__(self, name):
+            return Wrapper(getattr(self.__obj, name))
+
+    class C(object):
+        def __getclass(self):
+            return Wrapper(type(self))
+        __class__ = property(__getclass)
+
+    dir(C()) # This used to segfault
+
 binops = {
     'add': '+',
     'sub': '-',
diff --git a/Objects/object.c b/Objects/object.c
@@ -1520,14 +1520,22 @@ merge_class_dict(PyObject* dict, PyObject* aclass)
 	if (bases == NULL)
 		PyErr_Clear();
 	else {
+		/* We have no guarantee that bases is a real tuple */
 		int i, n;
-		assert(PyTuple_Check(bases));
-		n = PyTuple_GET_SIZE(bases);
-		for (i = 0; i < n; i++) {
-			PyObject *base = PyTuple_GET_ITEM(bases, i);
-			if (merge_class_dict(dict, base) < 0) {
-				Py_DECREF(bases);
-				return -1;
+		n = PySequence_Size(bases); /* This better be right */
+		if (n < 0)
+			PyErr_Clear();
+		else {
+			for (i = 0; i < n; i++) {
+				PyObject *base = PySequence_GetItem(bases, i);
+				if (base == NULL) {
+					Py_DECREF(bases);
+					return -1;
+				}
+				if (merge_class_dict(dict, base) < 0) {
+					Py_DECREF(bases);
+					return -1;
+				}
 			}
 		}
 		Py_DECREF(bases);
