bpo-41909: Enable previously disabled recursion checks. (GH-22536)

Enable recursion checks which were disabled when get __bases__ of
non-type objects in issubclass() and isinstance() and when intern
strings. It fixes a stack overflow when getting __bases__ leads
to infinite recursion.

Originally recursion checks was disabled for PyDict_GetItem() which
silences all errors including the one raised in case of detected
recursion and can return incorrect result. But now the code uses
PyDict_GetItemWithError() and PyDict_SetDefault() instead.

1 files changed, 10 insertions, 0 deletions

diff --git a/Lib/test/test_isinstance.py b/Lib/test/test_isinstance.py
index 91e79c2..109c3f8 100644
--- a/Lib/test/test_isinstance.py
+++ b/Lib/test/test_isinstance.py
@@ -303,6 +303,16 @@ class TestIsInstanceIsSubclass(unittest.TestCase):
 
         self.assertEqual(True, issubclass(B(), int))
 
+    def test_infinite_recursion_in_bases(self):
+        class X:
+            @property
+            def __bases__(self):
+                return self.__bases__
+
+        self.assertRaises(RecursionError, issubclass, X(), int)
+        self.assertRaises(RecursionError, issubclass, int, X())
+        self.assertRaises(RecursionError, isinstance, 1, X())
+
 
 def blowstack(fxn, arg, compare_to):
     # Make sure that calling isinstance with a deeply nested tuple for its