bpo-36473: add maximum iteration check for dict .values() and .items() (GH-12619)

Lib/test/test_dict.py

@@ -477,7 +477,25 @@ class DictTest(unittest.TestCase):
         with self.assertRaises(RuntimeError):
             for i in d:
                 del d[0]
-                d[1] = 1
+                d[0] = 0
+
+    def test_mutating_iteration_delete_over_values(self):
+        # change dict content during iteration
+        d = {}
+        d[0] = 0
+        with self.assertRaises(RuntimeError):
+            for i in d.values():
+                del d[0]
+                d[0] = 0
+
+    def test_mutating_iteration_delete_over_items(self):
+        # change dict content during iteration
+        d = {}
+        d[0] = 0
+        with self.assertRaises(RuntimeError):
+            for i in d.items():
+                del d[0]
+                d[0] = 0
 
     def test_mutating_lookup(self):
         # changing dict during a lookup (issue #14417)

Misc/NEWS.d/next/Core and Builtins/2019-03-27-23-53-00.bpo-36452.xhK2lT.rst

-Changing `dict` keys during iteration will now be detected in certain corner cases where the number of keys isn't changed (but they keys themselves are), and a `RuntimeError` will be raised.
\ No newline at end of file
+Changing ``dict`` keys during iteration of the dict itself, ``keys()``,
+``values()``, or ``items()`` will now be detected in certain corner cases where
+keys are deleted/added so that the number of keys isn't changed.
+A `RuntimeError` will be raised after ``len(dict)`` iterations.
+Contributed by Thomas Perl.

Objects/dictobject.c

@@ -3630,6 +3630,12 @@ dictiter_iternextvalue(dictiterobject *di)
             goto fail;
         value = entry_ptr->me_value;
     }
+    // We found an element, but did not expect it
+    if (di->len == 0) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "dictionary keys changed during iteration");
+        goto fail;
+    }
     di->di_pos = i+1;
     di->len--;
     Py_INCREF(value);
@@ -3713,6 +3719,12 @@ dictiter_iternextitem(dictiterobject *di)
         key = entry_ptr->me_key;
         value = entry_ptr->me_value;
     }
+    // We found an element, but did not expect it
+    if (di->len == 0) {
+        PyErr_SetString(PyExc_RuntimeError,
+                        "dictionary keys changed during iteration");
+        goto fail;
+    }
     di->di_pos = i+1;
     di->len--;
     Py_INCREF(key);