bpo-36452: dictiter: track maximum iteration count (GH-12596)

Lib/test/test_dict.py

@@ -470,6 +470,15 @@ class DictTest(unittest.TestCase):
             for i in d:
                 d[i+1] = 1
 
+    def test_mutating_iteration_delete(self):
+        # change dict content during iteration
+        d = {}
+        d[0] = 0
+        with self.assertRaises(RuntimeError):
+            for i in d:
+                del d[0]
+                d[1] = 1
+
     def test_mutating_lookup(self):
         # changing dict during a lookup (issue #14417)
         class NastyKey:

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

@@ -0,0 +1 @@
+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.

Objects/dictobject.c

@@ -3543,6 +3543,12 @@ dictiter_iternextkey(dictiterobject *di)
             goto fail;
         key = entry_ptr->me_key;
     }
+    // We found an element (key), 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);