From feaf0f228d28cce2a6baa2a796ae807328ec71b4 Mon Sep 17 00:00:00 2001
From: Thomas Perl <m@thp.io>
Date: Thu, 28 Mar 2019 00:27:23 +0100
Subject: [PATCH] dictiterobject: Track maximum iteration count via di->len

---
 Lib/test/test_dict.py | 9 +++++++++
 Objects/dictobject.c  | 6 ++++++
 2 files changed, 15 insertions(+)

diff --git a/Lib/test/test_dict.py b/Lib/test/test_dict.py
index 03afd5b2a6..eecdc8beec 100644
--- a/Lib/test/test_dict.py
+++ b/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:
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index e2603e190b..7ea979cd17 100644
--- a/Objects/dictobject.c
+++ b/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);
-- 
2.16.3