From 796cc6e3ad3617c1ea9e528663aac1a206230a28 Mon Sep 17 00:00:00 2001
From: Thomas Perl <m@thp.io>
Date: Thu, 28 Mar 2019 07:03:25 +0100
Subject: [PATCH] bpo-36452: dictiter: track maximum iteration count (GH-12596)

---
 Lib/test/test_dict.py                                    | 9 +++++++++
 .../2019-03-27-23-53-00.bpo-36452.xhK2lT.rst             | 1 +
 Objects/dictobject.c                                     | 6 ++++++
 3 files changed, 16 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2019-03-27-23-53-00.bpo-36452.xhK2lT.rst

diff --git a/Lib/test/test_dict.py b/Lib/test/test_dict.py
index 03afd5b2a6c..eecdc8beec6 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/Misc/NEWS.d/next/Core and Builtins/2019-03-27-23-53-00.bpo-36452.xhK2lT.rst b/Misc/NEWS.d/next/Core and Builtins/2019-03-27-23-53-00.bpo-36452.xhK2lT.rst
new file mode 100644
index 00000000000..37c0c503ede
--- /dev/null
+++ b/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.
\ No newline at end of file
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index e2603e190b6..7ea979cd176 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);