gh-92930: _pickle.c: Acquire strong references before calling save()

Lib/test/pickletester.py

@@ -3035,6 +3035,35 @@ def check_array(arr):
         # 2-D, non-contiguous
         check_array(arr[::2])
 
+    def test_evil_class_mutating_dict(self):
+        from random import getrandbits
+
+        global Bad
+        class Bad:
+            def __eq__(self, other):
+                return ENABLED
+            def __hash__(self):
+                return 42
+            def __reduce__(self):
+                if getrandbits(6) == 0:
+                    collection.clear()
+                return (Bad, ())
+
+        for proto in protocols:
+            for _ in range(20):
+                ENABLED = False
+                collection = {Bad(): Bad() for _ in range(20)}
+                for bad in collection:
+                    bad.bad = bad
+                    bad.collection = collection
+                ENABLED = True
+                try:
+                    data = self.dumps(collection, proto)
+                    self.loads(data)
+                except RuntimeError as e:
+                    expected = "changed size during iteration"
+                    self.assertIn(expected, str(e))
+
 
 class BigmemPickleTests:
 

Misc/NEWS.d/next/Core and Builtins/2022-05-18-18-34-45.gh-issue-92930.kpYPOb.rst

@@ -0,0 +1 @@
+Fixed a crash when pickling objects that mutate collections during ``__reduce__``.

Modules/_pickle.c

@@ -3259,10 +3259,16 @@ batch_dict_exact(PicklerObject *self, PyObject *obj)
     /* Special-case len(d) == 1 to save space. */
     if (dict_size == 1) {
         PyDict_Next(obj, &ppos, &key, &value);
-        if (save(self, key, 0) < 0)
-            return -1;
-        if (save(self, value, 0) < 0)
-            return -1;
+        Py_INCREF(key);
+        Py_INCREF(value);
+        if (save(self, key, 0) < 0) {
+            goto error;
+        }
+        if (save(self, value, 0) < 0) {
+            goto error;
+        }
+        Py_CLEAR(key);
+        Py_CLEAR(value);
         if (_Pickler_Write(self, &setitem_op, 1) < 0)
             return -1;
         return 0;
@@ -3274,10 +3280,16 @@ batch_dict_exact(PicklerObject *self, PyObject *obj)
         if (_Pickler_Write(self, &mark_op, 1) < 0)
             return -1;
         while (PyDict_Next(obj, &ppos, &key, &value)) {
-            if (save(self, key, 0) < 0)
-                return -1;
-            if (save(self, value, 0) < 0)
-                return -1;
+            Py_INCREF(key);
+            Py_INCREF(value);
+            if (save(self, key, 0) < 0) {
+                goto error;
+            }
+            if (save(self, value, 0) < 0) {
+                goto error;
+            }
+            Py_CLEAR(key);
+            Py_CLEAR(value);
             if (++i == BATCHSIZE)
                 break;
         }
@@ -3292,6 +3304,10 @@ batch_dict_exact(PicklerObject *self, PyObject *obj)
 
     } while (i == BATCHSIZE);
     return 0;
+error:
+    Py_XDECREF(key);
+    Py_XDECREF(value);
+    return -1;
 }
 
 static int