gh-125004: fix unpickling for dataclasses in a cycle via hashing

Lib/dataclasses.py

@@ -1184,17 +1184,21 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen,
     return cls
 
 
-# _dataclass_getstate and _dataclass_setstate are needed for pickling frozen
-# classes with slots.  These could be slightly more performant if we generated
+# _dataclass_reduce and _dataclass_produce are needed because pickle will
+# crash when there is a cycle in the object graph and the object serves
+# as a key for a dict, set or frozenset in one of its descendants.
+# cf. gh python/cpython#124937.
+#
+# These could be slightly more performant if we generated
 # the code instead of iterating over fields.  But that can be a project for
 # another day, if performance becomes an issue.
-def _dataclass_getstate(self):
-    return [getattr(self, f.name) for f in fields(self)]
+def _dataclass_reduce(self):
+    return _dataclass_produce, ([getattr(self, f.name) for f in fields(self)],)
 
 
-def _dataclass_setstate(self, state):
-    for field, value in zip(fields(self), state):
-        # use setattr because dataclass may be frozen
+def _dataclass_produce(self, data):
+    for field, value in zip(fields(self), data):
+        # use `object.__setattr__` because dataclass may be frozen.
         object.__setattr__(self, field.name, value)
 
 
@@ -1305,12 +1309,8 @@ def _add_slots(cls, is_frozen, weakref_slot, defined_fields):
     if qualname is not None:
         newcls.__qualname__ = qualname
 
-    if is_frozen:
-        # Need this for pickling frozen classes with slots.
-        if '__getstate__' not in cls_dict:
-            newcls.__getstate__ = _dataclass_getstate
-        if '__setstate__' not in cls_dict:
-            newcls.__setstate__ = _dataclass_setstate
+    if '__reduce__' not in cls_dict:
+        newcls.__reduce__ = _dataclass_reduce
 
     # Fix up any closures which reference __class__.  This is used to
     # fix zero argument super so that it points to the correct class

Lib/test/test_dataclasses/__init__.py

@@ -2194,6 +2194,28 @@ class R:
                     self.assertEqual(new_sample.x, another_new_sample.x)
                     self.assertEqual(sample.y, another_new_sample.y)
 
+    def test_dataclasses_hash_pickleable(self):
+        global P, Q
+        class Q:
+            def __init__(self):
+                self.p = set()
+
+        @dataclass(frozen=True)
+        class P:
+            q: Q
+
+        q = Q()
+        q.p = P(q)
+        q.p.add(q)
+        new_q = pickle.loads(pickle.dumps(q))
+
+        self.assertEqual(q, new_q)
+        self.assertIsNot(q, new_q)
+
+        self.assertEqual(q.p, new_q.p)
+        self.assertIsNot(q.p, new_q.p)
+
+
     def test_dataclasses_qualnames(self):
         @dataclass(order=True, unsafe_hash=True, frozen=True)
         class A:

Misc/NEWS.d/next/Library/2024-10-05-10-24-48.gh-issue-125004.noUctj.rst

@@ -0,0 +1,2 @@
+Fix unpickling for :mod:`dataclasses` with hash-based data structures in their
+descendants in the presence of cycles.