Fix slotted reference cycles on 3.14 (#1446)

* Fix slotted reference cycles on 3.14

Ref python/cpython#136893 &
python/cpython#135228

Co-authored-by: Jelle Zijlstra <[email protected]>

* Add news fragment

* Bump version tag to CMA

Co-authored-by: Brandt Bucher
<[email protected]>
Co-authored-by: Jelle Zijlstra
<[email protected]>

* Apply review comment

#1446 (comment)

* EAFP

---------

Co-authored-by: Jelle Zijlstra <[email protected]>

Author: hynek

changelog.d/1446.change.md

@@ -0,0 +1,2 @@
+On 3.14, the cycles in slotted classes are now manually broken.
+An explicit call to `gc.collect()` is still necessary, unfortunately.

src/attr/_make.py

@@ -24,6 +24,7 @@
     PY_3_10_PLUS,
     PY_3_11_PLUS,
     PY_3_13_PLUS,
+    PY_3_14_PLUS,
     _AnnotationExtractor,
     _get_annotations,
     get_generic_base,
@@ -618,6 +619,17 @@ def evolve(*args, **changes):
     return cls(**changes)
 
 
+# Hack to the get the underlying dict out of a mappingproxy
+# Use it with: cls.__dict__ | _deproxier
+# See: https://github.com/python/cpython/pull/136893
+class _Deproxier:
+    def __ror__(self, other):
+        return other
+
+
+_deproxier = _Deproxier()
+
+
 class _ClassBuilder:
     """
     Iteratively build *one* class.
@@ -845,6 +857,20 @@ def _create_slots_class(self):
             if k not in (*tuple(self._attr_names), "__dict__", "__weakref__")
         }
 
+        if PY_3_14_PLUS:
+            # Clean up old dict to avoid leaks.
+            old_cls_dict = self._cls.__dict__ | _deproxier
+            old_cls_dict.pop("__dict__", None)
+            if "__weakref__" in self._cls.__dict__:
+                del self._cls.__weakref__
+
+            # Manually bump internal version tag.
+            try:
+                self._cls.__abstractmethods__ = self._cls.__abstractmethods__
+            except AttributeError:
+                self._cls.__abstractmethods__ = frozenset({"__init__"})
+                del self._cls.__abstractmethods__
+
         # If our class doesn't have its own implementation of __setattr__
         # (either from the user or by us), check the bases, if one of them has
         # an attrs-made __setattr__, that needs to be reset. We don't walk the

tests/test_make.py

@@ -23,7 +23,7 @@
 import attr
 
 from attr import _config
-from attr._compat import PY_3_10_PLUS, PY_3_14_PLUS
+from attr._compat import PY_3_10_PLUS
 from attr._make import (
     Attribute,
     Factory,
@@ -1939,7 +1939,6 @@ class C2(C):
 
         assert [C2] == C.__subclasses__()
 
-    @pytest.mark.xfail(PY_3_14_PLUS, reason="Currently broken on nightly.")
     def test_no_references_to_original_when_using_cached_property(self):
         """
         When subclassing a slotted class and using cached property, there are