gh-159283: Keep the propagating exception in PyObject_ClearManagedDict()

When an object whose __dict__ still pointed at its inline values was
freed while an exception was propagating, a failed allocation while
detaching the dict called PyErr_NoMemory(), which replaced that
exception, and PyErr_FormatUnraisable() then cleared it. A debug build
aborted in py_dealloc() and a release build went on with no exception
set, which ended in SystemError.

Save the current exception before detaching the dict and restore it
afterwards.

Author: christianaurichzm

Lib/test/test_class.py

@@ -1047,6 +1047,41 @@ def __init__(self):
             self.fail("MemoryError was not raised while detaching "
                       f"the dictionary: {seen}")
 
+    @support.nomemtest
+    def test_detach_materialized_dict_no_memory_keeps_exception(self):
+        # A MemoryError raised while detaching the dictionary must not
+        # replace the exception which is propagating when the object is freed.
+        import _testcapi
+
+        class A:
+            def __init__(self):
+                self.a = 1
+                self.b = 2
+
+        def f(lst):
+            # The object is only referenced from the evaluation stack,
+            # so it is freed while ZeroDivisionError propagates.
+            [lst.pop(), 1/0]
+
+        seen = []
+        for n in range(20):
+            lst = [A()]
+            d = lst[0].__dict__
+            with support.catch_unraisable_exception() as ex:
+                with self.assertRaises((ZeroDivisionError, MemoryError)) as cm:
+                    _testcapi.call_with_nomemory(n, n + 1, f, lst)
+                if ex.unraisable is None:
+                    continue
+                err_msg = ex.unraisable.err_msg
+            seen.append((n, type(cm.exception), err_msg))
+            if (err_msg ==
+                    'Exception ignored while clearing an object managed dict'):
+                self.assertIs(type(cm.exception), ZeroDivisionError)
+                break
+        else:
+            self.fail("MemoryError was not raised while detaching "
+                      f"the dictionary: {seen}")
+
 class DefinitionOrderTests(unittest.TestCase):
     # PEP 520: Preserving Class Attribute Definition Order
 

Misc/NEWS.d/next/Core_and_Builtins/2026-10-12-23-18-16.gh-issue-159283.VdOijg.rst

@@ -0,0 +1,4 @@
+Fix a :exc:`SystemError` (a fatal error in a debug build) when an object
+whose :attr:`~object.__dict__` has been accessed is freed while an exception
+propagates and detaching the dictionary fails with :exc:`MemoryError`. The
+propagating exception is now kept. Patch by Christian Aurich Zanettini Martins.

Objects/dictobject.c

@@ -8018,6 +8018,9 @@ PyObject_ClearManagedDict(PyObject *obj)
             // We have a materialized object which points at the inline
             // values. We need to materialize the keys. Nothing can modify
             // this object, but we need to lock the dictionary.
+            // The object can be freed while an exception is propagating:
+            // don't let a MemoryError from detaching the dict replace it.
+            PyObject *exc = PyErr_GetRaisedException();
             int err;
             Py_BEGIN_CRITICAL_SECTION(dict);
             err = detach_dict_from_object(dict, obj);
@@ -8038,6 +8041,7 @@ PyObject_ClearManagedDict(PyObject *obj)
                 clear_inline_values(_PyObject_InlineValues(obj));
                 Py_END_CRITICAL_SECTION();
             }
+            PyErr_SetRaisedException(exc);
         }
     }
     Py_CLEAR(_PyObject_ManagedDictPointer(obj)->dict);