gh-159285: Fix crash when copying an empty dict fails with MemoryError (GH-159286)

copy_lock_held_untracked() asserted that the new empty dict was not
tracked by the GC without checking whether its allocation had failed,
so a debug build dereferenced NULL instead of raising MemoryError. This
affected dict.copy(), PyDict_Copy() and _PyDict_CopyAsDict(), which
type() uses for the class namespace.
(cherry picked from commit f9132f1)

Co-authored-by: Christian Aurich Zanettini Martins <christian.aurichzm@gmail.com>

Author: miss-islington

Lib/test/test_dict.py

@@ -459,6 +459,18 @@ def test_copy_noncompact(self):
         d2 = d.copy()
         self.assertEqual(d2, d)
 
+    @support.nomemtest
+    def test_copy_empty_no_memory(self):
+        import _testcapi
+        d = {}
+        d_copy = d.copy
+        with self.assertRaises(MemoryError):
+            try:
+                _testcapi.set_nomemory(0)
+                d_copy()
+            finally:
+                _testcapi.remove_mem_hooks()
+
     def test_get(self):
         d = {}
         self.assertIs(d.get('c'), None)

Misc/NEWS.d/next/Core_and_Builtins/2026-10-12-23-32-50.gh-issue-159285.nlLuwQ.rst

@@ -0,0 +1,2 @@
+Fix a crash in a debug build when copying an empty :class:`dict` fails with
+:exc:`MemoryError`. Patch by Christian Aurich Zanettini Martins.

Objects/dictobject.c

@@ -4448,7 +4448,7 @@ copy_lock_held_untracked(PyObject *o, int as_frozendict)
         else {
             d = dict_new_untracked(&PyDict_Type);
         }
-        assert(!_PyObject_GC_IS_TRACKED(d));
+        assert(d == NULL || !_PyObject_GC_IS_TRACKED(d));
         return d;
     }