gh-158525: Avoid reacquiring dict lock when copying

Author: n1snt

Misc/NEWS.d/next/Core_and_Builtins/2026-10-01-00-00-00.gh-issue-158525.Xw3K9a.rst

@@ -0,0 +1,2 @@
+Avoid reacquiring the source dictionary lock when copying a sparse dictionary
+in the free-threaded build.

Objects/dictobject.c

@@ -4575,7 +4575,15 @@ copy_lock_held_untracked(PyObject *o, int as_frozendict)
     }
     if (copy == NULL)
         return NULL;
-    if (dict_merge(copy, o, 1, NULL) < 0) {
+    int res;
+    if (!PyFrozenDict_Check(o) && Py_TYPE(mp)->tp_iter == dict_iter) {
+        // The source is already locked and the copy is not yet visible.
+        res = dict_dict_merge((PyDictObject *)copy, mp, 1, NULL);
+    }
+    else {
+        res = dict_merge(copy, o, 1, NULL);
+    }
+    if (res < 0) {
         Py_DECREF(copy);
         return NULL;
     }