gh-151722: Do not track the dict in the GC in _PyDict_FromKeys()

dict_merge() no longer requires the dictionary to be tracked by the
GC.

Co-authored-by: Donghee Na <donghee.na@python.org>

Author: vstinner

Misc/NEWS.d/next/Core_and_Builtins/2026-06-24-13-36-31.gh-issue-151722.lWKfE1.rst

@@ -0,0 +1,3 @@
+:meth:`!frozendict.fromkeys` now creates a :class:`frozendict` which is not
+tracked by the garbage collector, and only tracks it once the dictionary is
+fully initialized. Patch by Donghee Na and Victor Stinner.

Objects/dictobject.c

@@ -140,6 +140,7 @@ static PyObject* frozendict_new(PyTypeObject *type, PyObject *args,
                                 PyObject *kwds);
 static PyObject* frozendict_new_untracked(PyTypeObject *type);
 static PyObject* dict_new(PyTypeObject *type, PyObject *args, PyObject *kwds);
+static PyObject* dict_new_untracked(PyTypeObject *type);
 static int dict_merge(PyObject *a, PyObject *b, int override, PyObject **dupkey);
 static int dict_contains(PyObject *op, PyObject *key);
 static int dict_merge_from_seq2(PyObject *d, PyObject *seq2, int override);
@@ -3414,40 +3415,49 @@ dict_set_fromkeys(PyDictObject *mp, PyObject *iterable, PyObject *value)
 PyObject *
 _PyDict_FromKeys(PyObject *cls, PyObject *iterable, PyObject *value)
 {
-    PyObject *it;       /* iter(iterable) */
+    PyObject *it = NULL;       /* iter(iterable) */
     PyObject *key;
     PyObject *d;
     int status;
 
-    d = _PyObject_CallNoArgs(cls);
+    PyTypeObject *cls_type = _PyType_CAST(cls);
+    if (PyObject_IsSubclass(cls, (PyObject*)&PyFrozenDict_Type)
+        && cls_type->tp_new == frozendict_new)
+    {
+        // gh-151722: Create a frozendict copy which is not tracked by the GC.
+        d = frozendict_new_untracked(cls_type);
+    }
+    else {
+        d = _PyObject_CallNoArgs(cls);
+    }
     if (d == NULL) {
         return NULL;
     }
 
-    // If cls is a dict or frozendict subclass with overridden constructor,
-    // copy the frozendict.
-    PyTypeObject *cls_type = _PyType_CAST(cls);
-    if (PyFrozenDict_Check(d) && cls_type->tp_new != frozendict_new) {
+    // gh-151722: If cls constructor returns a frozendict which is tracked by
+    // the GC, create a frozendict copy which is not tracked by the GC.
+    if (PyFrozenDict_Check(d) && _PyObject_GC_IS_TRACKED(d)) {
         // Subclass-friendly copy
         PyObject *copy;
         if (PyObject_IsSubclass(cls, (PyObject*)&PyFrozenDict_Type)) {
-            copy = frozendict_new(cls_type, NULL, NULL);
+            copy = frozendict_new_untracked(cls_type);
         }
         else {
-            copy = dict_new(cls_type, NULL, NULL);
+            copy = dict_new_untracked(cls_type);
         }
         if (copy == NULL) {
-            Py_DECREF(d);
-            return NULL;
+            goto Fail;
         }
         if (dict_merge(copy, d, 1, NULL) < 0) {
-            Py_DECREF(d);
             Py_DECREF(copy);
-            return NULL;
+            goto Fail;
         }
         Py_SETREF(d, copy);
     }
     assert(!PyFrozenDict_Check(d) || can_modify_dict((PyDictObject*)d));
+    if (PyFrozenDict_Check(d)) {
+        assert(!_PyObject_GC_IS_TRACKED(d));
+    }
 
     if (PyDict_CheckExact(d)) {
         if (PyDict_CheckExact(iterable)) {
@@ -3456,23 +3466,23 @@ _PyDict_FromKeys(PyObject *cls, PyObject *iterable, PyObject *value)
             Py_BEGIN_CRITICAL_SECTION2(d, iterable);
             d = (PyObject *)dict_dict_fromkeys(mp, iterable, value);
             Py_END_CRITICAL_SECTION2();
-            return d;
+            goto Done;
         }
         else if (PyFrozenDict_CheckExact(iterable)) {
             PyDictObject *mp = (PyDictObject *)d;
 
             Py_BEGIN_CRITICAL_SECTION(d);
             d = (PyObject *)dict_dict_fromkeys(mp, iterable, value);
             Py_END_CRITICAL_SECTION();
-            return d;
+            goto Done;
         }
         else if (PyAnySet_CheckExact(iterable)) {
             PyDictObject *mp = (PyDictObject *)d;
 
             Py_BEGIN_CRITICAL_SECTION2(d, iterable);
             d = (PyObject *)dict_set_fromkeys(mp, iterable, value);
             Py_END_CRITICAL_SECTION2();
-            return d;
+            goto Done;
         }
     }
     else if (PyFrozenDict_CheckExact(d)) {
@@ -3482,27 +3492,26 @@ _PyDict_FromKeys(PyObject *cls, PyObject *iterable, PyObject *value)
             Py_BEGIN_CRITICAL_SECTION(iterable);
             d = (PyObject *)dict_dict_fromkeys(mp, iterable, value);
             Py_END_CRITICAL_SECTION();
-            return d;
+            goto Done;
         }
         else if (PyFrozenDict_CheckExact(iterable)) {
             PyDictObject *mp = (PyDictObject *)d;
             d = (PyObject *)dict_dict_fromkeys(mp, iterable, value);
-            return d;
+            goto Done;
         }
         else if (PyAnySet_CheckExact(iterable)) {
             PyDictObject *mp = (PyDictObject *)d;
 
             Py_BEGIN_CRITICAL_SECTION(iterable);
             d = (PyObject *)dict_set_fromkeys(mp, iterable, value);
             Py_END_CRITICAL_SECTION();
-            return d;
+            goto Done;
         }
     }
 
     it = PyObject_GetIter(iterable);
     if (it == NULL){
-        Py_DECREF(d);
-        return NULL;
+        goto Fail;
     }
 
     if (PyDict_CheckExact(d)) {
@@ -3541,12 +3550,19 @@ dict_iter_exit:;
     if (PyErr_Occurred())
         goto Fail;
     Py_DECREF(it);
-    return d;
+    goto Done;
 
 Fail:
-    Py_DECREF(it);
+    Py_XDECREF(it);
     Py_DECREF(d);
     return NULL;
+
+Done:
+    // d can be NULL
+    if (d != NULL && !_PyObject_GC_IS_TRACKED(d)) {
+        _PyObject_GC_TRACK(d);
+    }
+    return d;
 }
 
 /* Methods */
@@ -4147,9 +4163,6 @@ dict_dict_merge(PyDictObject *mp, PyDictObject *other, int override, PyObject **
             set_keys(mp, keys);
             STORE_USED(mp, other->ma_used);
             ASSERT_CONSISTENT(mp);
-            if (PyDict_Check(mp)) {
-                assert(_PyObject_GC_IS_TRACKED(mp));
-            }
             return 0;
         }
     }
@@ -4316,7 +4329,12 @@ dict_merge_api(PyObject *a, PyObject *b, int override, PyObject **dupkey)
         }
         return -1;
     }
-    return dict_merge(a, b, override, dupkey);
+
+    int res = dict_merge(a, b, override, dupkey);
+    if (PyDict_Check(a)) {
+        assert(_PyObject_GC_IS_TRACKED(a));
+    }
+    return res;
 }
 
 int
@@ -4475,10 +4493,16 @@ copy_lock_held(PyObject *o, int as_frozendict)
     }
     if (copy == NULL)
         return NULL;
-    if (dict_merge(copy, o, 1, NULL) == 0)
-        return copy;
-    Py_DECREF(copy);
-    return NULL;
+    if (dict_merge(copy, o, 1, NULL) < 0) {
+        Py_DECREF(copy);
+        return NULL;
+
+    }
+
+    if (PyDict_Check(copy)) {
+        assert(_PyObject_GC_IS_TRACKED(copy));
+    }
+    return copy;
 }
 
 PyObject *