gh-152235: Defer GC tracking in set.union and set.difference

Author: corona10

Misc/NEWS.d/next/Core_and_Builtins/2026-06-26-23-09-34.gh-issue-152235.ArozdF.rst

@@ -0,0 +1,2 @@
+Defer GC tracking in :meth:`set.union`, :meth:`set.difference` and
+:meth:`set.__sub__`. Patch by Donghee Na.

Objects/setobject.c

@@ -1577,6 +1577,23 @@ _PySet_Freeze(PyObject *set)
     return Py_NewRef(set);
 }
 
+// Shallow copy of `so` left GC-untracked; the caller must _PyObject_GC_TRACK()
+// it once any further filling is done.
+static PyObject *
+set_copy_untracked_lock_held(PySetObject *so)
+{
+    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(so);
+    PyObject *copy = make_new_set_basetype_untracked(Py_TYPE(so), NULL);
+    if (copy == NULL) {
+        return NULL;
+    }
+    if (set_merge_lock_held((PySetObject *)copy, (PyObject *)so) < 0) {
+        Py_DECREF(copy);
+        return NULL;
+    }
+    return copy;
+}
+
 /*[clinic input]
 @critical_section
 set.copy
@@ -1589,14 +1606,9 @@ static PyObject *
 set_copy_impl(PySetObject *so)
 /*[clinic end generated code: output=c9223a1e1cc6b041 input=c169a4fbb8209257]*/
 {
-    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(so);
-    PyObject *copy = make_new_set_basetype(Py_TYPE(so), NULL);
-    if (copy == NULL) {
-        return NULL;
-    }
-    if (set_merge_lock_held((PySetObject *)copy, (PyObject *)so) < 0) {
-        Py_DECREF(copy);
-        return NULL;
+    PyObject *copy = set_copy_untracked_lock_held(so);
+    if (copy != NULL) {
+        _PyObject_GC_TRACK(copy);
     }
     return copy;
 }
@@ -1652,7 +1664,8 @@ set_union_impl(PySetObject *so, PyObject * const *others,
     PyObject *other;
     Py_ssize_t i;
 
-    result = (PySetObject *)set_copy((PyObject *)so, NULL);
+    result = (PySetObject *)make_new_set_basetype_untracked(Py_TYPE(so),
+                                                            (PyObject *)so);
     if (result == NULL)
         return NULL;
 
@@ -1665,6 +1678,7 @@ set_union_impl(PySetObject *so, PyObject * const *others,
             return NULL;
         }
     }
+    _PyObject_GC_TRACK(result);
     return (PyObject *)result;
 }
 
@@ -2059,7 +2073,7 @@ set_copy_and_difference(PySetObject *so, PyObject *other)
 {
     PyObject *result;
 
-    result = set_copy_impl(so);
+    result = set_copy_untracked_lock_held(so);
     if (result == NULL)
         return NULL;
     if (set_difference_update_internal((PySetObject *) result, other) == 0)
@@ -2118,7 +2132,6 @@ set_difference(PySetObject *so, PyObject *other)
             }
             Py_DECREF(key);
         }
-        _PyObject_GC_TRACK(result);
         return result;
     }
 
@@ -2142,7 +2155,6 @@ set_difference(PySetObject *so, PyObject *other)
         }
         Py_DECREF(key);
     }
-    _PyObject_GC_TRACK(result);
     return result;
 }
 
@@ -2185,6 +2197,7 @@ set_difference_multi_impl(PySetObject *so, PyObject * const *others,
             return NULL;
         }
     }
+    _PyObject_GC_TRACK(result);
     return result;
 }
 
@@ -2199,6 +2212,9 @@ set_sub(PyObject *self, PyObject *other)
     Py_BEGIN_CRITICAL_SECTION2(so, other);
     rv = set_difference(so, other);
     Py_END_CRITICAL_SECTION2();
+    if (rv != NULL) {
+        _PyObject_GC_TRACK(rv);
+    }
     return rv;
 }