gh-158600: Avoid copying in set.intersection_update()

Calculate all intersections before swapping the final result into the target. Keep the calculation and swap within the same critical section so concurrent updates are not lost.

This keeps the existing unchanged-on-error behavior while removing
the O(len(self)) initial copy.

Author: CaQtiml

Objects/setobject.c

@@ -1861,20 +1861,36 @@ set_intersection_update_multi_impl(PySetObject *so, PyObject * const *others,
                                    Py_ssize_t others_length)
 /*[clinic end generated code: output=d768b5584675b48d input=782e422fc370e4fc]*/
 {
-    PyObject *copy;
-    PyObject *result = NULL;
+    Py_ssize_t i;
+    PyObject *other;
+    PyObject *result;
 
-    Py_BEGIN_CRITICAL_SECTION(so);
-    copy = set_copy_untracked_lock_held(so);
-    if (copy != NULL) {
-        result = set_intersection_multi_impl((PySetObject *)copy,
-                                             others, others_length);
-        Py_DECREF(copy);
+    if (others_length == 0) {
+        Py_RETURN_NONE;
+    }
+
+    other = others[0];
+    Py_BEGIN_CRITICAL_SECTION2(so, other);
+    result = set_intersection(so, other);
+    if (result != NULL) {
+        for (i = 1; i < others_length; i++) {
+            PyObject *newresult;
+
+            other = others[i];
+            Py_BEGIN_CRITICAL_SECTION(other);
+            newresult = set_intersection((PySetObject *)result, other);
+            Py_END_CRITICAL_SECTION();
+            if (newresult == NULL) {
+                Py_CLEAR(result);
+                break;
+            }
+            Py_SETREF(result, newresult);
+        }
         if (result != NULL) {
             set_swap_bodies(so, (PySetObject *)result);
         }
     }
-    Py_END_CRITICAL_SECTION();
+    Py_END_CRITICAL_SECTION2();
 
     if (result == NULL) {
         return NULL;