Issue #24583: Fix crash when set is mutated while being updated.

Lib/test/test_set.py

@@ -1742,6 +1742,19 @@ class TestWeirdBugs(unittest.TestCase):
         s.update(range(100))
         list(si)
 
+    def test_merge_and_mutate(self):
+        class X:
+            def __hash__(self):
+                return hash(0)
+            def __eq__(self, o):
+                other.clear()
+                return False
+
+        other = set()
+        other = {X() for i in range(10)}
+        s = {0}
+        s.update(other)
+
 # Application tests (based on David Eppstein's graph recipes ====================================
 
 def powerset(U):

Misc/NEWS

@@ -12,6 +12,8 @@ Core and Builtins
 
 - Issue #24569: Make PEP 448 dictionary evaluation more consistent.
 
+- Issue #24583: Fix crash when set is mutated while being updated.
+
 - Issue #24407: Fix crash when dict is mutated while being updated.
 
 Library

Objects/setobject.c

@@ -600,7 +600,8 @@ set_merge(PySetObject *so, PyObject *otherset)
     }
 
     /* We can't assure there are no duplicates, so do normal insertions */
-    for (i = 0; i <= other->mask; i++, other_entry++) {
+    for (i = 0; i <= other->mask; i++) {
+        other_entry = &other->table[i];
         key = other_entry->key;
         if (key != NULL && key != dummy) {
             Py_INCREF(key);