gh-141805: avoid increase the set->used twice when the __eq__ trigger another add

Lib/test/test_set.py

@@ -675,6 +675,33 @@ def __hash__(self):
         with self.assertRaises(KeyError):
             myset.discard(elem2)
 
+    def test_set_add_to_dummy_slot(self):
+        # gh-141805
+        tasks = set()
+
+        class Dummy:
+            def __hash__(self):
+                return 0
+
+        class CorruptTrigger:
+            triggered = False
+
+            def __hash__(self):
+                return 0
+
+            def __eq__(self, value):
+                if not self.triggered:
+                    self.triggered = True
+                    tasks.add(self)
+                return False
+
+        tasks.add(Dummy())
+        tasks.add(Dummy())
+        tasks.pop()
+        self.assertEqual(len(tasks), 1)
+        tasks.add(CorruptTrigger())
+        self.assertEqual(len(tasks), 2)
+
 
 class SetSubclass(set):
     pass

Misc/NEWS.d/next/Core_and_Builtins/2025-11-27-17-47-09.gh-issue-141805.TT0biF.rst

@@ -0,0 +1,2 @@
+Fix set length corruption when set additions occur within
+element comparison (:meth:`object.__eq__`) methods.

Objects/setobject.c

@@ -298,6 +298,7 @@ set_add_entry_takeref(PySetObject *so, PyObject *key, Py_hash_t hash)
             else if (entry->hash == -1) {
                 assert (entry->key == dummy);
                 freeslot = entry;
+                goto found_unused_or_dummy;
             }
             entry++;
         } while (probes--);