gh-141805: Fix crash after concurrent addition objects with the same hash to set (GH-143815)

This happens when the set contained several elements with the same hash,
and then some of them were removed.

Lib/test/test_set.py

@@ -1853,6 +1853,7 @@ class TestWeirdBugs(unittest.TestCase):
         list(si)
 
     def test_merge_and_mutate(self):
+        # gh-141805
         class X:
             def __hash__(self):
                 return hash(0)
@@ -1865,6 +1866,33 @@ class TestWeirdBugs(unittest.TestCase):
         s = {0}
         s.update(other)
 
+    def test_hash_collision_concurrent_add(self):
+        class X:
+            def __hash__(self):
+                return 0
+        class Y:
+            flag = False
+            def __hash__(self):
+                return 0
+            def __eq__(self, other):
+                if not self.flag:
+                    self.flag = True
+                    s.add(X())
+                return self is other
+
+        a = X()
+        s = set()
+        s.add(a)
+        s.add(X())
+        s.remove(a)
+        # Now the set contains a dummy entry followed by an entry
+        # for an object with hash 0.
+        s.add(Y())
+        # The following operations should not crash.
+        repr(s)
+        list(s)
+        set() | s
+
 
 class TestOperationsMutating:
     """Regression test for bpo-46615"""

Misc/NEWS.d/next/Core_and_Builtins/2026-01-13-22-26-49.gh-issue-141805.QzIKPS.rst

@@ -0,0 +1,3 @@
+Fix crash in :class:`set` when objects with the same hash are concurrently
+added to the set after removing an element with the same hash while the set
+still contains elements with the same hash.

Objects/setobject.c

@@ -308,6 +308,9 @@ set_add_entry_takeref(PySetObject *so, PyObject *key, Py_hash_t hash)
   found_unused_or_dummy:
     if (freeslot == NULL)
         goto found_unused;
+    if (freeslot->hash != -1) {
+        goto restart;
+    }
     FT_ATOMIC_STORE_SSIZE_RELAXED(so->used, so->used + 1);
     FT_ATOMIC_STORE_SSIZE_RELAXED(freeslot->hash, hash);
     FT_ATOMIC_STORE_PTR_RELEASE(freeslot->key, key);