gh-159244: Fix crash in dict() on a concurrently mutated key/value pair (#159245)

In the free-threaded build, dict() and dict.update() checked that a
list key/value pair had two items and then read them with borrowed
references without holding its lock, so another thread could empty
the list in between and the items were read from a freed or NULL
array.

Author: christianaurichzm

Lib/test/test_free_threading/test_dict.py

@@ -532,5 +532,28 @@ def race(iter_times: int,
         CHURN_SIZE = 512
         race(ITER_TIMES, CHURN_SIZE, APPENDER_THREADS, REPLACER_THREADS)
 
+    def test_racing_update_from_mutating_pair(self):
+        # gh-159244: dict() must not read the items of a key/value pair
+        # that another thread empties concurrently.
+        pair = [object(), object()]
+        done = Event()
+
+        def writer():
+            try:
+                for _ in range(10000):
+                    pair.clear()
+                    pair.extend((object(), object()))
+            finally:
+                done.set()
+
+        def reader():
+            while not done.is_set():
+                try:
+                    dict([pair])
+                except ValueError:
+                    pass
+
+        threading_helper.run_concurrently([writer] + [reader] * 4)
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2026-10-12-16-00-34.gh-issue-159244.ThiobE.rst

@@ -0,0 +1,3 @@
+Fix a crash in :class:`dict` and :meth:`dict.update` in the
+:term:`free-threaded build` when another thread concurrently mutates a list
+used as a key/value pair. Patch by Christian Aurich Zanettini Martins.

Objects/dictobject.c

@@ -4113,7 +4113,7 @@ merge_from_seq2_lock_held(PyObject *d, PyObject *seq2, int override)
         return -1;
 
     for (i = 0; ; ++i) {
-        PyObject *key, *value;
+        PyObject *key = NULL, *value = NULL;
         Py_ssize_t n;
 
         fast = NULL;
@@ -4135,7 +4135,13 @@ merge_from_seq2_lock_held(PyObject *d, PyObject *seq2, int override)
             }
             goto Fail;
         }
+        Py_BEGIN_CRITICAL_SECTION_SEQUENCE_FAST(item);
         n = PySequence_Fast_GET_SIZE(fast);
+        if (n == 2) {
+            key = Py_NewRef(PySequence_Fast_GET_ITEM(fast, 0));
+            value = Py_NewRef(PySequence_Fast_GET_ITEM(fast, 1));
+        }
+        Py_END_CRITICAL_SECTION_SEQUENCE_FAST();
         if (n != 2) {
             PyErr_Format(PyExc_ValueError,
                          "dictionary update sequence element #%zd "
@@ -4145,10 +4151,6 @@ merge_from_seq2_lock_held(PyObject *d, PyObject *seq2, int override)
         }
 
         /* Update/merge with this (key, value) pair. */
-        key = PySequence_Fast_GET_ITEM(fast, 0);
-        value = PySequence_Fast_GET_ITEM(fast, 1);
-        Py_INCREF(key);
-        Py_INCREF(value);
         if (override) {
             if (setitem_lock_held((PyDictObject *)d, key, value) < 0) {
                 Py_DECREF(key);