gh-145142: Make str.maketrans safe under free-threading

Author: VanshAgarwal24036

Lib/test/test_free_threading/test_maketrans_threading.py

@@ -0,0 +1,26 @@
+import unittest
+from test.support import threading_helper
+
+threading_helper.requires_working_threading(module=True)
+
+
+class TestUnicodeFreeThreading(unittest.TestCase):
+    @threading_helper.reap_threads
+    def test_maketrans_dict_concurrent_modification(self):
+        number_of_iterations = 5
+        number_of_attempts = 100
+        for _ in range(number_of_iterations):
+            d = {2000: 'a'}
+            
+            def work_iterator(dct):
+                for i in range(number_of_attempts):
+                    str.maketrans(dct)
+                    idx = 2000 + i
+                    dct[idx] = chr(idx % 16)
+                    dct.pop(idx, None)
+
+            threading_helper.run_concurrently(
+                work_iterator,
+                nthreads=5,
+                args=(d,),
+            )

Misc/NEWS.d/next/Core_and_Builtins/2026-02-23-23-18-28.gh-issue-145142.T-XbVe.rst

@@ -0,0 +1,3 @@
+Fix a crash in free-threaded builds when str.maketrans() iterates over a
+dictionary that is concurrently modified. Wrap PyDict_Next iteration in a
+critical section.

Objects/unicodeobject.c

@@ -13154,8 +13154,14 @@ unicode_maketrans_impl(PyObject *x, PyObject *y, PyObject *z)
                             "to maketrans it must be a dict");
             goto err;
         }
+        PyObject *items = PyDict_Items(x);
+        if(items == NULL) goto err;
+        Py_ssize_t n = PyList_GET_SIZE(items);
         /* copy entries into the new dict, converting string keys to int keys */
-        while (PyDict_Next(x, &i, &key, &value)) {
+        for (i = 0; i < n; i++) {
+            PyObject *pair = PyList_GET_ITEM(items, i);
+            key = PyTuple_GET_ITEM(pair, 0);
+            value = PyTuple_GET_ITEM(pair, 1);
             if (PyUnicode_Check(key)) {
                 /* convert string keys to integer keys */
                 PyObject *newkey;
@@ -13183,6 +13189,7 @@ unicode_maketrans_impl(PyObject *x, PyObject *y, PyObject *z)
                 goto err;
             }
         }
+        Py_DECREF(items);
     }
     return new;
   err: