gh-135410: Use a critical section around StringIO.__next__

Lib/test/test_memoryio.py

@@ -5,13 +5,15 @@
 
 import unittest
 from test import support
+from test.support import threading_helper
 
 import gc
 import io
 import _pyio as pyio
 import pickle
 import sys
 import weakref
+import threading
 
 class IntLike:
     def __init__(self, num):
@@ -723,6 +725,22 @@ def test_newline_argument(self):
         for newline in (None, "", "\n", "\r", "\r\n"):
             self.ioclass(newline=newline)
 
+    @unittest.skipUnless(support.Py_GIL_DISABLED, "only meaningful under free-threading")
+    @threading_helper.requires_working_threading()
+    def test_concurrent_use(self):
+        memio = self.ioclass("")
+
+        def use():
+            memio.write("x" * 10)
+            memio.readlines()
+
+        threads = [threading.Thread(target=use) for _ in range(8)]
+        with threading_helper.catch_threading_exception() as cm:
+            with threading_helper.start_threads(threads):
+                pass
+
+            self.assertIsNone(cm.exc_value)
+
 
 class PyStringIOTest(MemoryTestMixin, MemorySeekTestMixin,
                      TextIOTestMixin, unittest.TestCase):
@@ -890,6 +908,7 @@ def test_setstate(self):
         self.assertRaises(ValueError, memio.__setstate__, ("closed", "", 0, None))
 
 
+
 class CStringIOPickleTest(PyStringIOPickleTest):
     UnsupportedOperation = io.UnsupportedOperation
 

Misc/NEWS.d/next/Library/2025-06-11-19-05-49.gh-issue-135410.E89Boi.rst

@@ -0,0 +1,2 @@
+Fix a crash when iterating over :class:`io.StringIO` on the :term:`free
+threaded <free threading>` build.

Modules/_io/stringio.c

@@ -404,7 +404,7 @@ _io_StringIO_readline_impl(stringio *self, Py_ssize_t size)
 }
 
 static PyObject *
-stringio_iternext(PyObject *op)
+stringio_iternext_lock_held(PyObject *op)
 {
     PyObject *line;
     stringio *self = stringio_CAST(op);
@@ -441,6 +441,16 @@ stringio_iternext(PyObject *op)
     return line;
 }
 
+static PyObject *
+stringio_iternext(PyObject *op)
+{
+    PyObject *res;
+    Py_BEGIN_CRITICAL_SECTION(op);
+    res = stringio_iternext_lock_held(op);
+    Py_END_CRITICAL_SECTION();
+    return res;
+}
+
 /*[clinic input]
 @critical_section
 _io.StringIO.truncate