python · kumaraditya303 · Jun 12, 2025 · Jun 11, 2025 · Jun 11, 2025 · Jun 11, 2025
diff --git a/Lib/test/test_memoryio.py b/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


diff --git a/Misc/NEWS.d/next/Library/2025-06-11-19-05-49.gh-issue-135410.E89Boi.rst b/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.
diff --git a/Modules/_io/stringio.c b/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
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		Fix a crash when iterating over :class:`io.StringIO` on the :term:`free
		threaded <free threading>` build.