gh-135410: use a critical section around `StringIO.__next__` (#135412)

diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index 63998a86c45b53c61827396256b38cd76acb6d86..249e0f3ba32f29a674de68ace66e2a68f386af9e 100644 (file)
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -5,6 +5,7 @@ BytesIO -- for bytes
 
 import unittest
 from test import support
+from test.support import threading_helper
 
 import gc
 import io
@@ -12,6 +13,7 @@ import _pyio as pyio
 import pickle
 import sys
 import weakref
+import threading
 
 class IntLike:
     def __init__(self, num):
@@ -723,6 +725,22 @@ class TextIOTestMixin:
         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 @@ class CStringIOTest(PyStringIOTest):
         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
new file mode 100644 (file)
index 0000000..a5917fb
--- /dev/null
+++ 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
index 56913fafefba8b0670f4e1eeb39867c88b0a28ed..8482c268176c5b6207055c33607a9075a30328e9 100644 (file)
--- 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