From 5f3825b8baface39cb40038a0b81f13fa6d6601b Mon Sep 17 00:00:00 2001
From: Peter Bierma <zintensitydev@gmail.com>
Date: Wed, 11 Jun 2025 19:05:02 -0400
Subject: [PATCH 1/3] Fix thread safety in StringIO.

---
 Modules/_io/stringio.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/Modules/_io/stringio.c b/Modules/_io/stringio.c
index 56913fafefba8b..9cf1e0e3e7c278 100644
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -79,6 +79,7 @@ static int _io_StringIO___init__(PyObject *self, PyObject *args, PyObject *kwarg
 static int
 resize_buffer(stringio *self, size_t size)
 {
+    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
     /* Here, unsigned types are used to avoid dealing with signed integer
        overflow, which is undefined in C. */
     size_t alloc = self->buf_size;
@@ -131,6 +132,7 @@ resize_buffer(stringio *self, size_t size)
 static PyObject *
 make_intermediate(stringio *self)
 {
+    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
     PyObject *intermediate = PyUnicodeWriter_Finish(self->writer);
     self->writer = NULL;
     self->state = STATE_REALIZED;
@@ -153,6 +155,7 @@ make_intermediate(stringio *self)
 static int
 realize(stringio *self)
 {
+    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
     Py_ssize_t len;
     PyObject *intermediate;
 
@@ -188,6 +191,7 @@ realize(stringio *self)
 static Py_ssize_t
 write_str(stringio *self, PyObject *obj)
 {
+    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
     Py_ssize_t len;
     PyObject *decoded = NULL;
 
@@ -355,6 +359,7 @@ _io_StringIO_read_impl(stringio *self, Py_ssize_t size)
 static PyObject *
 _stringio_readline(stringio *self, Py_ssize_t limit)
 {
+    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
     Py_UCS4 *start, *end, old_char;
     Py_ssize_t len, consumed;
 
@@ -404,8 +409,9 @@ _io_StringIO_readline_impl(stringio *self, Py_ssize_t size)
 }
 
 static PyObject *
-stringio_iternext(PyObject *op)
+stringio_iternext_lock_held(PyObject *op)
 {
+    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(op);
     PyObject *line;
     stringio *self = stringio_CAST(op);
 
@@ -441,6 +447,16 @@ 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

From 25d2f2723fe5ea711857ab8975b4e7ca5db3052c Mon Sep 17 00:00:00 2001
From: Peter Bierma <zintensitydev@gmail.com>
Date: Wed, 11 Jun 2025 19:05:53 -0400
Subject: [PATCH 2/3] Add blurb.

---
 .../next/Library/2025-06-11-19-05-49.gh-issue-135410.E89Boi.rst | 2 ++
 1 file changed, 2 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2025-06-11-19-05-49.gh-issue-135410.E89Boi.rst

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
index 00000000000000..a5917fba3f7bb9
--- /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.

From 455bfba87b715c60e4a48dd26910823fef34d6be Mon Sep 17 00:00:00 2001
From: Peter Bierma <zintensitydev@gmail.com>
Date: Wed, 11 Jun 2025 19:15:36 -0400
Subject: [PATCH 3/3] Add a test.

---
 Lib/test/test_memoryio.py | 19 +++++++++++++++++++
 Modules/_io/stringio.c    |  6 ------
 2 files changed, 19 insertions(+), 6 deletions(-)

diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index 63998a86c45b53..249e0f3ba32f29 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -5,6 +5,7 @@
 
 import unittest
 from test import support
+from test.support import threading_helper
 
 import gc
 import io
@@ -12,6 +13,7 @@
 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/Modules/_io/stringio.c b/Modules/_io/stringio.c
index 9cf1e0e3e7c278..8482c268176c5b 100644
--- a/Modules/_io/stringio.c
+++ b/Modules/_io/stringio.c
@@ -79,7 +79,6 @@ static int _io_StringIO___init__(PyObject *self, PyObject *args, PyObject *kwarg
 static int
 resize_buffer(stringio *self, size_t size)
 {
-    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
     /* Here, unsigned types are used to avoid dealing with signed integer
        overflow, which is undefined in C. */
     size_t alloc = self->buf_size;
@@ -132,7 +131,6 @@ resize_buffer(stringio *self, size_t size)
 static PyObject *
 make_intermediate(stringio *self)
 {
-    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
     PyObject *intermediate = PyUnicodeWriter_Finish(self->writer);
     self->writer = NULL;
     self->state = STATE_REALIZED;
@@ -155,7 +153,6 @@ make_intermediate(stringio *self)
 static int
 realize(stringio *self)
 {
-    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
     Py_ssize_t len;
     PyObject *intermediate;
 
@@ -191,7 +188,6 @@ realize(stringio *self)
 static Py_ssize_t
 write_str(stringio *self, PyObject *obj)
 {
-    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
     Py_ssize_t len;
     PyObject *decoded = NULL;
 
@@ -359,7 +355,6 @@ _io_StringIO_read_impl(stringio *self, Py_ssize_t size)
 static PyObject *
 _stringio_readline(stringio *self, Py_ssize_t limit)
 {
-    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
     Py_UCS4 *start, *end, old_char;
     Py_ssize_t len, consumed;
 
@@ -411,7 +406,6 @@ _io_StringIO_readline_impl(stringio *self, Py_ssize_t size)
 static PyObject *
 stringio_iternext_lock_held(PyObject *op)
 {
-    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(op);
     PyObject *line;
     stringio *self = stringio_CAST(op);
 

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