From 00d23bb6355543e7a23bce27e06de5e318c0c024 Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Sat, 12 Sep 2026 02:38:47 +0200
Subject: [PATCH 1/2] gh-157242: Do not close io.BytesIO on MemoryError

Replace _PyBytes_Resize() with _PyBytes_ResizeKeepOnError().

Fix truncate(): only set string_size on resize success.
---
 Lib/test/test_io/test_memoryio.py             | 30 +++++++++++++++++++
 ...-09-12-03-14-57.gh-issue-157242.ycbtjC.rst |  2 ++
 Modules/_io/bytesio.c                         |  8 +++--
 3 files changed, 37 insertions(+), 3 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-09-12-03-14-57.gh-issue-157242.ycbtjC.rst

diff --git a/Lib/test/test_io/test_memoryio.py b/Lib/test/test_io/test_memoryio.py
index 624fd78327cae10..423b99779bc6e78 100644
--- a/Lib/test/test_io/test_memoryio.py
+++ b/Lib/test/test_io/test_memoryio.py
@@ -5,6 +5,7 @@
 
 import unittest
 from test import support
+from test.support import import_helper
 
 import gc
 import io
@@ -753,6 +754,35 @@ def __buffer__(self, flags):
         self.assertEqual(memio.getvalue(), b"01AAA56789")
         self.assertEqual(memio.tell(), 5)
 
+    def test_memory_error(self):
+        # gh-157242: io.BytesIO() must not close the file on MemoryError
+        _testcapi = import_helper.import_module('_testcapi')
+
+        # write()
+        stream = self.ioclass()
+        stream.write(self.buftype('abc'))
+        with self.assertRaises(MemoryError):
+            try:
+                data = self.buftype('def')
+                _testcapi.set_nomemory(0)
+                stream.write(data)
+            finally:
+                _testcapi.remove_mem_hooks()
+        stream.write(self.buftype('123'))
+        self.assertEqual(stream.getvalue(), self.buftype('abc123'))
+
+        # truncate()
+        data = self.buftype('x' * 100)
+        stream = self.ioclass()
+        stream.write(data)
+        with self.assertRaises(MemoryError):
+            try:
+                _testcapi.set_nomemory(0)
+                stream.truncate(5)
+            finally:
+                _testcapi.remove_mem_hooks()
+        self.assertEqual(stream.getvalue(), data)
+
 
 class TextIOTestMixin:
 
diff --git a/Misc/NEWS.d/next/Library/2026-09-12-03-14-57.gh-issue-157242.ycbtjC.rst b/Misc/NEWS.d/next/Library/2026-09-12-03-14-57.gh-issue-157242.ycbtjC.rst
new file mode 100644
index 000000000000000..54ada2892f76fac
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-09-12-03-14-57.gh-issue-157242.ycbtjC.rst
@@ -0,0 +1,2 @@
+:class:`io.BytesIO` is no longer closed on ``write()`` and ``truncate()``
+failure (:exc:`MemoryError`). Patch by Victor Stinner.
diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index ab54df8c76758f9..9ecb61af40cc23e 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -1,4 +1,5 @@
 #include "Python.h"
+#include "pycore_bytesobject.h"       // _PyBytes_ResizeKeepOnError()
 #include "pycore_critical_section.h"  // Py_BEGIN_CRITICAL_SECTION()
 #include "pycore_object.h"
 #include "pycore_pyatomic_ft_wrappers.h"
@@ -108,7 +109,7 @@ resize_unshared_buffer_lock_held(bytesio *self, Py_ssize_t size)
        Callers must detach first. */
     assert(!self->buf_shared);
 #endif
-    int ret = _PyBytes_Resize(&self->buf, size);
+    int ret = _PyBytes_ResizeKeepOnError(&self->buf, size);
     if (ret == 0) {
         clear_shared_buf(self);
     }
@@ -758,9 +759,10 @@ _io_BytesIO_truncate_impl(bytesio *self, PyObject *size)
     }
 
     if (new_size < self->string_size) {
-        self->string_size = new_size;
-        if (resize_buffer_lock_held(self, new_size) < 0)
+        if (resize_buffer_lock_held(self, new_size) < 0) {
             return NULL;
+        }
+        self->string_size = new_size;
     }
 
     return PyLong_FromSsize_t(new_size);

From 6fda0a7c0f40131113b945465092fcebb9cc5d4f Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Sat, 12 Sep 2026 05:40:17 +0200
Subject: [PATCH 2/2] Fix string_size

Revert the string_size change. Instead, restore string_size to its
previous value on error.
---
 Modules/_io/bytesio.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Modules/_io/bytesio.c b/Modules/_io/bytesio.c
index 9ecb61af40cc23e..b487d6d7beca93f 100644
--- a/Modules/_io/bytesio.c
+++ b/Modules/_io/bytesio.c
@@ -759,10 +759,12 @@ _io_BytesIO_truncate_impl(bytesio *self, PyObject *size)
     }
 
     if (new_size < self->string_size) {
+        Py_ssize_t old_string_size = self->string_size;
+        self->string_size = new_size;
         if (resize_buffer_lock_held(self, new_size) < 0) {
+            self->string_size = old_string_size;
             return NULL;
         }
-        self->string_size = new_size;
     }
 
     return PyLong_FromSsize_t(new_size);