gh-156312: Prevent mailbox.MH replacement failures from corrupting messages

Lib/mailbox.py

@@ -1069,19 +1069,44 @@ def __setitem__(self, key, message):
                 raise KeyError('No message with key: %s' % key)
             else:
                 raise
+        file_closed = False
         try:
             if self._locked:
                 _lock_file(f)
             try:
-                os.close(os.open(path, os.O_WRONLY | os.O_TRUNC))
-                self._dump_message(message, f)
+                new_file = _create_temporary(path)
+                try:
+                    self._dump_message(message, new_file)
+                    _sync_close(new_file)
+                    info = os.fstat(f.fileno())
+                    try:
+                        os.chown(new_file.name, info.st_uid, info.st_gid)
+                    except (AttributeError, OSError):
+                        pass
+                    os.chmod(new_file.name, info.st_mode)
+                    if os.name == 'nt':
+                        # Windows cannot replace an open file.
+                        f.close()
+                        file_closed = True
+                    os.replace(new_file.name, path)
+                except BaseException:
+                    try:
+                        new_file.close()
+                    except OSError:
+                        pass
+                    try:
+                        os.remove(new_file.name)
+                    except OSError:
+                        pass
+                    raise
                 if isinstance(message, MHMessage):
                     self._dump_sequences(message, key)
             finally:
                 if self._locked:
                     _unlock_file(f)
         finally:
-            _sync_close(f)
+            if not file_closed:
+                f.close()
 
     def get_message(self, key):
         """Return a Message representation or raise a KeyError."""

Lib/test/test_mailbox.py

@@ -1,4 +1,5 @@
 import os
+import stat
 import sys
 import time
 import socket
@@ -1338,6 +1339,57 @@ class TestMH(TestMailbox, unittest.TestCase):
     def assertMailboxEmpty(self):
         self.assertEqual(os.listdir(self._path), ['.mh_sequences'])
 
+    def test_set_item_nonascii_string_raises_without_modifying_message(self):
+        key = self._box.add(self._template % 'original')
+        original = self._box.get_bytes(key)
+        with self.assertRaisesRegex(ValueError, "ASCII-only"):
+            self._box[key] = self._nonascii_msg
+        self.assertEqual(self._box.get_bytes(key), original)
+        self._box.close()
+        self._box = self._factory(self._path)
+        self.assertEqual(self._box.get_bytes(key), original)
+
+    def test_set_item_read_error_does_not_modify_message(self):
+        class CustomError(Exception):
+            pass
+
+        class FaultyMessage:
+            def __init__(self):
+                self.first_read = True
+
+            def read(self):
+                raise AssertionError
+
+            def readline(self):
+                if self.first_read:
+                    self.first_read = False
+                    return b'Subject: replacement\n'
+                raise CustomError
+
+        key = self._box.add(self._template % 'original')
+        original = self._box.get_bytes(key)
+        original_files = set(os.listdir(self._path))
+        with self.assertRaises(CustomError):
+            self._box[key] = FaultyMessage()
+        self.assertEqual(self._box.get_bytes(key), original)
+        self.assertEqual(set(os.listdir(self._path)), original_files)
+        self._box.close()
+        self._box = self._factory(self._path)
+        self.assertEqual(self._box.get_bytes(key), original)
+
+    @unittest.skipUnless(hasattr(os, 'chown'), 'requires os.chown')
+    def test_set_item_preserves_mode(self):
+        key = self._box.add(self._template % 'original')
+        path = os.path.join(self._path, str(key))
+        mode = os.stat(path).st_mode | stat.S_ISUID
+        os.chmod(path, mode)
+        if os.stat(path).st_mode != mode:
+            self.skipTest('filesystem does not support set-user-ID mode')
+
+        self._box[key] = self._template % 'replacement'
+
+        self.assertEqual(os.stat(path).st_mode, mode)
+
     def test_list_folders(self):
         # List folders
         self._box.add_folder('one')

Misc/NEWS.d/next/Library/2026-08-24-19-37-56.gh-issue-156312.tF9p9m.rst

@@ -0,0 +1,3 @@
+Prevent :class:`mailbox.MH` message replacement from truncating or partially
+overwriting the original message when serializing the replacement raises an
+exception.