gh-160598: Preserve Babyl labels when reopening mailboxes

Lib/mailbox.py

@@ -1426,9 +1426,11 @@ def _generate_toc(self):
                 if len(stops) < len(starts):
                     stops.append(line_pos - len(linesep))
                 starts.append(next_pos)
-                labels = [label.strip() for label
-                                        in self._file.readline()[1:].split(b',')
-                                        if label.strip()]
+                labels = [
+                    label.strip().decode('utf-8', 'surrogateescape')
+                    for label in self._file.readline()[1:].split(b',')
+                    if label.strip()
+                ]
                 label_lists.append(labels)
             elif line == b'\037' or line == b'\037' + linesep:
                 if len(stops) < len(starts):
@@ -1447,7 +1449,7 @@ def _pre_mailbox_hook(self, f):
         babyl = b'BABYL OPTIONS:' + linesep
         babyl += b'Version: 5' + linesep
         labels = self.get_labels()
-        labels = (label.encode() for label in labels)
+        labels = (label.encode('utf-8', 'surrogateescape') for label in labels)
         babyl += b'Labels:' + b','.join(labels) + linesep
         babyl += b'\037'
         f.write(babyl)
@@ -1473,10 +1475,12 @@ def _install_message(self, message):
                     labels.append(label)
             self._file.write(b'1')
             for label in special_labels:
-                self._file.write(b', ' + label.encode())
+                self._file.write(
+                    b', ' + label.encode('utf-8', 'surrogateescape'))
             self._file.write(b',,')
             for label in labels:
-                self._file.write(b' ' + label.encode() + b',')
+                self._file.write(
+                    b' ' + label.encode('utf-8', 'surrogateescape') + b',')
             self._file.write(linesep)
         else:
             self._file.write(b'1,,' + linesep)

Lib/test/test_mailbox.py

@@ -1521,6 +1521,59 @@ def test_labels(self):
         self._box.remove(key1)
         self.assertEqual(set(self._box.get_labels()), set(['blah']))
 
+    def test_labels_reopened(self):
+        labels = ['unseen', 'work', 'caf\u00e9']
+        msg = mailbox.BabylMessage(self._template % 0)
+        msg.set_labels(labels)
+        self._box.add(msg)
+        self._box.add(self._template % 1)
+        self._box.close()
+        self._box = self._factory(self._path)
+
+        msg = self._box[0]
+        self.assertCountEqual(msg.get_labels(), labels)
+        self.assertCountEqual(self._box.get_labels(), ['work', 'caf\u00e9'])
+        self.assertNotIn('S', mailbox.MaildirMessage(msg).get_flags())
+        self.assertNotIn('R', mailbox.mboxMessage(msg).get_flags())
+
+        # Copying a message and rewriting the mailbox must preserve labels.
+        self._box.add(msg)
+        self._box.remove(1)
+        self._box.flush()
+        self._box.close()
+        self._box = self._factory(self._path)
+        self.assertEqual(len(self._box), 2)
+        for msg in self._box:
+            self.assertCountEqual(msg.get_labels(), labels)
+        self.assertCountEqual(self._box.get_labels(), ['work', 'caf\u00e9'])
+
+    def test_labels_non_utf8(self):
+        msg = mailbox.BabylMessage(self._template % 0)
+        msg.set_labels(['unseen', 'work'])
+        self._box.add(msg)
+        self._box.add(self._template % 1)
+        self._box.close()
+        with open(self._path, 'rb') as f:
+            data = f.read().replace(b'work', b'w\xffrk')
+        with open(self._path, 'wb') as f:
+            f.write(data)
+        self._box = self._factory(self._path)
+
+        labels = ['unseen', 'w\udcffrk']
+        msg = self._box[0]
+        self.assertCountEqual(msg.get_labels(), labels)
+        self.assertEqual(self._box.get_labels(), ['w\udcffrk'])
+        self._box.add(msg)
+        self._box.remove(1)
+        self._box.flush()
+        self._box.close()
+        self._box = self._factory(self._path)
+        self.assertEqual(len(self._box), 2)
+        for msg in self._box:
+            self.assertCountEqual(msg.get_labels(), labels)
+        with open(self._path, 'rb') as f:
+            self.assertIn(b'Labels:w\xffrk' + mailbox.linesep, f.read())
+
     def test_visible_headers(self):
         # gh-157880: visible headers are written to the file and read back
         msg = mailbox.BabylMessage(self._template % 0)

Misc/NEWS.d/next/Library/2026-10-25-00-00-00.gh-issue-160598.Babyl.rst

@@ -0,0 +1,3 @@
+Fix :class:`mailbox.Babyl` returning message labels as bytes after reopening a
+mailbox, which could cause incorrect read flags and errors when rewriting or
+copying messages. Preserve non-UTF-8 label bytes using ``surrogateescape``.