[3.12] gh-117467: Add preserving of mailbox owner on flush (GH-117510) (GH-117537)

(cherry picked from commit 3f5bcc86d0)

Co-authored-by: Tony Mountifield <tony@mountifield.org>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Lib/test/test_mailbox.py

@@ -9,6 +9,7 @@ import re
 import io
 import tempfile
 from test import support
+from test.support import import_helper
 from test.support import os_helper
 from test.support import socket_helper
 import unittest
@@ -981,6 +982,47 @@ class _TestSingleFile(TestMailbox):
 
         self.assertEqual(os.stat(self._path).st_mode, mode)
 
+    @unittest.skipUnless(hasattr(os, 'chown'), 'requires os.chown')
+    def test_ownership_after_flush(self):
+        # See issue gh-117467
+
+        pwd = import_helper.import_module('pwd')
+        grp = import_helper.import_module('grp')
+        st = os.stat(self._path)
+
+        for e in pwd.getpwall():
+            if e.pw_uid != st.st_uid:
+                other_uid = e.pw_uid
+                break
+        else:
+            self.skipTest("test needs more than one user")
+
+        for e in grp.getgrall():
+            if e.gr_gid != st.st_gid:
+                other_gid = e.gr_gid
+                break
+        else:
+            self.skipTest("test needs more than one group")
+
+        try:
+            os.chown(self._path, other_uid, other_gid)
+        except OSError:
+            self.skipTest('test needs root privilege')
+        # Change permissions as in test_permissions_after_flush.
+        mode = st.st_mode | 0o666
+        os.chmod(self._path, mode)
+
+        self._box.add(self._template % 0)
+        i = self._box.add(self._template % 1)
+        # Need to remove one message to make flush() create a new file
+        self._box.remove(i)
+        self._box.flush()
+
+        st = os.stat(self._path)
+        self.assertEqual(st.st_uid, other_uid)
+        self.assertEqual(st.st_gid, other_gid)
+        self.assertEqual(st.st_mode, mode)
+
 
 class _TestMboxMMDF(_TestSingleFile):