--- Lib/mailbox.py.copyback	2007-01-13 15:45:49.000000000 +0000
+++ Lib/mailbox.py	2007-01-13 17:06:39.000000000 +0000
@@ -21,6 +21,7 @@
 import rfc822
 import StringIO
 import shutil
+import warnings
 try:
     if sys.platform == 'os2emx':
         # OS/2 EMX fcntl() not adequate
@@ -583,6 +584,12 @@
                                      '(expected %i, found %i)' %
                                      (self._file_length, cur_len))
         
+        if fcntl and self._locked and not hasattr(self._file, 'truncate'):
+            warnings.warn('as file.truncate() is unavailable, flush() may '
+                          'momentarily release the fcntl lock; if you depend '
+                          'on fcntl locking, you should regard flush() as '
+                          'invalidating the message keys', RuntimeWarning,
+                          stacklevel=2)
         orig_file = self._file
         remove_temp_file = True
         new_file = _create_temporary(self._path)