--- Lib/mailbox.py.unified2	2007-01-21 13:48:02.000000000 +0000
+++ Lib/mailbox.py	2007-01-21 21:17:48.000000000 +0000
@@ -558,10 +558,7 @@
         if not self._locked:
             _lock_file(self._file)
             self._locked = True
-            if self._pending:
-                warnings.warn("mailbox .lock() method called with pending changes; "
-                              "should have been locked before making changes",
-                              stacklevel=2)
+            self._update_toc()
 
     def unlock(self):
         """Unlock the mailbox if it is locked."""
@@ -654,6 +651,7 @@
                     orig_file.close()
                     if self._locked:
                         _lock_file(self._file, dotlock=False)
+                        self._update_toc()
 
     def _pre_mailbox_hook(self, f):
         """Called before writing the mailbox to file f."""
@@ -677,13 +675,53 @@
     def _lookup(self, key=None):
         """Return (start, stop) or raise KeyError."""
         if self._toc is None:
-            self._generate_toc()
+            self._update_toc()
         if key is not None:
             try:
                 return self._toc[key]
             except KeyError:
                 raise KeyError('No message with key: %s' % key)
 
+    def _update_toc(self):
+        """Update self._toc and self._file_length from contents of file.
+        Raise an exception if called with pending changes.
+        """
+        # See bug #1599254 for why this method does what it does.
+        if self._pending:
+            raise Error('you must lock the mailbox before making any changes '
+                        '(_update_toc() was called with self._pending set)')
+        our_toc = self._toc
+        next_unused_key = self._next_key
+        try:
+            self._toc = None
+            self._next_key = 0
+            self._generate_toc()
+            new_toc = self._toc
+        finally:
+            self._toc = our_toc
+            self._next_key = next_unused_key
+        if our_toc is None:
+            our_toc = {}
+        old_keys = sorted(our_toc.keys())
+        new_keys = sorted(new_toc.keys())
+        old_count = len(our_toc)
+        new_count = len(new_toc)
+        if new_count < old_count:
+            for i in range(new_count):
+                our_toc[old_keys[i]] = new_toc[new_keys[i]]
+            for key in old_keys[new_count:]:
+                del our_toc[key]
+        else:
+            for i in range(old_count):
+                our_toc[old_keys[i]] = new_toc[new_keys[i]]
+            for key in new_keys[old_count:]:
+                assert self._next_key not in our_toc
+                our_toc[self._next_key] = new_toc[key]
+                self._next_key += 1
+        self._toc = our_toc
+        self._file.seek(0, 2)
+        self._file_length = self._file.tell()
+
     def _append_message(self, message):
         """Append message to mailbox and return (start, stop) offsets."""
         self._file.seek(0, 2)
@@ -783,7 +821,6 @@
                 break
         self._toc = dict(enumerate(zip(starts, stops)))
         self._next_key = len(self._toc)
-        self._file_length = self._file.tell()
 
 
 class MMDF(_mboxMMDF):
@@ -827,8 +864,6 @@
                 break
         self._toc = dict(enumerate(zip(starts, stops)))
         self._next_key = len(self._toc)
-        self._file.seek(0, 2)
-        self._file_length = self._file.tell()
 
 
 class MH(Mailbox):
@@ -1254,8 +1289,6 @@
         self._toc = dict(enumerate(zip(starts, stops)))
         self._labels = dict(enumerate(label_lists))
         self._next_key = len(self._toc)
-        self._file.seek(0, 2)
-        self._file_length = self._file.tell()
         
     def _pre_mailbox_hook(self, f):
         """Called before writing the mailbox to file f."""