bug#4623: 23.1.50; rmail changes encoding of characters on save

[Markus Rost <rost@math.uni-bielefeld.de>]

With rmail I observed random encoding changes in mbox files:  some
non-ascii characters are changed depending on which message you look
at while saving.

Providing a test case is a bit tiresome, so I omit one for now.

The reason for the problem is that rmail-expunge-and-save uses the
coding system of the current message when saving the file.  This is
certainly wrong:  the coding system for writing the mbox file must not
depend on the currently selected message.

Since rmail reads the mbox file with coding-system 'no-conversion, it
should save it with coding-system 'no-conversion.

Perhaps the problem sits in rmail-swap-buffers:  Shouldn't
rmail-swap-buffers exchange the value of buffer-file-coding-system in
both ways?  Or perhaps it should ensure that the buffer which contains
the full mbox file has always the same buffer-file-coding-system,
namely 'no-conversion.

Anyway, the following ad hoc patch fixes the problem for me.

*** rmail.el.~1.552.~	2009-09-26 21:21:07.000000000 +0200
--- rmail.el	2009-10-02 16:14:30.000000000 +0200
***************
*** 1458,1464 ****
    (rmail-expunge)
    ;; No need to swap buffers: rmail-write-region-annotate takes care of it.
    ;; (rmail-swap-buffers-maybe)
!   (save-buffer)
    (if (rmail-summary-exists)
        (rmail-select-summary (set-buffer-modified-p nil))))
  
--- 1458,1465 ----
    (rmail-expunge)
    ;; No need to swap buffers: rmail-write-region-annotate takes care of it.
    ;; (rmail-swap-buffers-maybe)
!   (let ((coding-system-for-write 'no-conversion))
!     (save-buffer))
    (if (rmail-summary-exists)
        (rmail-select-summary (set-buffer-modified-p nil))))


In GNU Emacs 23.1.50.1 (i686-pc-linux-gnu, GTK+ Version 2.16.1)
 of 2009-10-02 on laptop
Windowing system distributor `The X.Org Foundation', version 11.0.10600000
configured using `configure  '--prefix=/home/rost/local/cvs''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: C
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t