bug#65071: 29.1; RMAIL changed on disk leads to rmail-error-bad-format

[Eli Zaretskii <eliz@gnu.org>]

> From: emacs-bug@rulnick.com
> Cc: 65071@debbugs.gnu.org
> Date: Sat, 05 Aug 2023 14:07:40 -0400
> 
> I hope the receipe below is clearer. In Emacs 29.1, starting with no
> ~/RMAIL file and empty primary mail spool file (please note that '$'
> indicates Bash prompt in Terminal independent of Emacs):
> 
>   $ mail me@foo ... # send a message to myself
> 
>   $ emacs -Q
> 
>   (in Emacs) M-x rmail RET # retrieves/reads message
> 
>   $ cat /dev/null > ~/RMAIL # clear RMAIL file
> 
>   $ mail me@foo ... # send another message to myself
> 
>   (in Emacs) g
> 
>     "File RMAIL changed on disk. Reread from disk? (yes or no) " yes RET
> 
>     "rmail-error-bad-format: Message is not a valid RFC 822 (or later)
>     message"; Emacs mode changes from RMAIL to Fundamental
> 
> At this point the ~/RMAIL file on disk looks good (proper RFC 822 file),
> but the ~/RMAIL file as visited in buffer looks bad, as can be seen
> (e.g., upon immediate save) to be missing top "From " and other header
> lines ("Return-Path:", "Received:", etc.). I have many years experience
> with Emacs and RMAIL and this behavior is new as of 29.1 or another very
> recent version of Emacs.

Thanks, this should be fixed now on the emacs-29 branch.  The reason
for the bug was a change in revert-buffer we made during Emacs 29
development.