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

[Eli Zaretskii <eliz@gnu.org>]

> From: <emacs-bug@rulnick.com>
> Date: Fri, 04 Aug 2023 14:18:52 -0400
> 
> In Emacs 29.1, starting with no ~/RMAIL file and empty primary mail
> spool file ('$' indicates command line terminal prompt outside Emacs):
> 
>   $ mail me@foo ... # send a message to myself
> 
>   emacs -Q
> 
>   M-x RMAIL # retrieves/reads message
> 
>   $ cat /dev/null > ~/RMAIL # clear (or otherwise modify) RMAIL file
> 
>   $ mail me@foo ... # send another message to myself
> 
>   M-x RMAIL
> 
>   "File RMAIL changed on disk. Reread from disk? (yes or no) " yes
> 
>     "rmail-error-bad-format: Message is not a valid RFC 822 (or later)
>     message" and:
> 
>       mode changes from RMAIL to fundamental
> 
>       ~/RMAIL file on disk looks good (proper RFC 822 file)
> 
>       ~/RMAIL file as visited in buffer looks bad, can be seen (e.g.,
>         upon immediate save) to be missing top "From " and other header
>         lines ("Return-Path:", "Received:", etc.)

Does using "M-x revert-buffer RET" avoid the problem?

In any case, this is not how you are supposed to read new email into
an existing Rmail buffer.  You are supposed to use the 'g' command.