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

[emacs-bug]

Eli Zaretskii <eliz@gnu.org> writes:

>> 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.

1. M-x revert-buffer RET does not avoid or solve the problem completely;
   the contents from disk appear to be restored as expected but the
   buffer remains in Fundamental mode. However, following this with
   RMAIL-mode RET does appear to at least be a fix.

2. Yes, I should have said 'g' rather than 'M-x rmail RET' in the second
   instance. The bug report applies in this case as well.