bug#56469: 29.0.50; Unibyte dir in directory_files_internal

[Eli Zaretskii]

> Date: Sat, 09 Jul 2022 13:44:52 -0400
> From:  Stefan Monnier via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> If you have a directory named "/tmp/\303a" with a file named "fée"
> inside, then (directory-files "/tmp/\303a" 'full) is likely to return
> a funny string which is multibyte but contains an invalid
> utf-8 sequence (its bytes spell "/tmp/\303a/f\303\251e").
> That strings seems to be printed as "/tmp/¡/fée" which corresponds
> to "/tmp/\303\241/f\303\251e".
> 
> Such a string with an invalid UTF-8 sequence is handled quite graciously
> by Emacs, so I wasn't able to get an actual crash out of it, but it's
> still something we should avoid.
> 
> I suggest the patch below.  In a comment I suggest we don't try to use
> unibyte strings when a multibyte string would work as well.  This is
> because for those ASCII-only strings, it's cheaper to test bytes==chars
> to (re)discover that they are ASCII-only (when they're multibyte) than
> having to loop through the bytes (when they're unibyte).

Please bootstrap Emacs in a directory with such a name, and if that
works, I'm okay with installing this change.

Thanks.