bug#73307: Fix ctype(3) usage

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 17 Sep 2024 17:05:29 -0700
> Cc: 73307-done@debbugs.gnu.org, Thomas Klausner <wiz@gatalith.at>,
>  Po Lu <luangruo@yahoo.com>
> From: Paul Eggert <eggert@cs.ucla.edu>
> 
> On 2024-09-17 04:52, Eli Zaretskii wrote:
> > However, I'm not sure this is the right fix, the function is
> > defined with argument type of 'int'.  Paul, any comments?
> 
> Although that patch was an improvement it still had problems, as it 
> incorrectly assumed the string does not end in a multibyte space, and 
> that Emacs's locale matches the system's.
> 
> Emacs itself should not use <ctype.h> unless it knows the string is 
> unibyte and the system locale matches Emacs's. I scanned through its 
> source code looking for all problematic instances of <ctype.h> that have 
> crept in (except I didn't scan the MS-Windows code, where you're the 
> expert), and found five other places where ctype.h was obviously 
> misused. I installed the attached to fix these glitches and am boldly 
> closing this the report.
> 
> I can't easily test patch 0003, which fixes Android-specific code. 
> Although I think it's an improvement, in unlikely cases I suspect it 
> still doesn't exactly match what the Android kernel does with #! lines. 
> I don't know whether that matters. I'll CC this to Po Lu (my goto person 
> for Android) as a heads-up.

Thanks.

This part doesn't look right to me:

> -  {
> -    char *s = SSDATA (prompt);
> -    ptrdiff_t len = strlen (s);
> -    if ((len > 0) && !isspace (s[len - 1]))
> -      prompt = CALLN (Fconcat, prompt, build_string (" "));
> -  }
> -  prompt = CALLN (Fconcat, prompt, Vyes_or_no_prompt);
> +  ptrdiff_t promptlen = SCHARS (prompt);
> +  bool prompt_ends_in_nonspace
> +    = (0 < promptlen
> +       && (SYNTAX (XFIXNAT (Faref (prompt, make_fixnum (promptlen - 1))))
> +	   != Swhitespace));
> +  AUTO_STRING (space_string, " ");
> +  prompt = CALLN (Fconcat, prompt,
> +		  prompt_ends_in_nonspace ? space_string : empty_unibyte_string,
> +		  Vyes_or_no_prompt);

You are testing characters for whitespace syntax, which is AFAIU
subject to buffer-local syntax tables.  Thus, a strange enough setting
of buffer syntax could make this code return unexpected results.

Stefan, am I right?

Why can't we use something closer to the original code, which doesn't
depend on buffer-local customizations?  In particular, what's the
problem with using c_isspace (and what do you mean by "multibyte
space"?)