bug#25419: give better message when trying to go to forbidden areas

[Eli Zaretskii <eliz@gnu.org>]

> From: npostavs@users.sourceforge.net
> Cc: 積丹尼 Dan Jacobson <jidanni@jidanni.org>,
>   25419@debbugs.gnu.org
> Date: Wed, 18 Jan 2017 22:28:17 -0500
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> From: 積丹尼 Dan Jacobson
> >> 	<jidanni@jidanni.org>
> >> Date: Wed, 11 Jan 2017 22:44:08 +0800
> >> 
> >> Do C-x C-f ~root
> >> You will see:
> >> Setting current directory: Success, /root/
> >
> > That's because /root has a file-name handler,
> > tramp-completion-file-name-handler, and although that handler turns
> > around and calls file-accessible-directory-p, the value of errno is
> > lost in transition:
> >
> >   /* If the file name has special constructs in it,
> >      call the corresponding file handler.  */
> >   handler = Ffind_file_name_handler (absname, Qfile_accessible_directory_p);
> >   if (!NILP (handler))
> >     {
> >       Lisp_Object r = call2 (handler, Qfile_accessible_directory_p, absname);
> >       errno = 0;
> >       return r;
> >     }
> >
> > Is there any better way than arbitrarily set errno to EACCES if the
> > handler returns something other than t?
> 
> AFAICT, not without changing the interface of
> file-accessible-directory-p file handler, no.

OK, thanks.  I made that change, and I'm marking this bug done.