bug#27634: 25.2.1; C-g does not quit register-read-with-preview

[Eli Zaretskii <eliz@gnu.org>]

> From: Paul Rankin <hello@paulwrankin.com>
> Cc: 27634@debbugs.gnu.org, Eli Zaretskii <eliz@gnu.org>
> Date: Mon, 10 Jul 2017 17:20:55 +1000
> 
> On Mon, 10 Jul 2017, at 04:33 PM, Tino Calancha wrote:
> > How about the following?
> > 
> > @@ -164,6 +164,8 @@ register-read-with-preview
> >  		       help-chars)
> >  	    (unless (get-buffer-window buffer)
> >  	      (register-preview buffer 'show-empty)))
> > +          (when (eq (string-to-char "\C-g") last-input-event)
> > +            (keyboard-quit))
> >  	  (if (characterp last-input-event) last-input-event
> >  	    (error "Non-character input-event")))
> >        (and (timerp timer) (cancel-timer timer))
> 
> I think that's more a bandaid than fixing the root of the problem, which is that the manual tells the user that register names are alphanumeric characters (and I assume 99% of users only use alphanumeric characters) but the function doesn't test for this. e.g. testing for C-g doesn't catch for ^L or ^M, etc.

FWIW, I actually agree with Tino's solution, and was about to propose
something similar.  It's true that control characters are not
alphanumeric, but we could fix the documentation to be more accurate
if we care about that.  OTOH, we've supported control characters as
register names for many years, and by now it should be quite clear it
didn't bother anyone yet.

C-g needs indeed to generate keyboard quit, and perhaps ESC ESC as
well.  I'd look at read-char-choice for inspiration.

> If we want to be strict about it, this might work:
> 
> @@ -164,8 +164,8 @@
>  		       help-chars)
>  	    (unless (get-buffer-window buffer)
>  	      (register-preview buffer 'show-empty)))
> -	  (if (characterp last-input-event) last-input-event
> -	    (error "Non-character input-event")))
> +	  (if (= (char-syntax last-input-event) 119) last-input-event
> +	    (error "Register name must be alphanumeric")))
>        (and (timerp timer) (cancel-timer timer))
>        (let ((w (get-buffer-window buffer)))
>          (and (window-live-p w) (delete-window w)))
> 
> That prohibits anything except "a-zA-Z0-9", although users may want to save registers to "$" or "*".

Why would we want to be so strict when the only real problem is that
C-g doesn't quit?