bug#21747: 25.0.50; while-no-input breaks kbd event handling when called from post-command-hook

[Eli Zaretskii <eliz@gnu.org>]

> From: Tassilo Horn <tsdh@gnu.org>
> Cc: Stefan Monnier <monnier@iro.umontreal.ca>,  21747@debbugs.gnu.org, Kim F. Storm <storm@cua.dk>
> Date: Sat, 24 Oct 2015 11:49:01 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >> From: Tassilo Horn <tsdh@gnu.org>
> >> Cc: 21747@debbugs.gnu.org
> >> Date: Sat, 24 Oct 2015 10:53:12 +0200
> >> 
> >> > Given that, maybe I'm missing something, but what did you expect?  The
> >> > above literally says that Emacs shall loop indefinitely after
> >> > performing each command until there's more input.  And that's what you
> >> > get.  Right?
> >> 
> >> Correct, but when the input eventually arrives, I expect to see its
> >> effects as if it had arrived outside of the `while-no-input'.
> >
> > That effect was not yet produced, because the arriving input was not
> > yet consumed by the time while-no-input returns, that input is still
> > "pending".  For it to be consumed and acted upon, you need another
> > crank of the Emacs main loop and another redisplay cycle (which is
> > again delayed by the while-no-input loop).  So the one-character delay
> > goes on forever.
> 
> Ah, ok.  I changed `while-no-input' locally to
> 
> (defmacro while-no-input (&rest body)
>   "Execute BODY only as long as there's no pending input.
> If input arrives, that ends the execution of BODY,
> and `while-no-input' returns t.  Quitting makes it return nil.
> If BODY finishes, `while-no-input' returns whatever value BODY produced."
>   (declare (debug t) (indent 0))
>   (let ((catch-sym (make-symbol "input")))
>     `(with-local-quit
>        (catch ',catch-sym
> 	 (let ((throw-on-input ',catch-sym))
> 	   (or (input-pending-p)
> 	       (progn
> 		 (sit-for 0)   ;; <== just inserted that
> 		 ,@body)))))))
> 
> which seems to fix the issue somehow.  With your description, what I
> think it does is that it forces the display of the effects of the
> command which has interrupted the `while-no-input' in the previous
> cycle.

More accurately, it invokes redisplay without waiting for more input
to arrive.

> >> So the question is: should `while-no-input' call (sit-for 0) as the
> >> first statement in the `progn' or should functions using
> >> `while-no-input' do that on their own?  I'd prefer the former because
> >> the current behavior is not really obvious (at least not to me nor
> >> Artur).
> >
> > I don't have enough experience in using while-no-input to answer that.
> > Perhaps others could chime in and voice their opinions.  Maybe we
> > should have a discussion on emacs-devel about this (because many
> > people who read emacs-devel don't read the bug list).
> 
> Yes, I think that would be a good idea.  Originally, `while-no-input'
> used (not (sit-for 0 0 t)) instead of (input-pending-p) which I think is
> pretty equivalent except that the former forces a redisplay.  I've added
> Kim to the Cc, so maybe he can speak up.
> 
> --8<---------------cut here---------------start------------->8---
> commit 790e0ef78e306edc0664b8fa5a584c62ec01b444
> Author: Kim F. Storm <storm@cua.dk>
> Date:   Mon Sep 11 22:21:55 2006 +0000
> 
>     (sit-for): Rework to use input-pending-p and cond.
>     Return nil input is pending on entry also for SECONDS <= 0.
>     (while-no-input): Use input-pending-p instead of sit-for.

This discussion on emacs-devel might be relevant (I think it was the
motivation for this change):

  http://lists.gnu.org/archive/html/emacs-devel/2006-09/msg00300.html