Re: Timer scheduling and cancel-timer

[Michael Heerdegen <michael_heerdegen@web.de>]

Tomohiro Matsuyama <tomo@cx4a.org> writes:

> > >     (setq my-timer
> > >           (run-with-timer
> > >            nil 0.1
> > >            (lambda ()
> > >              (when my-timer
> > >                (cancel-timer my-timer)
> > >                (setq my-timer nil)
> > >                (sit-for 0.3)))))
> > > 
> > > After evaluating this code several times, you may see "zombie" timers
> > > in timer-list, though the code intends to keep at most one timer.

I see that, too.

To reveal what's happening, I ran the following experiment:

--8<---------------cut here---------------start------------->8---
(defvar my-timer nil)

(defun start-the-timer ()
  (interactive)
  (setq my-timer
        (run-with-timer
         0 0.1
         (lambda ()
           (cancel-timer my-timer)
           (sit-for 0.3)))))

(advice-add 'timer-event-handler :before
            (lambda (timer)
              (when (and (eq timer my-timer)
                         (not (memq my-timer timer-list)))
                (message "Why is this ever reached?"))))
--8<---------------cut here---------------end--------------->8---

If you call `start-the-timer', you get the message "Why is this ever
reached?" over and over.

Obviously, although the timer object created has been successfully
canceled (i.e., removed from `timer-list'), the C code still calls it
repeatedly with `timer-event-handler'.  Dunno why, but that's not good.


Regards,

Michael.