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Re: sleep-for documentation and how to pause reliably

From: Thierry Volpiatto
Subject: Re: sleep-for documentation and how to pause reliably
Date: Fri, 15 Feb 2013 11:19:27 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.2.93 (gnu/linux)

Hi Eli,

Eli Zaretskii <address@hidden> writes:

> Am I missing something, or is our current docs of sleep-for misleading?
> The doc string says:
>   (sleep-for SECONDS &optional MILLISECONDS)
>   Pause, without updating display, for SECONDS seconds.
> The only way I can interpret this is that sleep-for _always_ pauses
> for that number of seconds.
> The ELisp manual goes even further:
>    -- Function: sleep-for seconds &optional millisec
>        This function simply pauses for SECONDS seconds without updating
>        the display.  It pays no attention to available input.  It returns
>        `nil'.        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>        [...]
>        Use `sleep-for' when you wish to guarantee a delay.
>                                         ^^^^^^^^^^^^^^^^^
> "Guarantee a delay".  No "buts".
> However, in fact, sleep-for will return as soon as any input from any
> subprocess arrives.  E.g., try this in *scratch*:
>   (progn (setq proc (start-process-shell-command "ls" nil "ls"))
>          (sleep-for 20)
>        (message "hi"))
> You will see no delay at all before the message is displayed.
> Am I missing something?  If not, apart of fixing the docs, _is_ there
> any way to wait reliably when async subprocesses are running and
> producing output?
What about using a sentinel?

  (let ((proc (start-process-shell-command "ls" nil "ls")))
    (set-process-sentinel proc #'(lambda (process event)
                                   (when (string= event "finished\n"))
                                   (sleep-for 20)
                                   (message "hi after 20s sleeping"))))
  (message "Hi now sleeping 20s"))

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