bug#39638: 26.3; recentf-auto-cleanup deceptive

[Eli Zaretskii <eliz@gnu.org>]

> From: Allen Li <darkfeline@felesatra.moe>
> Date: Sun, 16 Feb 2020 23:33:36 -0800
> 
> This covers two bugs around recentf-auto-cleanup being
> deceptive/unintuitive.
> 
> Both bugs are present at 26.3 and on Emacs master as of
> 556cc727e5076d590f8286406e4f46cff3cee41e
> at Sun, 16 Feb 2020 11:37:07 -0800
> 
> 1. When setting recentf-auto-cleanup to a string, the timer does not
> repeat.  It is only set once.  This is in contrast to midnight-mode,
> which repeats its timer every day.  The documentation for
> recentf-auto-cleanup does not make this clear, and I'm not even sure if
> this was the intended behavior.
> 
> (defun recentf-auto-cleanup ()
>   "Automatic cleanup of the recent list."
>   (when (timerp recentf-auto-cleanup-timer)
>     (cancel-timer recentf-auto-cleanup-timer))
>   (when recentf-mode
>     (setq recentf-auto-cleanup-timer
>           (cond
>            ;; snipped
>            ((stringp recentf-auto-cleanup)
>             (run-at-time
>              recentf-auto-cleanup nil 'recentf-cleanup))))))
> 
> 2. Due to the behavior of run-at-time, if the time string set was in the
> past for today, recentf-cleanup runs immediately when recentf-mode is
> turned on (e.g., at Emacs startup).  This makes it pointless to set it
> to something like "3:00am" if I want recentf-cleanup to run at a time
> when I'm likely not using Emacs and I have also set
> recentf-max-saved-items to something large like 2000.  The docstring
> does not make this obvious.  This is also how one would usually
> customize midnight-mode.
> 
> 2a. midnight-mode suffers from the same problem of using run-at-time,
> but the default behavior of midnight-mode does not make it expensive.
> But this means that adding recentf-cleanup to midnight-hook when using a large
> recentf-max-saved-items will still be expensive at startup.
> 
> I have attached a number of patches:
> 
> 1. Simply fix some awkward wording that is not directly related to this bug.
> 2. Document the current behavior.
> 3. Make recentf-auto-cleanup repeat for time strings.
> 
> The third patch can be skipped if deemed too aggressive, but I think
> that's the more reasonable behavior to expect.
> 
> I have not fixed the problem of recentf-cleanup running immediately if
> the time is in the past for today, since I'm not sure the best way to do
> it.

Juanma, any comments?  Did you indeed mean for the cleanup feature to
work as it does, or are those omissions?

Thanks.