Re: [PATCH] maintenance: use systemd timers on Linux

[Phillip Wood <phillip.wood123@gmail.com>]

Hi Đoàn

On 05/05/2021 13:19, Đoàn Trần Công Danh wrote:
> On 2021-05-02 15:10:05+0100, Phillip Wood <phillip.wood123@gmail.com> wrote:
>> On 02/05/2021 07:45, Eric Sunshine wrote:
>>> On Sat, May 1, 2021 at 10:59 AM Lénaïc Huard <lenaic@lhuard.fr> wrote:
>>>> The existing mechanism for scheduling background maintenance is done
>>>> through cron. On Linux systems managed by systemd, systemd provides an
>>>> alternative to schedule recurring tasks: systemd timers.
>>>
>>> Thanks for working on this. While `cron` has been the go-to standard
>>> for decades, `systemd` is certainly widespread enough that it makes
>>> sense to support it, as well.
>>
>> Yes, thank you for working on this, it will be very useful to users like me
>> who use a linux distribution that does not install a cron daemon by default
>> but relies on systemd instead.
>>
>>>> The main motivations to implement systemd timers in addition to cron
>>>> are:
>>>> * cron is optional and Linux systems running systemd might not have it
>>>>     installed.
>>>> * The execution of `crontab -l` can tell us if cron is installed but not
>>>>     if the daemon is actually running.
>>
>> Can we use systemctl to see if it is running (and enabled so we know it will
>> be restarted after a reboot)?
> 
> Not sure if I understand this suggestion.
> However, non-systemd systems doesn't have systemctl command to begin
> with.

I was wondering if on systems with both cron and systemd installed we 
could use systemctl to determine if crond is actually running as Lénaïc 
pointed out that being able to run `crontab -l` does not tell us if 
crond is running.

Best Wishes

Phillip

>>>> * With systemd, each service is run in its own cgroup and its logs are
>>>>     tagged by the service inside journald. With cron, all scheduled tasks
>>>>     are running in the cron daemon cgroup and all the logs of the
>>>>     user-scheduled tasks are pretended to belong to the system cron
>>>>     service.
>>>>     Concretely, a user that doesn’t have access to the system logs won’t
>>>>     have access to the log of its own tasks scheduled by cron whereas he
>>>>     will have access to the log of its own tasks scheduled by systemd
>>>>     timer.
>>>
>>> The last point is somewhat compelling. A potential counterargument is
>>> that `cron` does send email to the user by default if any output is
>>> generated by the cron job. However, it seems quite likely these days
>>> that many systems either won't have local mail service enabled or the
>>> user won't bother checking the local mailbox. It's a minor point, but
>>> if you re-roll it might make sense for the commit message to expand
>>> the last point by saying that although `cron` attempts to send email,
>>> that email may go unseen by the user.
>>>
>>>> In order to schedule git maintenance, we need two unit template files:
>>>> * ~/.config/systemd/user/git-maintenance@.service
>>>>     to define the command to be started by systemd and
>>>> * ~/.config/systemd/user/git-maintenance@.timer
>>>>     to define the schedule at which the command should be run.
> 
> I think it would be better to change ~/.config here to
> $XDG_CONFIG_HOME, as others also points out in another comments.
> 
> [..snip..]
> 
>>>> +`git maintenance start` will overwrite these files and start the timer
>>>> +again with `systemctl --user`, so any customization should be done by
>>>> +creating a drop-in file
>>>> +`~/.config/systemd/user/git-maintenance@.service.d/*.conf`.
> 
> Ditto.
> 
>>> Will `systemd` users generally understand what filename to create in
>>> the "...@.service.d/" directory, and will they know what to populate
>>> the file with? (Genuine question; I've never dealt with that.)
>>
>> I think it would be helpful to explicitly mention the file names (I don't
>> think I could tell you what they are without reading the relevant systemd
>> man page)
> 
> [..snip..]
> 
>>>> +static char *systemd_timer_timer_filename()
>>>> +{
>>>> +       const char *filename = "~/.config/systemd/user/git-maintenance@.timer";
>>>> +       char *expanded = expand_user_path(filename, 0);
>>>> +       if (!expanded)
>>>> +               die(_("failed to expand path '%s'"), filename);
>>>> +
>>>> +       return expanded;
>>>> +}
>>>
>>> I was curious whether this would fail if `.config/systemd/user/`
>>> didn't already exist, but looking at the implementation of
>>> expand_user_path() , I see that it doesn't require the path to already
>>> exist if you pass 0 for the second argument as you do here. Okay.
>>
>> Do we need to worry about $XDG_CONFIG_HOME rather than hard coding
>> "~/.config/". There is a function xdg_config_home() that takes care of this.
>