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

> 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.
> 
> 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.
> * 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.
> 
> 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.
> 
> Those units are templates that are parametrized by the frequency.
> 
> Based on those templates, 3 timers are started:
> * git-maintenance@hourly.timer
> * git-maintenance@daily.timer
> * git-maintenance@weekly.timer
> 
> The command launched by those three timers are the same than with the
> other scheduling methods:
> 
> git for-each-repo --config=maintenance.repo maintenance run
> --schedule=%i
> 
> with the full path for git to ensure that the version of git launched
> for the scheduled maintenance is the same as the one used to run
> `maintenance start`.
> 
> The timer unit contains `Persistent=true` so that, if the computer is
> powered down when a maintenance task should run, the task will be run
> when the computer is back powered on.
> 
> Signed-off-by: Lénaïc Huard <lenaic@lhuard.fr>