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maintenance: add support for systemd timers on Linux

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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 their own tasks scheduled by cron whereas
  they will have access to the log of their own tasks scheduled by
  systemd timer.
  Although `cron` attempts to send email, that email may go unseen by
  the user because these days, local mailboxes are not heavily used
  anymore.

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 parameterized 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 as with the
other scheduling methods:

/path/to/git for-each-repo --exec-path=/path/to
--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>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Lénaïc Huard
2021-09-04 22:55:00 +02:00
committed by Junio C Hamano
parent eba1ba9d32
commit b681b191f9
3 changed files with 330 additions and 11 deletions
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58
Documentation/git-maintenance.txt
View File

@ -179,14 +179,16 @@ OPTIONS
`maintenance.<task>.enabled` configured as `true` are considered.
See the 'TASKS' section for the list of accepted `<task>` values.
--scheduler=auto|crontab|launchctl|schtasks::
--scheduler=auto|crontab|systemd-timer|launchctl|schtasks::
When combined with the `start` subcommand, specify the scheduler
for running the hourly, daily and weekly executions of
`git maintenance run`.
Possible values for `<scheduler>` are `auto`, `crontab` (POSIX),
`launchctl` (macOS), and `schtasks` (Windows).
When `auto` is specified, the appropriate platform-specific
scheduler is used. Default is `auto`.
Possible values for `<scheduler>` are `auto`, `crontab`
(POSIX), `systemd-timer` (Linux), `launchctl` (macOS), and
`schtasks` (Windows). When `auto` is specified, the
appropriate platform-specific scheduler is used; on Linux,
`systemd-timer` is used if available, otherwise
`crontab`. Default is `auto`.
TROUBLESHOOTING
@ -286,6 +288,52 @@ schedule to ensure you are executing the correct binaries in your
schedule.
BACKGROUND MAINTENANCE ON LINUX SYSTEMD SYSTEMS
-----------------------------------------------
While Linux supports `cron`, depending on the distribution, `cron` may
be an optional package not necessarily installed. On modern Linux
distributions, systemd timers are superseding it.
If user systemd timers are available, they will be used as a replacement
of `cron`.
In this case, `git maintenance start` will create user systemd timer units
and start the timers. The current list of user-scheduled tasks can be found
by running `systemctl --user list-timers`. The timers written by `git
maintenance start` are similar to this:
-----------------------------------------------------------------------
$ systemctl --user list-timers
NEXT LEFT LAST PASSED UNIT ACTIVATES
Thu 2021-04-29 19:00:00 CEST 42min left Thu 2021-04-29 18:00:11 CEST 17min ago git-maintenance@hourly.timer git-maintenance@hourly.service
Fri 2021-04-30 00:00:00 CEST 5h 42min left Thu 2021-04-29 00:00:11 CEST 18h ago git-maintenance@daily.timer git-maintenance@daily.service
Mon 2021-05-03 00:00:00 CEST 3 days left Mon 2021-04-26 00:00:11 CEST 3 days ago git-maintenance@weekly.timer git-maintenance@weekly.service
-----------------------------------------------------------------------
One timer is registered for each `--schedule=<frequency>` option.
The definition of the systemd units can be inspected in the following files:
-----------------------------------------------------------------------
~/.config/systemd/user/git-maintenance@.timer
~/.config/systemd/user/git-maintenance@.service
~/.config/systemd/user/timers.target.wants/git-maintenance@hourly.timer
~/.config/systemd/user/timers.target.wants/git-maintenance@daily.timer
~/.config/systemd/user/timers.target.wants/git-maintenance@weekly.timer
-----------------------------------------------------------------------
`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, i.e. a `.conf` suffixed file in the
`~/.config/systemd/user/git-maintenance@.service.d` directory.
`git maintenance stop` will stop the user systemd timers and delete
the above mentioned files.
For more details, see `systemd.timer(5)`.
BACKGROUND MAINTENANCE ON MACOS SYSTEMS
---------------------------------------