Commits on Sep 7, 2021

1. cache.h: Introduce a generic "xdg_config_home_for(…)" function …

   Current implementation of `xdg_config_home(filename)` returns
   `$XDG_CONFIG_HOME/git/$filename`, with the `git` subdirectory inserted
   between the `XDG_CONFIG_HOME` environment variable and the parameter.
   
   This patch introduces a `xdg_config_home_for(subdir, filename)` function
   which is more generic. It only concatenates "$XDG_CONFIG_HOME", or
   "$HOME/.config" if the former isn’t defined, with the parameters,
   without adding `git` in between.
   
   `xdg_config_home(filename)` is now implemented by calling
   `xdg_config_home_for("git", filename)` but this new generic function can
   be used to compute the configuration directory of other programs.
   
   Signed-off-by: Lénaïc Huard <lenaic@lhuard.fr>
   Acked-by: Derrick Stolee <dstolee@microsoft.com>
   Signed-off-by: Junio C Hamano <gitster@pobox.com>

   

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2. maintenance: git maintenance run learned --scheduler=<scheduler> …

   Depending on the system, different schedulers can be used to schedule
   the hourly, daily and weekly executions of `git maintenance run`:
   * `launchctl` for MacOS,
   * `schtasks` for Windows and
   * `crontab` for everything else.
   
   `git maintenance run` now has an option to let the end-user explicitly
   choose which scheduler he wants to use:
   `--scheduler=auto|crontab|launchctl|schtasks`.
   
   When `git maintenance start --scheduler=XXX` is run, it not only
   registers `git maintenance run` tasks in the scheduler XXX, it also
   removes the `git maintenance run` tasks from all the other schedulers to
   ensure we cannot have two schedulers launching concurrent identical
   tasks.
   
   The default value is `auto` which chooses a suitable scheduler for the
   system.
   
   `git maintenance stop` doesn't have any `--scheduler` parameter because
   this command will try to remove the `git maintenance run` tasks from all
   the available schedulers.
   
   Signed-off-by: Lénaïc Huard <lenaic@lhuard.fr>
   Acked-by: Derrick Stolee <dstolee@microsoft.com>
   Signed-off-by: Junio C Hamano <gitster@pobox.com>

   

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3. maintenance: add support for 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 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>

   

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