maintenance: add --schedule option and config

Maintenance currently triggers when certain data-size thresholds are
met, such as number of pack-files or loose objects. Users may want to
run certain maintenance tasks based on frequency instead. For example,
a user may want to perform a 'prefetch' task every hour, or 'gc' task
every day. To help these users, update the 'git maintenance run' command
to include a '--schedule=<frequency>' option. The allowed frequencies
are 'hourly', 'daily', and 'weekly'. These values are also allowed in a
new config value 'maintenance.<task>.schedule'.

The 'git maintenance run --schedule=<frequency>' checks the '*.schedule'
config value for each enabled task to see if the configured frequency is
at least as frequent as the frequency from the '--schedule' argument. We
use the following order, for full clarity:

	'hourly' > 'daily' > 'weekly'

Use new 'enum schedule_priority' to track these values numerically.

The following cron table would run the scheduled tasks with the correct
frequencies:

  0 1-23 * * *    git -C <repo> maintenance run --schedule=hourly
  0 0    * * 1-6  git -C <repo> maintenance run --schedule=daily
  0 0    * * 0    git -C <repo> maintenance run --schedule=weekly

This cron schedule will run --schedule=hourly every hour except at
midnight. This avoids a concurrent run with the --schedule=daily that
runs at midnight every day except the first day of the week. This avoids
a concurrent run with the --schedule=weekly that runs at midnight on
the first day of the week. Since --schedule=daily also runs the
'hourly' tasks and --schedule=weekly runs the 'hourly' and 'daily'
tasks, we will still see all tasks run with the proper frequencies.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/Documentation/config/maintenance.txt b/Documentation/config/maintenance.txt
index 06db758172..70585564fa 100644
--- a/Documentation/config/maintenance.txt
+++ b/Documentation/config/maintenance.txt
@@ -10,6 +10,11 @@ maintenance.<task>.enabled::
 	`--task` option exists. By default, only `maintenance.gc.enabled`
 	is true.
 
+maintenance.<task>.schedule::
+	This config option controls whether or not the given `<task>` runs
+	during a `git maintenance run --schedule=<frequency>` command. The
+	value must be one of "hourly", "daily", or "weekly".
+
 maintenance.commit-graph.auto::
 	This integer config option controls how often the `commit-graph` task
 	should be run as part of `git maintenance run --auto`. If zero, then