maintenance: create maintenance.strategy config

To provide an on-ramp for users to use background maintenance without
several 'git config' commands, create a 'maintenance.strategy' config
option. Currently, the only important value is 'incremental' which
assigns the following schedule:

* gc: never
* prefetch: hourly
* commit-graph: hourly
* loose-objects: daily
* incremental-repack: daily

These tasks are chosen to minimize disruptions to foreground Git
commands and use few compute resources.

The 'maintenance.strategy' is intended as a baseline that can be
customzied further by manually assigning 'maintenance.<task>.enabled'
and 'maintenance.<task>.schedule' config options, which will override
any recommendation from 'maintenance.strategy'. This operates similarly
to config options like 'feature.experimental' which operate as "meta"
config options that change default config values.

This presents a way forward for updating the 'incremental' strategy in
the future or adding new strategies. For example, a potential strategy
could be to include a 'full' strategy that runs the 'gc' task weekly
and no other tasks by default.

Helped-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/maintenance.txt

@@ -3,6 +3,21 @@ maintenance.auto::
 	`git maintenance run --auto` after doing their normal work. Defaults
 	to true.
 
+maintenance.strategy::
+	This string config option provides a way to specify one of a few
+	recommended schedules for background maintenance. This only affects
+	which tasks are run during `git maintenance run --schedule=X`
+	commands, provided no `--task=<task>` arguments are provided.
+	Further, if a `maintenance.<task>.schedule` config value is set,
+	then that value is used instead of the one provided by
+	`maintenance.strategy`. The possible strategy strings are:
++
+* `none`: This default setting implies no task are run at any schedule.
+* `incremental`: This setting optimizes for performing small maintenance
+  activities that do not delete any data. This does not schedule the `gc`
+  task, but runs the `prefetch` and `commit-graph` tasks hourly and the
+  `loose-objects` and `incremental-repack` tasks daily.
+
 maintenance.<task>.enabled::
 	This boolean config option controls whether the maintenance task
 	with name `<task>` is run when no `--task` option is specified to

builtin/gc.c

@@ -1308,12 +1308,35 @@ static int maintenance_run_tasks(struct maintenance_run_opts *opts)
 	return result;
 }
 
-static void initialize_task_config(void)
+static void initialize_maintenance_strategy(void)
+{
+	char *config_str;
+
+	if (git_config_get_string("maintenance.strategy", &config_str))
+		return;
+
+	if (!strcasecmp(config_str, "incremental")) {
+		tasks[TASK_GC].schedule = SCHEDULE_NONE;
+		tasks[TASK_COMMIT_GRAPH].enabled = 1;
+		tasks[TASK_COMMIT_GRAPH].schedule = SCHEDULE_HOURLY;
+		tasks[TASK_PREFETCH].enabled = 1;
+		tasks[TASK_PREFETCH].schedule = SCHEDULE_HOURLY;
+		tasks[TASK_INCREMENTAL_REPACK].enabled = 1;
+		tasks[TASK_INCREMENTAL_REPACK].schedule = SCHEDULE_DAILY;
+		tasks[TASK_LOOSE_OBJECTS].enabled = 1;
+		tasks[TASK_LOOSE_OBJECTS].schedule = SCHEDULE_DAILY;
+	}
+}
+
+static void initialize_task_config(int schedule)
 {
 	int i;
 	struct strbuf config_name = STRBUF_INIT;
 	gc_config();
 
+	if (schedule)
+		initialize_maintenance_strategy();
+
 	for (i = 0; i < TASK__COUNT; i++) {
 		int config_value;
 		char *config_str;
@@ -1389,7 +1412,6 @@ static int maintenance_run(int argc, const char **argv, const char *prefix)
 	memset(&opts, 0, sizeof(opts));
 
 	opts.quiet = !isatty(2);
-	initialize_task_config();
 
 	for (i = 0; i < TASK__COUNT; i++)
 		tasks[i].selected_order = -1;
@@ -1402,6 +1424,8 @@ static int maintenance_run(int argc, const char **argv, const char *prefix)
 	if (opts.auto_flag && opts.schedule)
 		die(_("use at most one of --auto and --schedule=<frequency>"));
 
+	initialize_task_config(opts.schedule);
+
 	if (argc != 0)
 		usage_with_options(builtin_maintenance_run_usage,
 				   builtin_maintenance_run_options);

t/t7900-maintenance.sh

@@ -300,6 +300,55 @@ test_expect_success '--schedule inheritance weekly -> daily -> hourly' '
 	test_subcommand git multi-pack-index write --no-progress <weekly.txt
 '
 
+test_expect_success 'maintenance.strategy inheritance' '
+	for task in commit-graph loose-objects incremental-repack
+	do
+		git config --unset maintenance.$task.schedule || return 1
+	done &&
+
+	test_when_finished git config --unset maintenance.strategy &&
+	git config maintenance.strategy incremental &&
+
+	GIT_TRACE2_EVENT="$(pwd)/incremental-hourly.txt" \
+		git maintenance run --schedule=hourly --quiet &&
+	GIT_TRACE2_EVENT="$(pwd)/incremental-daily.txt" \
+		git maintenance run --schedule=daily --quiet &&
+
+	test_subcommand git commit-graph write --split --reachable \
+		--no-progress <incremental-hourly.txt &&
+	test_subcommand ! git prune-packed --quiet <incremental-hourly.txt &&
+	test_subcommand ! git multi-pack-index write --no-progress \
+		<incremental-hourly.txt &&
+
+	test_subcommand git commit-graph write --split --reachable \
+		--no-progress <incremental-daily.txt &&
+	test_subcommand git prune-packed --quiet <incremental-daily.txt &&
+	test_subcommand git multi-pack-index write --no-progress \
+		<incremental-daily.txt &&
+
+	# Modify defaults
+	git config maintenance.commit-graph.schedule daily &&
+	git config maintenance.loose-objects.schedule hourly &&
+	git config maintenance.incremental-repack.enabled false &&
+
+	GIT_TRACE2_EVENT="$(pwd)/modified-hourly.txt" \
+		git maintenance run --schedule=hourly --quiet &&
+	GIT_TRACE2_EVENT="$(pwd)/modified-daily.txt" \
+		git maintenance run --schedule=daily --quiet &&
+
+	test_subcommand ! git commit-graph write --split --reachable \
+		--no-progress <modified-hourly.txt &&
+	test_subcommand git prune-packed --quiet <modified-hourly.txt &&
+	test_subcommand ! git multi-pack-index write --no-progress \
+		<modified-hourly.txt &&
+
+	test_subcommand git commit-graph write --split --reachable \
+		--no-progress <modified-daily.txt &&
+	test_subcommand git prune-packed --quiet <modified-daily.txt &&
+	test_subcommand ! git multi-pack-index write --no-progress \
+		<modified-daily.txt
+'
+
 test_expect_success 'register and unregister' '
 	test_when_finished git config --global --unset-all maintenance.repo &&
 	git config --global --add maintenance.repo /existing1 &&