builtin/maintenance: add a --detach flag

Same as the preceding commit, add a `--[no-]detach` flag to the
git-maintenance(1) command. This will be used in a subsequent commit to
fix backgrounding of that command when configured with a non-standard
set of tasks.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

builtin/gc.c

@@ -1426,6 +1426,10 @@ static int maintenance_run_tasks(struct maintenance_run_opts *opts,
 	}
 	free(lock_path);
 
+	/* Failure to daemonize is ok, we'll continue in foreground. */
+	if (opts->detach > 0)
+		daemonize();
+
 	for (i = 0; !found_selected && i < TASK__COUNT; i++)
 		found_selected = tasks[i].selected_order >= 0;
 
@@ -1552,6 +1556,8 @@ static int maintenance_run(int argc, const char **argv, const char *prefix)
 	struct option builtin_maintenance_run_options[] = {
 		OPT_BOOL(0, "auto", &opts.auto_flag,
 			 N_("run tasks based on the state of the repository")),
+		OPT_BOOL(0, "detach", &opts.detach,
+			 N_("perform maintenance in the background")),
 		OPT_CALLBACK(0, "schedule", &opts.schedule, N_("frequency"),
 			     N_("run tasks based on frequency"),
 			     maintenance_opt_schedule),

t/t7900-maintenance.sh

@@ -908,4 +908,43 @@ test_expect_success 'failed schedule prevents config change' '
 	done
 '
 
+test_expect_success '--no-detach causes maintenance to not run in background' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		# Prepare the repository such that git-maintenance(1) ends up
+		# outputting something.
+		test_commit something &&
+		git config set maintenance.gc.enabled false &&
+		git config set maintenance.loose-objects.enabled true &&
+		git config set maintenance.loose-objects.auto 1 &&
+		git config set maintenance.incremental-repack.enabled true &&
+
+		# We have no better way to check whether or not the task ran in
+		# the background than to verify whether it output anything. The
+		# next testcase checks the reverse, making this somewhat safer.
+		git maintenance run --no-detach >out 2>&1 &&
+		test_line_count = 1 out
+	)
+'
+
+test_expect_success '--detach causes maintenance to run in background' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		test_commit something &&
+		git config set maintenance.gc.enabled false &&
+		git config set maintenance.loose-objects.enabled true &&
+		git config set maintenance.loose-objects.auto 1 &&
+		git config set maintenance.incremental-repack.enabled true &&
+
+		git maintenance run --detach >out 2>&1 &&
+		test_must_be_empty out
+	)
+'
+
 test_done