builtin/maintenance: don't silently ignore invalid strategy

When parsing maintenance strategies we completely ignore the
user-configured value in case it is unknown to us. This makes it
basically undiscoverable to the user that scheduled maintenance is
devolving into a no-op.

Change this to instead die when seeing an unknown maintenance strategy.
While at it, pull out the parsing logic into a separate function so that
we can reuse it in a subsequent commit.

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

Author: pks-t

builtin/gc.c

@@ -1855,6 +1855,13 @@ static const struct maintenance_strategy incremental_strategy = {
 	},
 };
 
+static struct maintenance_strategy parse_maintenance_strategy(const char *name)
+{
+	if (!strcasecmp(name, "incremental"))
+		return incremental_strategy;
+	die(_("unknown maintenance strategy: '%s'"), name);
+}
+
 static void initialize_task_config(struct maintenance_run_opts *opts,
 				   const struct string_list *selected_tasks)
 {
@@ -1890,12 +1897,10 @@ static void initialize_task_config(struct maintenance_run_opts *opts,
 	 * override specific aspects of our strategy.
 	 */
 	if (opts->schedule) {
-		strategy = none_strategy;
-
-		if (!repo_config_get_string_tmp(the_repository, "maintenance.strategy", &config_str)) {
-			if (!strcasecmp(config_str, "incremental"))
-				strategy = incremental_strategy;
-		}
+		if (!repo_config_get_string_tmp(the_repository, "maintenance.strategy", &config_str))
+			strategy = parse_maintenance_strategy(config_str);
+		else
+			strategy = none_strategy;
 	} else {
 		strategy = default_strategy;
 	}

t/t7900-maintenance.sh

@@ -1263,6 +1263,11 @@ test_expect_success 'fails when running outside of a repository' '
 	nongit test_must_fail git maintenance unregister
 '
 
+test_expect_success 'fails when configured to use an invalid strategy' '
+	test_must_fail git -c maintenance.strategy=invalid maintenance run --schedule=hourly 2>err &&
+	test_grep "unknown maintenance strategy: .invalid." err
+'
+
 test_expect_success 'register and unregister bare repo' '
 	test_when_finished "git config --global --unset-all maintenance.repo || :" &&
 	test_might_fail git config --global --unset-all maintenance.repo &&