Hoist control for exiting script a level up (#348)

* Hoist control for exiting script a level up

* Do not accidentally nil out errors

* Log when running schedule

* Remove duplicate log line

* Warn on cron schedule that will never run

cmd/backup/cron.go

@@ -0,0 +1,29 @@
+// Copyright 2024 - Offen Authors <hioffen@posteo.de>
+// SPDX-License-Identifier: MPL-2.0
+
+package main
+
+import (
+	"time"
+
+	"github.com/robfig/cron/v3"
+)
+
+// checkCronSchedule detects whether the given cron expression will actually
+// ever be executed or not.
+func checkCronSchedule(expression string) (ok bool) {
+	defer func() {
+		if err := recover(); err != nil {
+			ok = false
+		}
+	}()
+	sched, err := cron.ParseStandard(expression)
+	if err != nil {
+		ok = false
+		return
+	}
+	now := time.Now()
+	sched.Next(now) // panics when the cron would never run
+	ok = true
+	return
+}