package main

import (
	"github.com/go-redis/redis"
	"github.com/jmoiron/sqlx"
	_ "github.com/lib/pq"
	"github.com/robfig/cron"
	"github.com/sirupsen/logrus"
	"time"
)

func init() {
	logrus.SetFormatter(&logrus.TextFormatter{})
	logrus.SetLevel(logrus.DebugLevel)
}


func main() {
	db := sqlx.MustConnect("postgres", "host=localhost port=5432 user=ben password=password dbname=globalcron sslmode=disable")
	_, err := db.Exec(` delete from task_log; `)
	if err != nil {
		panic(err)
	}

	client := redis.NewClient(&redis.Options{
		Addr:     "localhost:6379",
		Password: "", // no password set
		DB:       0,  // use default DB
	})

	c := cron.New()
	err = c.AddJob("*/5 * * * * *", newAtomicCronTask(
		"print",
		client,
		func() {
			logrus.Infof("Running job: the time is %s", time.Now().Format(time.RFC3339))
		},
		2 * time.Second))

	err = c.AddJob("*/5 * * * * *", newAtomicCronTask(
		"daily",
		client,
		onceDailyCron(db, "daily", func() {
			logrus.Println("Running once daily cron")
		}),
		2 * time.Second))

	if err != nil {
		panic(err)
	}

	c.Run()
}

// AtomicCronTask is a cron task that uses a schedule lock to
// ensure that it only runs 1 time across the entire cluster.
type AtomicCronTask struct {
	redis   *redis.Client
	name    string
	lockDur time.Duration
	fn      func()
}

func newAtomicCronTask(name string, r *redis.Client, fun func(), lockDur time.Duration) *AtomicCronTask {
	return &AtomicCronTask{
		redis:   r,
		name:    "atomic_job:" + name,
		fn:      fun,
		lockDur: lockDur,
	}
}

func (s *AtomicCronTask) Run() {
	err := s.redis.Watch(func(tx *redis.Tx) error {
		res := tx.Get(s.name)
		_, err := res.Result()
		if err == redis.Nil {
			pipe := tx.TxPipeline()
			pipe.Set(s.name, time.Now().Format(time.RFC3339Nano), s.lockDur)
			_, err := pipe.Exec()
			if err != nil {
				return err
			}
			logrus.Debugf("set key running task %s", s.name)

			s.fn()

			return nil
		} else if err != nil {
			logrus.Errorf("got err while getting key from redis: %v", err)
			return nil
		} else {
			logrus.Debugf("key exists, skipping task %s", s.name)
		}

		return nil
	}, s.name)
	if err != nil {
		if err == redis.TxFailedErr {
			logrus.Debugln("race condition avoided via transaction")
		} else {
			logrus.Errorf("got error during redis transaction: %v", err)
		}
	}
}

var _ cron.Job = &AtomicCronTask{}

func onceDailyCron(db *sqlx.DB, name string, fn func()) func() {
	return func() {
		var n int
		err := db.Get(&n, `
			select count(*)
			from task_log
			where task_name = $1
				and run_time > $2
		`, name, time.Now().Truncate(24*time.Hour))
		if err != nil {
			logrus.Errorf("Got err while getting task log entry: %v", err)
			return
		}

		if n < 1 {
			_, execErr := db.Exec(`
				insert into task_log (task_name, run_time)
				values ($1, $2);
			`, name, time.Now())
			if execErr != nil {
				logrus.Errorln("Failed to insert daily cron entry: %v", execErr)
				return
			}

			fn()
			return
		}

		logrus.Debugln("Skipping daily task due to existing entry")
	}
}