main.go

package main

import (
	"context"
	"fmt"
	"log"
	"time"

	"go.mongodb.org/mongo-driver/mongo"
	"go.mongodb.org/mongo-driver/mongo/options"
	"gopkg.in/mgo.v2/bson"
)

type LockObject struct {
	InTransaction bool   `json:"inTransaction" bson:"inTransaction"`
	Name          string `json:"name" bson:"name"`
}

var (
	url         = "mongodb+srv://mongodb:CONNECTION_STRING..."
	dbiName     = "locker"
	colname     = "locker"
	MongoClient *mongo.Client
	coll        *mongo.Collection
)

func main() {

	// you can specify lock name for more then one lock in collection
	lockName := "globalLock"

	fmt.Println("start")

	// blocked until getting lock
	if Lock(lockName) {
		// after getting lock
		fmt.Println("heja! locked by me!")
	}

	fmt.Println("and continue...")

	// uncomment for unlock
	// if you run with commented Unlock() will be blocked next app run
	// Unlock(lockName)

	_ = MongoClient.Disconnect(context.TODO())
}

func Lock(lockName string) (locked bool) {
	var (
		filter = bson.M{
			"inTransaction": bson.M{
				"$exists": true,
			},
			"name": lockName,
		}
		locker = bson.M{
			"$set": LockObject{
				InTransaction: true,
				Name:          lockName,
			},
		}
		upsert = true
		opt    = options.UpdateOptions{
			Upsert: &upsert,
		}
	)

	if MongoClient == nil {
		var err error
		if MongoClient, err = mongo.Connect(context.TODO(),options.Client().ApplyURI(url)); err != nil {
			log.Fatal(err)
		}
		coll = MongoClient.Database(dbiName).Collection(colname)
		fmt.Println("mongo connected")
	}

	for !locked {
		res, err := coll.UpdateOne(context.TODO(), filter, locker, &opt)
		if err != nil {
			return false
		}
		locked = res.UpsertedCount == 1
		if !locked {
			fmt.Println("locked by someone else")
			time.Sleep(1 * time.Second)
		}
	}

	return locked
}

func Unlock(lockName string) error {
	var (
		filter = bson.M{
			"inTransaction": bson.M{
				"$exists": true,
			},
			"name": lockName,
		}
	)
	res, err := coll.DeleteOne(context.TODO(), filter)
	if err != nil {
		return err
	}
	if res.DeletedCount != 1 {
		return fmt.Errorf("unlcck error")
	}

	return nil
}