lock.go

// mogul - distributed lock via mongoDB
//
// Copyright 2016 - Rene Kroon <kroon.r.w@gmail.com>
//

package mogul

import (
	"time"

	"github.com/globalsign/mgo"
	"github.com/globalsign/mgo/bson"
)

// Mutex is the object used for locking
type Mutex struct {
	doc        lock
	collection *mgo.Collection
}

type lock struct {
	Name         string    `bson:"_id"`
	User         string    `bson:"user"`
	ExpiresAtUtc time.Time `bson:"expires"`
}

// MutexCreator is a specialization of the Manager struct for locking
type MutexCreator interface {
	NewMutex(name string, user string) *Mutex
}

// Trylock will claim a lock if it is available. It also returns true when you already hold the lock.
// This extends the duration if you already hold the lock.
func (m *Mutex) TryLock(atMost time.Duration) (bool, error) {

	now := time.Now().UTC()
	until := now.Add(atMost)

	m.doc.ExpiresAtUtc = until

	var result lock

	selector := bson.M{"$or": []bson.M{m.identityClause(), bson.M{"$and": []bson.M{bson.M{"_id": m.doc.Name}, bson.M{"expires": bson.M{"$lt": now}}}}}}

	_, err := m.collection.Find(selector).Apply(mgo.Change{
		Update:    &m.doc,
		ReturnNew: true,
	}, &result)
	if err == mgo.ErrNotFound {
		err = m.collection.Insert(&m.doc)
	}

	return err == nil, err
}

// Unlock frees the lock, removing the corresponding record in the database
func (m *Mutex) Unlock() error {
	return m.collection.Remove(m.identityClause())
}

// IsExpired will return true when your lock time has expired
func (m *Mutex) IsExpired() bool {
	return m.doc.ExpiresAtUtc.Before(time.Now().UTC())
}

func (m *Mutex) identityClause() bson.M {
	return bson.M{"$and": []bson.M{bson.M{"_id": m.doc.Name}, bson.M{"user": m.doc.User}}}
}