SET key value [EX seconds] [PX milliseconds] [NX|XX]

-- KEYS[1]: 鎖key
-- ARGV[1]: 鎖value,隨機(jī)字符串
-- ARGV[2]: 過(guò)期時(shí)間
-- 判斷鎖key持有的value是否等于傳入的value
-- 如果相等說(shuō)明是再次獲取鎖并更新獲取時(shí)間，防止重入時(shí)過(guò)期
-- 這里說(shuō)明是“可重入鎖”
if redis.call("GET", KEYS[1]) == ARGV[1] then
    -- 設(shè)置
    redis.call("SET", KEYS[1], ARGV[1], "PX", ARGV[2])
    return "OK"

else
    -- 鎖key.value不等于傳入的value則說(shuō)明是第一次獲取鎖
    -- SET key value NX PX timeout : 當(dāng)key不存在時(shí)才設(shè)置key的值
    -- 設(shè)置成功會(huì)自動(dòng)返回“OK”，設(shè)置失敗返回“NULL Bulk Reply”
    -- 為什么這里要加“NX”呢，因?yàn)樾枰乐拱褎e人的鎖給覆蓋了
    return redis.call("SET", KEYS[1], ARGV[1], "NX", "PX", ARGV[2])
end

-- 釋放鎖
-- 不可以釋放別人的鎖
if redis.call("GET", KEYS[1]) == ARGV[1] then
    -- 執(zhí)行成功返回“1”
    return redis.call("DEL", KEYS[1])
else
    return 0
end

package redis

import (
    "math/rand"
    "strconv"
    "sync/atomic"
    "time"

    red "github.com/go-redis/redis"
    "github.com/tal-tech/go-zero/core/logx"
)

const (
    letters     = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
    lockCommand = `if redis.call("GET", KEYS[1]) == ARGV[1] then
    redis.call("SET", KEYS[1], ARGV[1], "PX", ARGV[2])
    return "OK"
else
    return redis.call("SET", KEYS[1], ARGV[1], "NX", "PX", ARGV[2])
end`
    delCommand = `if redis.call("GET", KEYS[1]) == ARGV[1] then
    return redis.call("DEL", KEYS[1])
else
    return 0
end`
    randomLen = 16
    // 默認(rèn)超時(shí)時(shí)間，防止死鎖
    tolerance       = 500 // milliseconds
    millisPerSecond = 1000
)

// A RedisLock is a redis lock.
type RedisLock struct {
    // redis客戶(hù)端
    store *Redis
    // 超時(shí)時(shí)間
    seconds uint32
    // 鎖key
    key string
    // 鎖value，防止鎖被別人獲取到
    id string
}

func init() {
    rand.Seed(time.Now().UnixNano())
}

// NewRedisLock returns a RedisLock.
func NewRedisLock(store *Redis, key string) *RedisLock {
    return &RedisLock{
        store: store,
        key:   key,
        // 獲取鎖時(shí)，鎖的值通過(guò)隨機(jī)字符串生成
        // 實(shí)際上go-zero提供更加高效的隨機(jī)字符串生成方式
        // 見(jiàn)core/stringx/random.go：Randn
        id:    randomStr(randomLen),
    }
}

// Acquire acquires the lock.
// 加鎖
func (rl *RedisLock) Acquire() (bool, error) {
    // 獲取過(guò)期時(shí)間
    seconds := atomic.LoadUint32(&rl.seconds)
    // 默認(rèn)鎖過(guò)期時(shí)間為500ms，防止死鎖
    resp, err := rl.store.Eval(lockCommand, []string{rl.key}, []string{
        rl.id, strconv.Itoa(int(seconds)*millisPerSecond + tolerance),
    })
    if err == red.Nil {
        return false, nil
    } else if err != nil {
        logx.Errorf("Error on acquiring lock for %s, %s", rl.key, err.Error())
        return false, err
    } else if resp == nil {
        return false, nil
    }

    reply, ok := resp.(string)
    if ok && reply == "OK" {
        return true, nil
    }

    logx.Errorf("Unknown reply when acquiring lock for %s: %v", rl.key, resp)
    return false, nil
}

// Release releases the lock.
// 釋放鎖
func (rl *RedisLock) Release() (bool, error) {
    resp, err := rl.store.Eval(delCommand, []string{rl.key}, []string{rl.id})
    if err != nil {
        return false, err
    }

    reply, ok := resp.(int64)
    if !ok {
        return false, nil
    }

    return reply == 1, nil
}

// SetExpire sets the expire.
// 需要注意的是需要在Acquire()之前調(diào)用
// 不然默認(rèn)為500ms自動(dòng)釋放
func (rl *RedisLock) SetExpire(seconds int) {
    atomic.StoreUint32(&rl.seconds, uint32(seconds))
}

func randomStr(n int) string {
    b := make([]byte, n)
    for i := range b {
        b[i] = letters[rand.Intn(len(letters))]
    }
    return string(b)
}