import "golang.org/x/sync/singleflight"
var g singleflight.Group

func GetUser(uid string) (string, error) {
    v, err, _ := g.Do("user:"+uid, func() (interface{}, error) {
        if v := redis.Get("user:" + uid); v != nil {
            return v, nil
        }
        row, err := db.Query(uid)     // 并发只执行一次
        if err == nil {
            redis.Set("user:"+uid, row, 300)
        }
        return row, err
    })
    return v.(string), err
}