## Singleflight并发获取数据时，如何避免访问穿透问题？

package main

import (
    "errors"
    "log"
    "sync"
    "golang.org/x/sync/singleflight"
)

var errornotexist = errors.new("not exist")
var g singleflight.group

func main() {
    var wg sync.waitgroup
    cpu_amount := 4
    wg.add(cpu_amount)

    for i := 0; i < cpu_amount; i++ {
        go func() {
            defer wg.done()
            data, err := getdata("key")
            if err != nil {
                log.print(err)
                return
            }
            log.println(data)
        }()
    }
    wg.wait()
}

func getdata(key string) (string, error) {
    data, err := getdatafromcache(key)
    if err == errornotexist {
        v, err, _ := g.do(key, func() (interface{}, error) {
            return getdatafromdb(key)
        })
        if err != nil {
            log.println(err)
            return "", err
        }
        data = v.(string)
    } else if err != nil {
        return "", err
    }
    return data, nil
}

func getdatafromcache(key string) (string, error) {
    return "", errornotexist
}

func getdatafromdb(key string) (string, error) {
    log.printf("get %s from database", key)
    return "data", nil
}

func getDataFromDB(key string) (string, error) {
    log.Printf("get %s from database", key)
    time.Sleep(time.Millisecond * 500) // 引入延迟
    return "data", nil
}