go get -u golang.org/x/sync/singleflight

type Group struct {
	mu sync.Mutex       //保護(hù)map對(duì)象m的并發(fā)安全
	m  map[string]*call //key-請(qǐng)求的唯一標(biāo)識(shí)，val-請(qǐng)求唯一標(biāo)識(shí)對(duì)應(yīng)要執(zhí)行的函數(shù)
}

type call struct {
	wg sync.WaitGroup //用來阻塞相同標(biāo)識(shí)對(duì)應(yīng)的請(qǐng)求中的第一個(gè)請(qǐng)求之外的請(qǐng)求
	val interface{} //第一個(gè)請(qǐng)求的執(zhí)行結(jié)果
	err error //第一個(gè)請(qǐng)求返回的錯(cuò)誤
	dups  int //第一個(gè)請(qǐng)求之外的其它請(qǐng)求數(shù)
	chans []chan<- Result //請(qǐng)求結(jié)果寫入通道
}

//
// Do
//  @Description: 一個(gè)唯一標(biāo)識(shí)對(duì)應(yīng)的請(qǐng)求在執(zhí)行過程中，相同唯一標(biāo)識(shí)對(duì)應(yīng)的請(qǐng)求會(huì)被阻塞，等待第一個(gè)請(qǐng)求執(zhí)行完并共享結(jié)果
//  @receiver g 
//  @param key 請(qǐng)求唯一標(biāo)識(shí)
//  @param fn 請(qǐng)求要執(zhí)行的函數(shù)
//  @return v 請(qǐng)求要執(zhí)行函數(shù)返回的結(jié)果
//  @return err 請(qǐng)求要執(zhí)行的函數(shù)返回的錯(cuò)誤
//  @return shared 是否有多個(gè)請(qǐng)求共享結(jié)果
//
func (g *Group) Do(key string, fn func() (interface{}, error)) (v interface{}, err error, shared bool) {
	g.mu.Lock() //保護(hù)map對(duì)象m的并發(fā)安全
	if g.m == nil {
		g.m = make(map[string]*call) //初始化m對(duì)象
	}
	if c, ok := g.m[key]; ok { //map中key存在說明這個(gè)key對(duì)應(yīng)的請(qǐng)求正在執(zhí)行中，這次請(qǐng)求不是第一個(gè)請(qǐng)求
		c.dups++ //等待共享結(jié)果數(shù)+1
		g.mu.Unlock()
		c.wg.Wait() //阻塞等待第一個(gè)請(qǐng)求執(zhí)行完

		if e, ok := c.err.(*panicError); ok {
			panic(e)
		} else if c.err == errGoexit {
			runtime.Goexit()
		}
		return c.val, c.err, true //返回第一個(gè)請(qǐng)求的執(zhí)行結(jié)果
	}
    
    //第一個(gè)請(qǐng)求的處理邏輯
	c := new(call)
	c.wg.Add(1) //計(jì)數(shù)+1
	g.m[key] = c //唯一標(biāo)識(shí)關(guān)聯(lián)對(duì)應(yīng)的函數(shù)對(duì)象
	g.mu.Unlock()

	g.doCall(c, key, fn) //執(zhí)行請(qǐng)求對(duì)應(yīng)的函數(shù)
	return c.val, c.err, c.dups > 0 //返回執(zhí)行結(jié)果
}

func (g *Group) doCall(c *call, key string, fn func() (interface{}, error)) {
	normalReturn := false
	recovered := false
	
	defer func() { //fn函數(shù)執(zhí)行完之后執(zhí)行
		if !normalReturn && !recovered {
			c.err = errGoexit
		}

		g.mu.Lock()
		defer g.mu.Unlock()
		c.wg.Done() //釋放計(jì)數(shù)
		if g.m[key] == c {
			delete(g.m, key) //刪除此次請(qǐng)求的唯一標(biāo)識(shí)相關(guān)信息
		}

		if e, ok := c.err.(*panicError); ok {
			if len(c.chans) > 0 {
				go panic(e)
				select {} 
			} else {
				panic(e)
			}
		} else if c.err == errGoexit {
			// Already in the process of goexit, no need to call again
		} else {
			for _, ch := range c.chans { //執(zhí)行結(jié)果寫入通道
				ch <- Result{c.val, c.err, c.dups > 0}
			}
		}
	}()

	func() {
		defer func() {
			if !normalReturn {
				if r := recover(); r != nil {
					c.err = newPanicError(r)
				}
			}
		}()

		c.val, c.err = fn() //執(zhí)行fn函數(shù)
		normalReturn = true
	}()

	if !normalReturn {
		recovered = true
	}
}

func (g *Group) Forget(key string) {
	g.mu.Lock()
	delete(g.m, key) //刪除map中的key，相同key對(duì)應(yīng)請(qǐng)求進(jìn)來會(huì)重新執(zhí)行，不等待第一個(gè)key對(duì)應(yīng)請(qǐng)求的執(zhí)行結(jié)果
	g.mu.Unlock()
}

func main() {
	var singleFlight singleflight.Group //初始化一個(gè)單次執(zhí)行對(duì)象
	var count uint64                    //用于測(cè)試是否被修改

	//為了并發(fā)執(zhí)行，這里測(cè)試唯一標(biāo)識(shí)都為xxx
	go func() {
		val1, _, shared1 := singleFlight.Do("xxx", func() (interface{}, error) {
			logger.Info("first count +1")

			atomic.AddUint64(&count, 1) //第一次執(zhí)行，將count+1

			time.Sleep(5 * time.Second) //增加第一次執(zhí)行時(shí)間

			return count, nil
		})

		//打印第一次執(zhí)行結(jié)果
		logger.Info("first count info", zap.Any("val1", val1), zap.Bool("shared1", shared1), zap.Uint64("count", count))
	}()

	time.Sleep(2 * time.Second) //為了防止下面的Do函數(shù)先執(zhí)行

	val2, _, shared2 := singleFlight.Do("xxx", func() (interface{}, error) {
		logger.Info("second count +1")

		atomic.AddUint64(&count, 1) //第2次執(zhí)行count+1

		return count, nil
	})

	//打印第二次執(zhí)行結(jié)果
	logger.Info("second count info", zap.Any("val2", val2), zap.Bool("shared2", shared2), zap.Uint64("count", count))
}

$ go run ./singlefight_demo/main.go
[2025-01-09 17:08:11.169] | INFO  | Goroutine:6  | [singlefight_demo/main.go:19] | first count +1
[2025-01-09 17:08:16.261] | INFO  | Goroutine:6  | [singlefight_demo/main.go:28] | first count info | {"val1": 1, "shared1": true, "count": 1}
[2025-01-09 17:08:16.261] | INFO  | Goroutine:1  | [singlefight_demo/main.go:41] | second count info | {"val2": 1, "shared2": true, "count": 1}