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Golang WaitGroup 用法源码阅读笔记

backend 2025/5/9 3:00:35

使用

sync.WaitGroup可以用来阻塞等待一组并发任务完成

下面是如何使用sync.WaitGroup的使用

最重要的就是不能并发调用Add()和Wait()

var wg sync.WaitGroupfor ... {wg.Add(1) // 不能和wg.Wait()并发执行go func() {// 不能在启动的函数里面执行wg.Add(), 否则会panicdefer wg.Done()// dosomething} ()
}wg.Wait()

下面是官方示例

package mainimport ("sync"
)type httpPkg struct{}func (httpPkg) Get(url string) {}var http httpPkgfunc main() {var wg sync.WaitGroupvar urls = []string{"http://www.golang.org/","http://www.google.com/","http://www.example.com/",}for _, url := range urls {// Increment the WaitGroup counter.wg.Add(1)// Launch a goroutine to fetch the URL.go func(url string) {// Decrement the counter when the goroutine completes.defer wg.Done()// Fetch the URL.http.Get(url)}(url)}// Wait for all HTTP fetches to complete.wg.Wait()
}

源码解读

结构体

type WaitGroup struct {noCopy noCopystate atomic.Uint64 // high 32 bits are counter, low 32 bits are waiter count.sema  uint32
}

noCopy标识结构体不能被复制

state的高32位表示counter的值，低32位表示waiter的值

sema用来阻塞和唤醒goroutine

方法

func(wg *WaitGroup) Done()
func(wg *WaitGroup) Add(delta int)
func(wg *WaitGroup) Wait()

Done()

// Done 将计数器（counter）值减 1
func (wg *WaitGroup) Done() {wg.Add(-1)
}

调用了add方法

Add()

func (wg *WaitGroup) Add(delta int) {state := wg.state.Add(uint64(delta) << 32)v := int32(state >> 32) // counter数量w := uint32(state) // waiter数量// counter < 0 -->> panicif v < 0 {panic("sync: negative WaitGroup counter")}// 并发调用Add、Wait-->> panicif w != 0 && delta > 0 && v == int32(delta) {panic("sync: WaitGroup misuse: Add called concurrently with Wait")}// 操作成功if v > 0 || w == 0 {return}// 并发调用panicif wg.state.Load() != state {panic("sync: WaitGroup misuse: Add called concurrently with Wait")}// v == 0 && counter != 0// 释放等待的waiter，将state的状态置为0wg.state.Store(0)for ; w != 0; w-- {runtime_Semrelease(&wg.sema, false, 0)}
}

从源码的panic来看，不能并发调用Add()&Wait()

不能让counter < 0

Wait()

func (wg *WaitGroup) Wait() {// CAS操作失败后for {state := wg.state.Load()v := int32(state >> 32)w := uint32(state)if v == 0 {// counter == 0 直接返回，没有需要等待的goroutinereturn}// if wg.state.CompareAndSwap(state, state+1) {runtime_SemacquireWaitGroup(&wg.sema)// 检验，唤醒前就已经将state置为0if wg.state.Load() != 0 {panic("sync: WaitGroup is reused before previous Wait has returned")}return}}
}