go sync.WaitGroup

1、数据结构

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

        计数器：原子变量，高32位用于为协程计数，低32位为等待计数（被Wait阻塞等待）。

        信号量：用于阻塞协程。

2、Add(x)

// 简化版// Add adds delta, which may be negative, to the [WaitGroup] counter.
// If the counter becomes zero, all goroutines blocked on [WaitGroup.Wait] are released.
// If the counter goes negative, Add panics.func (wg *WaitGroup) Add(delta int) {state := wg.state.Add(uint64(delta) << 32)v := int32(state >> 32)w := uint32(state)if v > 0 || w == 0 {return}// Reset waiters count to 0.wg.state.Store(0)for ; w != 0; w-- {runtime_Semrelease(&wg.sema, false, 0)}
}

        通过原子操作对协程计数器加减，若协程计数变为0，则将等待计数设置为0，并释放所有被信号量阻塞的协程。

3、Done()

// 简化版// Done decrements the [WaitGroup] counter by one.func (wg *WaitGroup) Done() {wg.Add(-1)
}

        计数器-1

4、Wait()

// 简化版// Wait blocks until the [WaitGroup] counter is zero.func (wg *WaitGroup) Wait() {for {state := wg.state.Load()v := int32(state >> 32)w := uint32(state)if v == 0 {// Counter is 0, no need to wait.return}// Increment waiters count.if wg.state.CompareAndSwap(state, state+1) {runtime_Semacquire(&wg.sema)return}}
}

        若协程计数为0，则直接跳过。

        若协程计数非0，则通过CAS将等待计数+1，并通过信号量阻塞协程。