go sync.WaitGroup

1、数据结构

type WaitGroup struct {
	noCopy noCopy

	state 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。