Usage

To wait for multiple goroutines to finish, we can use a wait group.

This is the function we’ll run in every goroutine. Note that a WaitGroup must be passed to functions by pointer.

On return, notify the WaitGroup that we’re done.

package main

import (
    "fmt"
    "sync"
    "time"
)

func worker(id int, wg *sync.WaitGroup) {

    defer wg.Done()

    fmt.Printf("Worker %d starting\n", id)

    time.Sleep(time.Second)
    fmt.Printf("Worker %d done\n", id)
}

func main() {

    var wg sync.WaitGroup

    for i := 1; i <= 5; i++ {
        wg.Add(1)
        // Launch several goroutines and increment the WaitGroup counter for each.
        go worker(i, &wg)
    }
		
    // Block until the WaitGroup counter goes back to 0; all the workers notified they’re done.
    wg.Wait()
}

Source Code

// A WaitGroup waits for a collection of goroutines to finish.
// The main goroutine calls Add to set the number of
// goroutines to wait for. Then each of the goroutines
// runs and calls Done when finished. At the same time,
// Wait can be used to block until all goroutines have finished.
//
// A WaitGroup must not be copied after first use.
type WaitGroup struct {
	noCopy noCopy

	// 64-bit value: high 32 bits are counter, low 32 bits are waiter count.
	// 64-bit atomic operations require 64-bit alignment, but 32-bit
	// compilers do not ensure it. So we allocate 12 bytes and then use
	// the aligned 8 bytes in them as state, and the other 4 as storage
	// for the sema.
	state1 [3]uint32
}

state1是个长度为3的数组，其中包含了state和一个信号量，而state实际上是两个计数器：

counter：当前还未执行结束的goroutine计数器
waiter count: 等待goroutine-group结束的goroutine数量，即有多少个等候者
semaphore: 信号量

考虑到字节是否对齐，三者出现的位置不同，为简单起见，依照字节已对齐情况下，三者在内存中的位置如下所示：

WaitGroup对外提供三个接口：

Add(delta int): 将delta值加到counter中
Wait()：waiter递增1，并阻塞等待信号量semaphore
Done()：counter递减1，按照waiter数值释放相应次数信号量

`Add(delta int)`

// Add adds delta, which may be negative, to the WaitGroup counter.
// If the counter becomes zero, all goroutines blocked on Wait are released.
// If the counter goes negative, Add panics.
//
// Note that calls with a positive delta that occur when the counter is zero
// must happen before a Wait. Calls with a negative delta, or calls with a
// positive delta that start when the counter is greater than zero, may happen
// at any time.
// Typically this means the calls to Add should execute before the statement
// creating the goroutine or other event to be waited for.
// If a WaitGroup is reused to wait for several independent sets of events,
// new Add calls must happen after all previous Wait calls have returned.
// See the WaitGroup example.
func (wg *WaitGroup) Add(delta int) {
	statep, semap := wg.state()
	if race.Enabled {
		_ = *statep // trigger nil deref early
		if delta < 0 {
			// Synchronize decrements with Wait.
			race.ReleaseMerge(unsafe.Pointer(wg))
		}
		race.Disable()
		defer race.Enable()
	}
	state := atomic.AddUint64(statep, uint64(delta)<<32)
	v := int32(state >> 32)
	w := uint32(state)
	if race.Enabled && delta > 0 && v == int32(delta) {
		// The first increment must be synchronized with Wait.
		// Need to model this as a read, because there can be
		// several concurrent wg.counter transitions from 0.
		race.Read(unsafe.Pointer(semap))
	}
	if v < 0 {
		panic("sync: negative WaitGroup counter")
	}
	if w != 0 && delta > 0 && v == int32(delta) {
		panic("sync: WaitGroup misuse: Add called concurrently with Wait")
	}
	if v > 0 || w == 0 {
		return
	}
	// This goroutine has set counter to 0 when waiters > 0.
	// Now there can't be concurrent mutations of state:
	// - Adds must not happen concurrently with Wait,
	// - Wait does not increment waiters if it sees counter == 0.
	// Still do a cheap sanity check to detect WaitGroup misuse.
	if *statep != state {
		panic("sync: WaitGroup misuse: Add called concurrently with Wait")
	}
	// Reset waiters count to 0.
	*statep = 0
	for ; w != 0; w-- {
		runtime_Semrelease(semap, false, 0)
	}
}

`Wait()`

// Wait blocks until the WaitGroup counter is zero.
func (wg *WaitGroup) Wait() {
	statep, semap := wg.state()
	if race.Enabled {
		_ = *statep // trigger nil deref early
		race.Disable()
	}
	for {
		state := atomic.LoadUint64(statep)
		v := int32(state >> 32)
		w := uint32(state)
		if v == 0 {
			// Counter is 0, no need to wait.
			if race.Enabled {
				race.Enable()
				race.Acquire(unsafe.Pointer(wg))
			}
			return
		}
		// Increment waiters count.
		if atomic.CompareAndSwapUint64(statep, state, state+1) {
			if race.Enabled && w == 0 {
				// Wait must be synchronized with the first Add.
				// Need to model this is as a write to race with the read in Add.
				// As a consequence, can do the write only for the first waiter,
				// otherwise concurrent Waits will race with each other.
				race.Write(unsafe.Pointer(semap))
			}
			runtime_Semacquire(semap)
			if *statep != 0 {
				panic("sync: WaitGroup is reused before previous Wait has returned")
			}
			if race.Enabled {
				race.Enable()
				race.Acquire(unsafe.Pointer(wg))
			}
			return
		}
	}
}

这里用到了CAS算法保证有多个goroutine同时执行Wait()时也能正确累加waiter。

`Done()`

Done()只做一件事，即把counter减1，我们知道Add()可以接受负值，所以Done实际上只是调用了Add(-1)。

源码如下：

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

Done()的执行逻辑就转到了Add()，实际上也正是最后一个完成的goroutine把等待者唤醒的。

【Golang】同步机制 - WaitGroup

Usage

Source Code

`Add(delta int)`

`Wait()`

`Done()`

Reference

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