Go WaitGroup with goroutine

Question

I wonder why we need to run the wg.Wait() in goroutine

// This one works as expected...
func main() {
    var wg sync.WaitGroup
    for i:=0; i<5; i++ {
        wg.Add(1)
        go func() {
            defer wg.Done()
        }()
        time.Sleep(time.Second)
    }
    go func() {
        wg.Wait()
    }()
}

But this one never ends waiting forever

func main() {
    var wg sync.WaitGroup
    for i:=0; i<5; i++ {
        wg.Add(1)
        go func() {
            defer wg.Done()
        }()
        time.Sleep(time.Second)
    }
    wg.Wait()
}

Can anybody explain why I need to wait in another goroutine?

Thanks!

Stabler

Are you sure? I'd guess something is broken in the not shown part of the code, especially as the first version doesn't wait IMHO. – Raindrop

My code actually does a lot more, which uses its own goroutines again. I get this pattern from github.com/rakyll/coop/blob/master/coop.go#L85 – Stabler

Here, she does wg.Wait() in goroutine. – Stabler

Answer 1

why we need to run the wg.Wait() in goroutine?

In the example you mention (coop/blob/master/coop.go#L85), the wait is in a goroutine in order to return immediately a channel that will indicate when all the other goroutines have completed. Those are the goroutines to be started:

for _, fn := range fns {
    go func(f func()) {
        f()
        wg.Done()
    }(fn)
}

They mention the completion through the var wg sync.WaitGroup.
That WaitGroup is set to wait for the right number of goroutines to finish:

wg.Add(len(fns))

The wait is done in a goroutine because it will in turn signal the global completion to a channel:

go func() {
    wg.Wait()
    doneSig(ch, true)
}()

But the channel is returned immediately.

ch := make(chan bool, 1)
...
return ch

That is why the Wait is done asynchronously: you don't want to wait in this function. You just want the channel.

Bieber