I've read a pipeline article from https://go.dev/blog/pipelines.

I've tried to remove the goroutine that covers wg.Wait() but it turns out that caused all goroutines are asleep - deadlock!

I really don't understand why.

func merge(cs ...<-chan int) <-chan int {
    var wg sync.WaitGroup
    out := make(chan int)

    // Start an output goroutine for each input channel in cs.  output
    // copies values from c to out until c is closed, then calls wg.Done.
    output := func(c <-chan int) {
        for n := range c {
            out <- n
        }
        wg.Done()
    }
    wg.Add(len(cs))
    for _, c := range cs {
        go output(c)
    }

    // Start a goroutine to close out once all the output goroutines are
    // done.  This must start after the wg.Add call.
    go func() {
        wg.Wait()
        close(out)
    }()
    return out
}

CodePudding user response：

out is a blocking channel. You need to return out and let something consume from out, or else the output goroutines will block as soon as they try to write anything. If merge waits on the waitgroup (i.e. waits for everything to finish) before the consumer even has a chance to start, that's an inevitable deadlock.