Go程序不会死锁，但也永远不会返回

Question

The attached gist is a simple program using channels in a producer / multi-consumer model. For some reason,

go run channels.go prints all the results but does not return (and does not deadlock or at least go doesn't give me that panic that a deadlock occurs.)

type walkietalkie struct {
    in chan int
    out chan int
    quit chan bool
}

var items []int = []int{
    0, 1, 2, 3, 4, 5,
}

func work1(q walkietalkie) {
    for {
        select {
        case a, more := <- q.in:
            if more {
                q.out <- a * 2
            }
        default:
            break
        }
    }
}

func work2(q walkietalkie) {
    for  {
        select {
        case a, more := <- q.in:
            if more {
                q.out <- a * -1
            }
        default:
            break
        }
    }
}

func work3(q walkietalkie) {
    for  {
        select {
        case a, more := <- q.in:
            if more {
                q.out <- a * 7
            }
            default:
                break
        }
    }
}

func main() {
    results := make(chan int, 18)
    defer close(results)

    w := []walkietalkie{
        walkietalkie{ in: make(chan int, 6), out: results, quit: make(chan bool, 1) },
        walkietalkie{ in: make(chan int, 6), out: results, quit: make(chan bool, 1) },
        walkietalkie{ in: make(chan int, 6), out: results, quit: make(chan bool, 1) },
    }

    go work1(w[0])
    go work2(w[1])
    go work3(w[2])

    // Iterate over work items
    l := len(items)
    for i, e := range items {
        // Send the work item to each worker
        for _, f := range w {
            f.in <- e // send the work item
            if i == l - 1 { // This is the last input, close the channel
                close(f.in)
            }
        }
    }

    // Read all the results from the workers
    for {
        select {
        case r, more := <-results:
            if more {
                fmt.Println(r)
            } else {
                continue
            }
        default:
            break
        }
    }

}

Answer 1

You have a few problems.

For 1, reading from a channel with multiple return values like

case a, more := <-q.in 

Will proceed on a closed channel, with more being set to false. In your case the default is never hit.

But those are in goroutines and wouldn't stop the program from exiting. The problem is your main goroutine is doing the same thing. Also, as it turns out, break will break out of selects as well as for loops. So if you want to break the for loop then you need to use a label and break LABEL.

As an alternative, you could also just return instead of breaking in your main goroutine.