concurrency - Priority in Go select statement workaround

Question

I wish to have a go routine listening on two channels, blocked when both channels are drained. However, if both channels contains data, I want one to be drained before the other is handled.

In the working example below I wish all out to be drained before exit is handled. I use a select-statement which doesn't have any priority order. How might I get around the problem, making all 10 out-values be handled before the exit?

package main

import "fmt"

func sender(out chan int, exit chan bool){
    for i := 1; i <= 10; i++ {
        out <- i
    } 
    exit <- true
}

func main(){
    out := make(chan int, 10)
    exit := make(chan bool)

    go sender(out, exit)

    L:
    for {
        select {
            case i := <-out:
                fmt.Printf("Value: %d
", i)
            case <-exit:
                fmt.Println("Exiting")
                break L
        }
    }
    fmt.Println("Did we get all 10? Most likely not")
}

question from:https://stackoverflow.com/questions/11117382/priority-in-go-select-statement-workaround

Answer 1

package main

import "fmt"

func sender(out chan int, exit chan bool) {
    for i := 1; i <= 10; i++ {
        out <- i
    }
    exit <- true
}

func main() {
    out := make(chan int, 10)
    exit := make(chan bool)

    go sender(out, exit)

    for {
        select {
        case i := <-out:
            fmt.Printf("Value: %d
", i)
            continue
        default:
        }
        select {
        case i := <-out:
            fmt.Printf("Value: %d
", i)
            continue
        case <-exit:
            fmt.Println("Exiting")
        }
        break
    }
    fmt.Println("Did we get all 10? I think so!")
}

The default case of the first select makes it non-blocking. The select will drain the out channel without looking at the exit channel, but otherwise will not wait. If the out channel is empty, it immediately drops to the second select. The second select is blocking. It will wait for data on either channel. If an exit comes, it handles it and allows the loop to exit. If data comes, it goes back up the top of the loop and back into drain mode.