Priority in Go select statement workaround

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\n", i)
            continue
        default:
        }
        select {
        case i := <-out:
            fmt.Printf("Value: %d\n", 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.

Another approach:

package main

import "fmt"

func sender(c chan int) chan int {
        go func() {
                for i := 1; i <= 15; i++ {
                        c <- i
                }
                close(c)
        }()
        return c
}

func main() {
        for i := range sender(make(chan int, 10)) {
                fmt.Printf("Value: %d\n", i)
        }
        fmt.Println("Did we get all 15? Surely yes")
}

---

$ go run main.go
Value: 1
Value: 2
Value: 3
Value: 4
Value: 5
Value: 6
Value: 7
Value: 8
Value: 9
Value: 10
Value: 11
Value: 12
Value: 13
Value: 14
Value: 15
Did we get all 15? Surely yes
$ 

The language supports this natively and no workaround is required. It's very simple: the quit channel should only be visible to the producer. On quit, the producer closes the channel. Only when the channel is empty and closed does the consumer quit. This is made possible by ranging over the channel.

Here is an example to illustrate:

package main

import (
    "fmt"
    "math/rand"
    "time"
)

var (
    produced  = 0
    processed = 0
)

func produceEndlessly(out chan int, quit chan bool) {
    defer close(out)
    for {
        select {
        case <-quit:
            fmt.Println("RECV QUIT")
            return
        default:
            out <- rand.Int()
            time.Sleep(time.Duration(rand.Int63n(5e6)))
            produced++
        }
    }
}

func quitRandomly(quit chan bool) {
    d := time.Duration(rand.Int63n(5e9))
    fmt.Println("SLEEP", d)
    time.Sleep(d)
    fmt.Println("SEND QUIT")
    quit <- true
}

func main() {
    vals, quit := make(chan int, 10), make(chan bool)
    go produceEndlessly(vals, quit)
    go quitRandomly(quit)
    for x := range vals {
        fmt.Println(x)
        processed++
        time.Sleep(time.Duration(rand.Int63n(5e8)))
    }
    fmt.Println("Produced:", produced)
    fmt.Println("Processed:", processed)
}