在具有select case和default的Goroutines中，一旦通道关闭，默认不应该执行

Question

我试图在默认情况下理解选择块中的闭合通道的行为，但是对此后续输出感到困惑。 这里正在调用50个goroutines并关闭终点频道。

func testClosedChannelBehavior() {
    const n = 50
    finish := make(chan bool)
    var done sync.WaitGroup
    for i := 0; i < n; i++ {
        done.Add(1)
        go func(x int) {
            select {
            case <-time.After(1 * time.Hour):
            case <-finish:
                fmt.Printf("received finish %d\n", x)
            default:
            fmt.Printf("I didnt wait %d\n", x)
            }
            done.Done()
        }(i)
    }
    t0 := time.Now()
    close(finish) 
    fmt.Println("finish closed")
    done.Wait() 
    fmt.Printf("Waited %v for %d goroutines to stop\n",     time.Since(t0), n)
}

我预计一旦任何goroutine打印＆＃34;收到完成＆＃34;，默认情况不应该由任何其他goroutines执行，即＆＃34;我没有等待＆＃34;不应该打印。 但输出并不一致。有时它表现得如预期，但在多次运行时，我可以看到意外的输出如下： =====output====== I didnt wait 0 received finish 7 finish closed received finish 13 received finish 10 received finish 32 received finish 5 received finish 14 received finish 33 received finish 42 received finish 11 received finish 4 received finish 23 received finish 44 received finish 49 received finish 15 received finish 24 received finish 31 received finish 16 received finish 40 received finish 41 received finish 6 received finish 26 I didnt wait 1 received finish 19 received finish 8 received finish 43 received finish 29 received finish 20 received finish 46 received finish 12 received finish 36 received finish 47 received finish 37 received finish 35 received finish 30 received finish 39 received finish 22 received finish 28 I didnt wait 2 received finish 17 received finish 45 I didnt wait 9 received finish 48 received finish 34 I didnt wait 3 received finish 25 received finish 38 received finish 27 received finish 18 received finish 21 Waited 394.999µs for 50 goroutines to stop

我正在经历this link期待接近（完成）会发出其他人，即那些仍在等待的人，表现出相似的行为。

Answer 1

对fmt.Printf的调用涉及系统调用。 Syscalls会自动导致重新安排goroutine，因为它必须等待操作系统完成该系统调用。这意味着其中一些goroutine很可能运行select语句并选择默认情况，但不打印到控制台。

编辑：此外，如果您在具有多个线程的系统上运行此操作，默认情况下，go运行时将并行运行多个go例程 （匹配OS线程的数量），这意味着其中一些goroutine可以在通道关闭的同时执行，并在主goroutine中关闭通道之前到达select语句。

如果您添加同步通道以确保在任何goroutine中发生select之前发生通道关闭操作，它将按预期工作：

https://play.golang.org/p/XtUYaihKgRT

func testClosedChannelBehavior() {
    const n = 50
    finish := make(chan bool)
    proceed := make(chan struct{})
    var done sync.WaitGroup
    for i := 0; i < n; i++ {
        done.Add(1)
        go func(x int) {
            <-proceed
            select {
            case <-time.After(1 * time.Hour):
            case <-finish:
                fmt.Printf("received finish %d\n", x)
            default:
                fmt.Printf("I didnt wait %d\n", x)
            }
            done.Done()
        }(i)
    }
    t0 := time.Now()
    close(finish)
    fmt.Println("finish closed")
    close(proceed)
    done.Wait()
    fmt.Printf("Waited %v for %d goroutines to stop\n", time.Since(t0), n)
}