go routine 神秘结束，channel 没有到达 close 语句就关闭了

Question

我创建了以下简单程序来测试使用通道的扇入扇出模式。它所做的是生成一些 go 例程来计算来自输入通道的数字的平方并将平方发送到输出通道。然后所有输出通道将合并为一个通道以打印 main.

中的正方形

func calculateSquare(in <-chan int) <-chan int {
    out := make(chan int)

    go func() {
       for num := range in {
           fmt.Printf("Receving num %v\n", num)
           out <- num * num
           fmt.Printf("Sending square %v\n", num * num)
       }
       fmt.Println("Closing out")
       close(out)
    }()

    return out
}

func fanOut(in <-chan int, workerCount int) []<-chan int {
    outs := make([]<-chan int, 0, workerCount)

    for i := 0 ; i < workerCount ; i++ {
        outs = append(outs, calculateSquare(in))
    }

    return outs
}

func fanIn(outs []<-chan int) <-chan int {
    var wg sync.WaitGroup

    merge := make(chan int)

    for _, out := range outs {
        wg.Add(1)

        go func() {
            for result := range out {
                merge <- result
            }

            wg.Done()
        }()
    }

    go func() {
        wg.Wait()
        fmt.Println("Closing merge")
        close(merge)
    }()

    return merge
}

func main() {
    in := make(chan int)

    go func() {
        for i := 0 ; i < 4 ; i++ {
            fmt.Printf("Sending num %v\n", i)
            in <- i
        }
        close(in)
    }()

    outs := fanOut(in, 5)
    merge := fanIn(outs)

    for num := range merge {
        fmt.Printf("Final square %v\n", num)
    }
}

在 main 函数中，我将 4 个数字 0 -> 3 发送到输入通道，我希望在控制台中看到 4 个正方形。但是，当我运行程序时，即使输出有点波动，但我从未在控制台中看到 4 个平方数。

下面是我看到的示例输出。

Sending num 0
Sending num 1
Sending num 2
Sending num 3
Closing out
Receving num 0
Receving num 1
Receving num 2
Sending square  4
Closing out
Receving num 3
Final square 4
Closing merge

如果有人能向我解释为什么打印 Receving num 1 但 Sending square 1 永远不会出现，我将不胜感激。另外，如果不打印Sending square 1，output通道是怎么关闭的。我只看到 2 Closing out，但是，我合并结果的等待组结束了它的 Wait()。

一定是我哪里做错了。

Answer 1

您的问题出在下面的代码中，fanIn 函数中的 for 循环。

    for _, out := range outs {
        wg.Add(1)

        go func() {
            for result := range out {
                merge <- result
            }

            wg.Done()
        }()
    }

原因是你在gofunc中使用了out迭代器变量，当gofunc要使用它时，循环就结束了。

这在 go/wiki/CommonMistakes 子主题 Using goroutines on loop iterator variables

下进行了描述

更多示例 - 阅读 this

更正后的循环应该如下所示，

    for _, out := range outs {
        wg.Add(1)

        go func(c <- chan int) {
            for result := range c {
                merge <- result
            }

            wg.Done()
        }(out)
    }

Answer 2

修复：

for _, out := range outs {
    wg.Add(1)

    out := out // <- add this

为什么？

https://golang.org/doc/effective_go is an excellent resource and covers the exact closure bug (that @JimB mentioned) towards the end of the channels section:

It may seem odd to write

req := req

but it's legal and idiomatic in Go to do this. You get a fresh version of the variable with the same name, deliberately shadowing the loop variable locally but unique to each goroutine.

go routine 神秘结束，channel 没有到达 close 语句就关闭了

Go routine ending mysteriously, channel closed without reaching close statement

go

goroutine