fatal error: all goroutines are asleep - deadlock! when using a WaitGroup with a for-select loop

Question

go版本：go1.12.5linux/amd64

我试图了解 Go 中的 nil 通道。

package main

import (
    "fmt"
    "sync"
)

func main() {
    ch := make(chan int)
    ch2 := make(chan int)
    wg := sync.WaitGroup{}
    wg.Add(1)
    go func(c1 chan int, c2 chan int, w *sync.WaitGroup) {
        for c1 != nil || c2 != nil {
            fmt.Println("in for")
            fmt.Println(c1, c2)
            select {
            case v, ok := <-c1:
                if !ok {
                    c1 = nil
                    fmt.Println("c1 closed")
                } else {
                    fmt.Println(v, " recieved c1")
                }
            case v, ok := <-c2:
                if !ok {
                    c2 = nil
                } else {
                    fmt.Println(v, " recieved c2")
                }
            }
        }
        fmt.Println("called wg.Done")
        wg.Done()
    }(ch, ch2, &wg)
    for i := 0; i < 4; i++ {
        if i%2 == 0 {
            ch <- i
        } else {
            ch2 <- i
        }
    }
    close(ch)
    wg.Wait()
}

并在标准输出上收到此输出：

in for
0xc000084060 0xc0000840c0
0  recieved c1
in for
0xc000084060 0xc0000840c0
1  recieved c2
in for
0xc000084060 0xc0000840c0
2  recieved c1
in for
0xc000084060 0xc0000840c0
3  recieved c2
in for
0xc000084060 0xc0000840c0
c1 closed
in for
<nil> 0xc0000840c0
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [semacquire]:
sync.runtime_Semacquire(0xc00009a018)
        /usr/local/go/src/runtime/sema.go:56 +0x39
sync.(*WaitGroup).Wait(0xc00009a010)
        /usr/local/go/src/sync/waitgroup.go:130 +0x65
main.main()
        /home/ayush/projects/gojects/src/go-practice/nil-channels/main.go:44 +0x155

goroutine 18 [select]:
main.main.func1(0xc00009a010, 0xc000084060, 0xc0000840c0, 0xc00009a010)
        /home/ayush/projects/gojects/src/go-practice/nil-channels/main.go:17 +0x1e5
created by main.main
        /home/ayush/projects/gojects/src/go-practice/nil-channels/main.go:13 +0xce
exit status 2

但是根据代码和日志，wg.Done() 从未被调用，这意味着 goroutine 仍然存在。

谁能帮我理解这里发生了什么？

Answer 1

您只关闭了 ch，没有关闭 ch2。当 ch 关闭时，goroutine 将 c1 设置为 nil，但 c2 仍然不为 nil，因此 select 等待从 c2 接收，而 main goroutine 在 wg.Done() 等待，所以两个 goroutines 都睡着了。

Answer 2

发生这种情况是因为您没有关闭 ch2。 select 块仍在等待 ch2。

工作代码：

package main

import (
    "fmt"
    "sync"
)

func main() {
    ch := make(chan int)
    ch2 := make(chan int)
    wg := sync.WaitGroup{}
    wg.Add(1)

    go func(c1 chan int, c2 chan int, w *sync.WaitGroup) {
        for c1 != nil || c2 != nil {

            select {
            case v, ok := <-c1:
                if !ok {
                    c1 = nil
                    fmt.Println("c1 closed")
                } else {
                    fmt.Println(v, " recieved c1")
                }
            case v, ok := <-c2:
                if !ok {
                    c2 = nil
                } else {
                    fmt.Println(v, " recieved c2")
                }
            }
        }

        fmt.Println("called wg.Done")
        wg.Done()
    }(ch, ch2, &wg)

    for i := 0; i < 4; i++ {
        if i%2 == 0 {
            ch <- i
            fmt.Println("sending to c1 ", i)
        } else {
            ch2 <- i
            fmt.Println("sending to c2 ", i)
        }
    }
    close(ch)
    close(ch2)
    wg.Wait()
}