go - 使用带有 for-select 循环的 WaitGroup 时出现 fatal error : all goroutines are asleep - deadlock!

Question

转到版本: go1.12.5 linux/amd64

我试图理解 Go 中的 nil channel 。

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

但根据代码和日志， wg.Done()永远不会被调用，这意味着 goroutine 还活着。

谁能帮我理解这里发生了什么？

Answer 1

你只关闭ch ，而不是 ch2 .当ch关闭，goroutine 设置 c1为零，但 c2仍然不是零，所以 select等待接收来自 c2 , 主协程在 wg.Done() 等待，所以两个 goroutine 都处于休眠状态。