等待组。等待（） 导致死锁

Question

我试图弄清楚为什么我有一个带有等待组的死锁。等待（）package mainimport (    "fmt"    "sync")var wg sync.WaitGroupfunc foo(c chan int, i int) {    defer wg.Done()    c <- i}func main() {    ch := make(chan int)    for i := 0; i < 10; i++ {        wg.Add(1)        go foo(ch, i)    }    wg.Wait()    close(ch)    for item := range ch {        fmt.Println(item)    }}当我像这样运行它时，它会打印fatal error: all goroutines are asleep - deadlock!我试图更改为缓冲通道，这解决了问题。但我真的很想知道为什么会有死锁。ch

Answer 1

我已经注释掉了程序逻辑不正确的部分：

package main

import (

    "fmt"

    "sync"

)

var wg sync.WaitGroup

func foo(c chan int, i int) {

    defer wg.Done()

    c <- i

}

func main() {

    ch := make(chan int) // unbuffered channel

    for i := 0; i < 10; i++ {

        wg.Add(1)

        go foo(ch, i)

    }

    // wg.Wait is waiting for all goroutines to finish but that's

    // only possible if the send to channel succeeds. In this case,

    // it is not possible as your receiver "for item := range ch" is below

    // this. Hence, a deadlock.

    wg.Wait()

    // Ideally, it should be the sender's duty to close the channel.

    // And closing a channel before the receiver where the channel

    // is unbuffered is not correct.

    close(ch)

    for item := range ch {

        fmt.Println(item)

    }

}

更正的程序：

package main

import (

    "fmt"

    "sync"

)

var wg sync.WaitGroup

func foo(c chan int, i int) {

    defer wg.Done()

    c <- i

}

func main() {

    ch := make(chan int)

    go func() {

        for item := range ch {

            fmt.Println(item)

        }

    }()

    for i := 0; i < 10; i++ {

        wg.Add(1)

        go foo(ch, i)

    }

    wg.Wait()

    close(ch)

}