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如何实现主线程等待goroutine完成？

来源：stackoverflow 2024-02-19 10:03:26 0浏览收藏

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问题内容

我试图了解 go 关键字在调用方法时的真正作用。据我了解，在方法之前放置 go 关键字将触发 go 在 goroutine 上执行此函数，该 goroutine 可以位于同一处理器或不同处理器上。

但是当我练习以下代码时，我对没有返回预期输出的情况感到困惑，这些输出是“从 $some-time-stamp 开始”和“获取消息：$some-time-stamp”。

更具体的问题描述在代码注释中。谢谢。

package main

import (
    "fmt"
    "time"
)

func main() {
    message := make(chan string, 1)
// I am aware of if I make a channel this way, it's asynchronous non-blocking

    workOnWriteOnlyChannel(message)
    workOnReadOnlyChannel(message)
    /*
        start at 2009-11-10 23:00:05 +0000 UTC m=+5.000000001
        get message : done at 2009-11-10 23:00:05 +0000 UTC m=+5.000000001
    */
// the above one worked as expected, is because it runs synchronously, no blocking is happening

    go workOnWriteOnlyChannel(message)
    workOnReadOnlyChannel(message)
    /*
        start at 2009-11-10 23:00:05 +0000 UTC m=+5.000000001
        get message : done at 2009-11-10 23:00:05 +0000 UTC m=+5.000000001
    */
// main thread executes to create a goroutine(let's call it goroutine-2) which is responsible to execute method 
// workOnWriteOnlyChannel, then the main thread goes to the next line immediately
// main thread executes method workOnReadOnlyChannel, and main thread will block for 5 seconds because goroutine-2 won't update the shared channel until 5 seconds after
// so after 5 seconds, the main thread will unblock

    workOnWriteOnlyChannel(message)
    go workOnReadOnlyChannel(message)
    /*
        start at 2009-11-10 23:00:05 +0000 UTC m=+5.000000001
    */
// I thought after calling workOnWriteOnlyChannel, the shared channel has value, though the 
// method workOnReadOnlyChannel is executed by another goroutine, it should be two outputs, 
// but it didn't which confuses me

    go workOnReadOnlyChannel(message)
    workOnWriteOnlyChannel(message)
    /*
        start at 2009-11-10 23:00:05 +0000 UTC m=+5.000000001
    */
// I thought a new goroutine(let's call it goroutine-1) will execute method 
// workOnReadOnlyChannel, but at first 5 seconds, it will be blocked, 
// but the main thread is not blocked, and when the main thread goes to execute method 
// workOnWriteOnlyChannel, the channel will have one element, and goroutine-1 will unblock, 
// but it didn't which confuses me

    /*
    workOnReadOnlyChannel(message)
    go workOnWriteOnlyChannel(message)
    */
    // fatal error: all goroutines are asleep - deadlock!
// I think I know this one, correct me if I'm wrong
// main thread will block when it goes to execute method workOnReadOnlyChannel, 
// so can't go to the next line, 
// so deadlock happens
}

func workOnWriteOnlyChannel(messages chan<- string) {
    time.Sleep(5 * time.Second)
    messages<- "done"
    defer fmt.Printf("start at %s\n", time.Now())
}

func workOnReadOnlyChannel(messages <-chan string) {
    fmt.Printf("get message : %s at %s\n", <-messages, time.Now())
}