Golang-为什么使用完成频道

Question

这是Golang的示例代码之一。但是无法理解为什么在这种情况下需要“完成”渠道。

https://gobyexample.com/closing-channels

没有理由将真实消息发送到已完成的频道。当打印“已发送所有作业”消息时，我们可以知道作业通道已完成，不是吗？

我删除了相对于已完成频道的代码，结果仍然相同。

https://play.golang.org/p/xOmzobFpTQ

Answer 1

TL;DR：有一个竞争条件---你很幸运。

如果您没有done通道，则程序的输出是不确定的。

根据线程的执行顺序，主线程可能会在 goroutine 完成处理之前退出，从而导致 goroutine 在中途被杀死。

通过强制主线程从done通道读取，我们强制主线程等待，直到done通道中有一些数据要消耗。这为我们提供了一个简洁的同步机制，其中 goroutine 通知主线程它是通过写入done通道来完成的。这反过来导致主线程的阻塞<- done完成并导致程序终止。

Answer 2

没有结果是不一样的：在许多情况下（例如，不同的CPU负载，并且不确定性和系统相关的行为），
您的主goroutine会在goroutine之前退出received job，因此，您无法保证all jobs received，例如Add

time.Sleep(500)

之前

fmt.Println("received job", j)

要查看此内容，请在The Go Playground上尝试：

// _Closing_ a channel indicates that no more values
// will be sent on it. This can be useful to communicate
// completion to the channel's receivers.

package main

import (
    "fmt"
    "time"
)

// In this example we'll use a `jobs` channel to
// communicate work to be done from the `main()` goroutine
// to a worker goroutine. When we have no more jobs for
// the worker we'll `close` the `jobs` channel.
func main() {
    jobs := make(chan int, 5)
    //done := make(chan bool)

    // Here's the worker goroutine. It repeatedly receives
    // from `jobs` with `j, more := <-jobs`. In this
    // special 2-value form of receive, the `more` value
    // will be `false` if `jobs` has been `close`d and all
    // values in the channel have already been received.
    // We use this to notify on `done` when we've worked
    // all our jobs.
    go func() {
        for {
            j, more := <-jobs
            if more {
                time.Sleep(500)
                fmt.Println("received job", j)
            } else {
                fmt.Println("received all jobs")
                //done <- true
                return
            }
        }
    }()

    // This sends 3 jobs to the worker over the `jobs`
    // channel, then closes it.
    for j := 1; j <= 3; j++ {
        jobs <- j
        fmt.Println("sent job", j)
    }
    close(jobs)
    fmt.Println("sent all jobs")

    // We await the worker using the
    // [synchronization](channel-synchronization) approach
    // we saw earlier.
    //<-done
}

输出：

sent job 1
sent job 2
sent job 3
sent all jobs

代替：

sent job 1
received job 1
received job 2
sent job 2
sent job 3
received job 3
received all jobs
sent all jobs

请参阅：
如果time.sleep包括在内，Goroutine将不执行
为什么要运行某些goroutine需要time.sleep ？
go中奇怪的频道行为