如何确保 goroutine 在 main() 之前退出？不使用 WaitGroup [关闭]答案

【问题标题】：How to ensure a goroutine exits before main()? without using WaitGroup [closed]如何确保 goroutine 在 main() 之前退出？不使用 WaitGroup [关闭]
【发布时间】：2021-01-27 05:59:21
【问题描述】：

来自以下代码：

package main

import (
    "fmt"
    "runtime"
    "time"
)

var signal = make(chan struct{})

func printNumbers() {

    counter := 1
    for {
        select {
        case <-signal:
            fmt.Println("Received signal")
            // do some housekeeping
            return
        default:
            time.Sleep(100 * time.Millisecond)
            counter++
        }
    }
}

func main() {

    go printNumbers()

    fmt.Println("Before: active go-routines", runtime.NumGoroutine())
    time.Sleep(time.Second)
    close(signal)
    //time.Sleep(1 * time.Second) do some work
    fmt.Println("After: active go-routines", runtime.NumGoroutine())
    fmt.Println("Program exited")
}

实际输出为：

Before: active go-routines 2
After: active go-routines 2
Program exited
Received signal

预期输出是：

Before: active go-routines 2
After: active go-routines 1   
Program exited
Received signal

主要是活动的go-routines(after)输出应该是1

如何确保main() go-routine 仅在其他 go-routine 退出后才退出？

【问题讨论】：

标签： go channel goroutine

【解决方案1】：

使用一个通道来等待一个 goroutine 很容易，如下所示。等待多个 goroutine 时使用 WaitGroup。

package main

import (
    "fmt"
    "runtime"
    "time"
)

var signal = make(chan struct{})
var done = make(chan struct{})   // closed when goroutine is done.

func printNumbers() {
    defer close(done)
    counter := 1
    for {
        select {
        case <-signal:
            fmt.Println("Received signal")
            // do some housekeeping
            return
        default:
            time.Sleep(100 * time.Millisecond)
            counter++
        }
    }
}

func main() {

    go printNumbers()

    fmt.Println("Before: active go-routines", runtime.NumGoroutine())
    time.Sleep(time.Second)
    close(signal)
    //time.Sleep(1 * time.Second) do some work
    <-done   // wait for the goroutine to be returning.
    fmt.Println("After: active go-routines", runtime.NumGoroutine())
    fmt.Println("Program exited")
}

【讨论】：