go - 对goroutines中的延迟感到困惑

Question

我遇到了以下代码段，这些代码段演示了sync.Cond中的“广播”功能。摘录如下:

package main

import (
    "fmt"
    "sync"
)

func main() {
    type Button struct {
        Clicked *sync.Cond
    }
    button := Button{Clicked: sync.NewCond(&sync.Mutex{})}

    subscribe := func(c *sync.Cond, fn func()) {
        var goroutineRunning sync.WaitGroup
        goroutineRunning.Add(1)
        go func() {
            goroutineRunning.Done()
            c.L.Lock()
            defer c.L.Unlock()
            c.Wait()
            fn()
        }()
        goroutineRunning.Wait()
    }

    var clickRegistered sync.WaitGroup
    clickRegistered.Add(3)
    subscribe(button.Clicked, func() {
        fmt.Println("Maximizing window.")
        clickRegistered.Done()
    })
    subscribe(button.Clicked, func() {
        fmt.Println("Displaying annoying dialogue box!")
        clickRegistered.Done()
    })
    subscribe(button.Clicked, func() {
        fmt.Println("Mouse clicked.")
        clickRegistered.Done()
    })

    button.Clicked.Broadcast()

    clickRegistered.Wait()
}

输出如下:

Mouse clicked.
Maximizing window.
Displaying annoying dialogue box!

我更改了订阅中的goroutine，以在goroutine完成执行后推迟对gorroutineRunning WaitGroup 的“完成”调用。我的想法是，只有在执行go例程后， WaitGroup 才应递减。所以我将代码更改如下:

package main

import (
    "fmt"
    "sync"
)

func main() {
    ......
    subscribe := func(c *sync.Cond, fn func()) {
        var goroutineRunning sync.WaitGroup
        goroutineRunning.Add(1)
        go func() {
            //Adding the defer here
            defer goroutineRunning.Done()
            c.L.Lock()
            defer c.L.Unlock()
            c.Wait()
            fn()
        }()
        goroutineRunning.Wait()
    }

    ....
}

加上延后，我会感到以下 panic :

fatal error: all goroutines are asleep - deadlock!

goroutine 1 [semacquire]:
sync.runtime_Semacquire(0xc0000b6028)
        /usr/local/go/src/runtime/sema.go:56 +0x42
sync.(*WaitGroup).Wait(0xc0000b6020)
        /usr/local/go/src/sync/waitgroup.go:130 +0x64
main.main.func1(0xc00009e040, 0xc0000b4030)
        /Users/go/concur/button.go:24 +0x91
main.main()
        /Users/go/concur/button.go:29 +0xf4

goroutine 18 [sync.Cond.Wait]:
runtime.goparkunlock(...)
        /usr/local/go/src/runtime/proc.go:310
sync.runtime_notifyListWait(0xc00009e050, 0x0)
        /usr/local/go/src/runtime/sema.go:510 +0xf8
sync.(*Cond).Wait(0xc00009e040)
        /usr/local/go/src/sync/cond.go:56 +0x9d
main.main.func1.1(0xc0000b6020, 0xc00009e040, 0xc0000b4030)
        /Users/go/concur/button.go:21 +0xbb
created by main.main.func1
        /Users/go/concur/button.go:17 +0x83
exit status 2

有人可以引导我理解为什么添加延迟会导致代码崩溃吗？

Answer 1

当goroutine开始运行时，原始代码将释放 WaitGroup 。当subscribe函数返回时，goroutine仍处于 Activity 状态。

当您将其更改为defer goroutineRunning.Done()时，goroutine开始，并在c.Wait()处停止，因为它正在等待条件变量的广播。由于goroutine正在等待，因此不调用goroutineRunning.Done，并且subscribe函数在goroutineRunning.Wait处停止。因此，第一次调用subscribe时，它将创建一个等待cond的goroutine，subscribe本身开始在waitgroup中等待。有goroutines(主要程序和一个由订阅开始的程序)，都等待某个事件发生，但是没有其他goroutines在运行以使该事件发生，因此出现了死锁。