将sync.WaitGroup与外部功能配合使用的最佳方法

Question

I have some issues with the following code:

package main

import (
"fmt"
"sync"
)
// This program should go to 11, but sometimes it only prints 1 to 10.
func main() {
    ch := make(chan int)
    var wg sync.WaitGroup
    wg.Add(2)
    go Print(ch, wg) //
    go func(){

        for i := 1; i <= 11; i++ {
            ch <- i
        }

        close(ch) 
        defer wg.Done()


    }()

    wg.Wait() //deadlock here
}

// Print prints all numbers sent on the channel.
// The function returns when the channel is closed.
func Print(ch <-chan int, wg sync.WaitGroup) {
    for n := range ch { // reads from channel until it's closed
        fmt.Println(n)
    }
    defer wg.Done()
}

I get a deadlock at the specified place. I have tried setting wg.Add(1) instead of 2 and it solves my problem. My belief is that I'm not successfully sending the channel as an argument to the Printer function. Is there a way to do that? Otherwise, a solution to my problem is replacing the go Print(ch, wg)line with:

go func() {
Print(ch)
defer wg.Done()
}

and changing the Printer function to:

func Print(ch <-chan int) {
    for n := range ch { // reads from channel until it's closed
        fmt.Println(n)
    }

}

What is the best solution?

Answer 1

Well, first your actual error is that you're giving the Print method a copy of the sync.WaitGroup, so it doesn't call the Done() method on the one you're Wait()ing on.

Try this instead:

package main

import (
    "fmt"
    "sync"
)

func main() {    
    ch := make(chan int)

    var wg sync.WaitGroup
    wg.Add(2)    

    go Print(ch, &wg)

    go func() {  
        for i := 1; i <= 11; i++ {
            ch <- i
        }
        close(ch)
        defer wg.Done()
    }()          

    wg.Wait() //deadlock here
}                

func Print(ch <-chan int, wg *sync.WaitGroup) {
    for n := range ch { // reads from channel until it's closed
        fmt.Println(n)
    }            
    defer wg.Done()
}

Now, changing your Print method to remove the WaitGroup of it is a generally good idea: the method doesn't need to know something is waiting for it to finish its job.