等待同步.Waitgroup推迟

Question

I have the following code that goes into deadlock when sending on channel from a goroutine below:

package main

import (
    "fmt"
    "sync"
)

func main() {
    for a := range getCh(10) {
        fmt.Println("Got:", a)
    }
}

func getCh(n int) <-chan int {
    var wg sync.WaitGroup
    ch := make(chan int)
    defer func() {
        fmt.Println("closing")
        wg.Wait()
        close(ch)
    }()
    wg.Add(1)
    go func() {
        defer wg.Done()
        for i := 0; i < n; i++ {
            ch <- i
        }
    }()
    wg.Add(1)
    go func() {
        defer wg.Done()
        for i := n; i < 0; i-- {
            ch <- i
        }
    }()

    return ch
}

I know that it is legal to use wg.Wait() in defer. But I haven't been able to find a use in a function with channel as a return value.

Answer 1

I think the mistake you're making is that you think that the deferred function will run asynchronously too. But that is not the case, so the getCh() will block in its deferred part, waiting for the WaitGroup. But as there is no one reading from the channel, the goroutines which write into it can't return and thus the WaitGroup causes deadlock. Try something like this:

func getCh(n int) <-chan int {
    ch := make(chan int)
    go func() {
        var wg sync.WaitGroup
        wg.Add(1)
        go func(n int) {
            defer wg.Done()
            for i := 0; i < n; i++ {
                ch <- i
            }
        }(n)
        wg.Add(1)
        go func(n int) {
            defer wg.Done()
            for i := n; i > 0; i-- {
                ch <- i
            }
        }(n)
        wg.Wait()
        fmt.Println("closing")
        close(ch)
    }()

    return ch
}