Go语言多路复用所有goroutine都处于睡眠状态-死锁

Question

I wants to create a fan-in function using multiple go routines returning channel here is my code.

package main

import (
    "fmt"
    "math/rand"
    "sync"
    "time"
)

var wg, wg2 sync.WaitGroup

func main() {
    final := talk(boring("Joe"), boring("Ann"))
    for i := 0; i < 10; i++ {
        fmt.Println(<-final)
    }
    fmt.Println("You are both boring I'm leaving")
}

func talk(input1, input2 <-chan string) <-chan string {
    out := make(chan string)
    go func() {
        wg.Add(1)
        for {
            out <- <-input1
        }
    }()
    go func() {
        wg.Add(1)
        for {
            out <- <-input2
        }
    }()
    wg.Done()
    close(out)
    return out
}

func boring(msg string) <-chan string {
    c := make(chan string)
    for i := 0; i < 5; i++ {
        c <- fmt.Sprintf("%s%d
", msg, i)
        time.Sleep(time.Duration(rand.Intn(1e3)) * time.Millisecond)
    }
    return c
}

But I got an error after running above code

all goroutines are asleep - deadlock

I have tried to close channels but still it is giving me the error. I have tried to assign boring returned channels to Joe and Ann and then pass those channels to talk function for multiplexing still no success. I am new to go learning channels not clear on this concept.

Answer 1

Instead of wait groups, you can use select: https://tour.golang.org/concurrency/5

The select statement lets a goroutine wait on multiple communication operations.

A select blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready.

package main

import (
    "fmt"
    "math/rand"
    "time"
)

func main() {
    final := talk(boring("Joe"), boring("Ann"))
    for i := 0; i < 10; i++ {
        fmt.Println(<-final)
    }
    fmt.Println("You are both boring I'm leaving")
}

func talk(input1, input2 <-chan string) <-chan string {
    c := make(chan string)
    go func() {
        for {
            select {
            case s := <-input1:
                c <- s
            case s := <-input2:
                c <- s
            }
        }
    }()
    return c
}

func boring(msg string) <-chan string {
    c := make(chan string)
    go func() {
        for i := 0; i < 5; i++ {
            c <- fmt.Sprintf("%s: %d", msg, i)
            time.Sleep(time.Duration(rand.Intn(1e3)) * time.Millisecond)
        }
    }()
    return c
}

Try it on Playground

Edit:

In your given example, boring function doesn't use goroutine for repeated send over channel which will block forever, because: https://tour.golang.org/concurrency/2

By default, sends and receives block until the other side is ready. This allows goroutines to synchronize without explicit locks or condition variables.

Also, wg.Done() needs to be part of goroutine.

I got it working by doing above changes: https://play.golang.org/p/YN0kfBO6iT