waitgroup.Wait() causing dead lock

Question

I'm trying to figure out why I have a dead lock with waitgroup.Wait()

package main

import (
    "fmt"
    "sync"
)

var wg sync.WaitGroup

func foo(c chan int, i int) {
    defer wg.Done()
    c <- i
}

func main() {
    ch := make(chan int)
    for i := 0; i < 10; i++ {
        wg.Add(1)
        go foo(ch, i)
    }
    wg.Wait()
    close(ch)
    for item := range ch {
        fmt.Println(item)
    }
}

When I run it like this, it prints fatal error: all goroutines are asleep - deadlock!

I tried to change ch to a buffered channel and that solved the problem. But I really want to know why is there a dead lock.

Answer 1

I've commented out the parts where your program's logic is not correct:

package main

import (
    "fmt"
    "sync"
)

var wg sync.WaitGroup

func foo(c chan int, i int) {
    defer wg.Done()
    c <- i
}

func main() {
    ch := make(chan int) // unbuffered channel

    for i := 0; i < 10; i++ {
        wg.Add(1)
        go foo(ch, i)
    }

    // wg.Wait is waiting for all goroutines to finish but that's
    // only possible if the send to channel succeeds. In this case,
    // it is not possible as your receiver "for item := range ch" is below
    // this. Hence, a deadlock.
    wg.Wait()

    // Ideally, it should be the sender's duty to close the channel.
    // And closing a channel before the receiver where the channel
    // is unbuffered is not correct.
    close(ch)

    for item := range ch {
        fmt.Println(item)
    }
}

Corrected program:

package main

import (
    "fmt"
    "sync"
)

var wg sync.WaitGroup

func foo(c chan int, i int) {
    defer wg.Done()
    c <- i
}

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

    go func() {
        for item := range ch {
            fmt.Println(item)
        }
    }()

    for i := 0; i < 10; i++ {
        wg.Add(1)
        go foo(ch, i)
    }

    wg.Wait()
    close(ch)
}