WaitGroup goroutines with channel - go

Question

I am learning WaitGroup from the blog https://nathanleclaire.com/blog/2014/02/15/how-to-wait-for-all-goroutines-to-finish-executing-before-continuing/
the code:
package main
import (
"fmt"
"sync"
"time"
)
func main() {
messages := make(chan int)
var wg sync.WaitGroup
// you can also add these one at
// a time if you need to
wg.Add(3)
go func() {
defer wg.Done()
time.Sleep(time.Second * 3)
messages <- 1
}()
go func() {
defer wg.Done()
time.Sleep(time.Second * 2)
messages <- 2
}()
go func() {
defer wg.Done()
time.Sleep(time.Second * 1)
messages <- 3
}()
go func() {
for i := range messages {
fmt.Println(i)
}
}()
wg.Wait()
}
I think it should print 3, 2 and 1 in order. But it only prints 3, 2 but 1 is missing, what's the problem?
You can tree it on https://play.golang.org/p/kZCvDhykYM

Answer 1

Right after the latest messages <- 1, the deferred wg.Done() is invoked which releases wg.Wait() in the end of the program and the program quits. When a program quits all the goroutines get killed, so the printing goroutine does not have a chance to print the latest value.
If you put something like time.Sleep(time.Second * 1) right after wg.Done() you would be able to see all the output lines.

Answer 2

package main
import (
"fmt"
"sync"
"time"
)
func main() {
messages := make(chan int)
var wg sync.WaitGroup
wg.Add(3)
// created this goroutine to wait for other
// goroutines to complete and to close the channel
go func() {
wg.Wait()
close(messages)
}()
go func() {
defer wg.Done()
time.Sleep(time.Second * 3)
messages <- 1
}()
go func() {
defer wg.Done()
time.Sleep(time.Second * 2)
messages <- 2
}()
go func() {
defer wg.Done()
time.Sleep(time.Second * 1)
messages <- 3
}()
// this for loop is blocked in main goroutine.
// till it reads all messages and channel is closed.
for v := range messages {
fmt.Print(v)
}
}

Answer 3

The mentioned blog starts with following comment:
EDIT: As pointed out by effenn in this Reddit comment, a lot of information in this article is “dangerously inaccurate”. OOPS! I’ve written a followup/correction article here for your viewing pleasure, but I’m leaving this article up for “historical purposes”.
The Reddit comment and the followup article both describe the problem and give a solution to your problem. (Adding time.Sleep(...) to make the program work the way you expect is really hacky...)

Answer 4

package main
import (
"fmt"
"sync"
"time"
)
func main() {
messages := make(chan int)
var wg sync.WaitGroup
// you can also add these one at
// a time if you need to
wg.Add(3)
go func() {
defer wg.Done()
time.Sleep(time.Second * 3)
messages <- 1
}()
go func() {
defer wg.Done()
time.Sleep(time.Second * 2)
messages <- 2
}()
go func() {
defer wg.Done()
time.Sleep(time.Second * 1)
messages <- 3
}()
exit:
for {
select {
case i, ok := <-messages:
if !ok {
break exit
}
fmt.Println(i)
default:
time.Sleep(time.Second)
}
}
wg.Wait()
}