golang goroutines, chanel and sync.WaitGroup - go

Question

I use simple code, but I get deadlock all the time.
Please explain to the beginner what I am doing wrong.
package main
import (
"fmt"
"sync"
)
func main() {
var wg sync.WaitGroup
ok := make(chan int, 3)
for i := 0; i < 2; i++ {
wg.Add(1)
go func(i int, wg *sync.WaitGroup) {
for x := range ok {
fmt.Println(i, x)
if x >= 5 {
ok <- x - 1
}
}
wg.Done()
}(i, &wg)
}
ok <- 1
ok <- 3
ok <- 2
ok <- 5
ok <- 3
ok <- 9
wg.Wait()
}
fatal error: all goroutines are asleep - deadlock!

Answer 1

The background of the problem is that the range operator reads from the channel until the channel is closed. Hence the for-range loop keeps waiting for more input from the channel ok even though there are no more inputs to come at some point. At the same time wg.Wait() is waiting for the goroutines to finish. Hence a deadlock!
Either you'll have to close the ok channel at some point, where
there are no more incoming values to the channel ok.
Or you may use the worker pool just like,
package main
import (
"fmt"
"strconv"
"sync"
)
func worker(wg *sync.WaitGroup, ch chan string, i int) {
defer wg.Done()
ch <- "worker process value " + strconv.Itoa(i)
}
func workerMonitor(wg *sync.WaitGroup, ch chan string) {
wg.Wait()
close(ch)
}
func doWork(ch <-chan string, done chan<- bool) {
for i := range ch {
fmt.Println(i)
}
done <- true
}
func main() {
var wg sync.WaitGroup
ch := make(chan string)
var arr = [6]int{1, 3, 2, 5, 3, 9}
for i := 0; i < len(arr); i++ {
wg.Add(1)
if arr[i] >= 5 {
for j := arr[i]; j >= 5; j-- {
wg.Add(1)
go worker(&wg, ch, j-1)
}
}
go worker(&wg, ch, arr[i])
}
go workerMonitor(&wg, ch)
done := make(chan bool, 1)
go doWork(ch, done)
<-done
}
Play ground url