Deadlock error when trying to sync goroutines - go

Question

I'm facing an annoying problem. When I try to use wg.Add() to sync my routines, a deadlock error is raised.
package main
import (
"fmt"
"sync"
)
func hello(ch chan int, num int, wg *sync.WaitGroup) {
for {
i := <-ch
if i == num {
fmt.Println("Hello number:", i)
ch <- (num - 1)
defer wg.Done() // Same happens without defer
return
}
ch <- i
}
}
func main() {
fmt.Println("Start")
var wg sync.WaitGroup
ch := make(chan int)
for i := 0; i < 10; i++ {
wg.Add(1)
go hello(ch, i, &wg)
}
ch <- 9
wg.Wait()
fmt.Println("End")
}
Outputs:
Start
Hello number: 9
Hello number: 8
Hello number: 7
Hello number: 6
Hello number: 5
Hello number: 4
Hello number: 3
Hello number: 2
Hello number: 1
Hello number: 0
fatal error: all goroutines are asleep - deadlock!
goroutine 1 [semacquire]:
sync.runtime_Semacquire(0xc04203a20c)
C:/Go/src/runtime/sema.go:47 +0x3b
sync.(*WaitGroup).Wait(0xc04203a200)
C:/Go/src/sync/waitgroup.go:131 +0x81
main.main()
C:/Users/Augusto Dias/Documents/GoLang/MT.go:34 +0x1a0
goroutine 18 [chan send]:
main.hello(0xc0420380c0, 0x0, 0xc04203a200)
C:/Users/Augusto Dias/Documents/GoLang/MT.go:13 +0x197
created by main.main
C:/Users/Augusto Dias/Documents/GoLang/MT.go:29 +0x151
exit status 2
When I use wg.Add(9) outside the for block, I got no error.
func main() {
fmt.Println("Start")
var wg sync.WaitGroup
ch := make(chan int)
wg.Add(9) // Use wg.Add(10) will raise deadlock too
for i := 0; i < 10; i++ {
go hello(ch, i, &wg)
}
ch <- 9
wg.Wait()
fmt.Println("End")
}
Outputs:
Start
Hello number: 9
Hello number: 8
Hello number: 7
Hello number: 6
Hello number: 5
Hello number: 4
Hello number: 3
Hello number: 2
Hello number: 1
End
Why is this happening, I mean, why routines goes asleep when I wait for them all? Use the same channel for send and receive can be the source of this problem?

Answer 1

Channel 0 (the call go hello(ch, 0, &wg)) since it is the last channel alive gets stuck on this line
ch <- (num - 1)
It is attempting to send to a channel but no one is there to receive it. Thus the function will wait indefinitely and never be done.
Some suggestions for how to remove this problem
create a consumer in the main loop
make the channel ch non blocking