go concurrency all goroutines are asleep - deadlock - go

Question

Sorry about the noob question but I'm having a hard time wrapping my head around the concurrency part of go. Basically this program below is a simplified version of a larger one I'm writing, thus I want to keep the structure similar to below.
Basically instead of waiting 4 seconds I want to run addCount(..) concurrent using the unbuffered channel and when all elements in the int_slice has been processed I want to do another operation on them. However this program ends with a "panic: close of closed channel" and if I remove the closing of the channel I'm getting the output I'm expecting but it panics with: "fatal error: all goroutines are asleep - deadlock"
How can I implement the concurrency part correctly in this scenario?
Thanks in advance!
package main
import (
"fmt"
"time"
)
func addCount(num int, counter chan<- int) {
time.Sleep(time.Second * 2)
counter <- num * 2
}
func main() {
counter := make(chan int)
int_slice := []int{2, 4}
for _, item := range int_slice {
go addCount(item, counter)
close(counter)
}
for item := range counter {
fmt.Println(item)
}
}

Answer 1

Here are the issues I spotted in the code, and below a working version based on your implementation.
If a goroutine tries to write to an "unbuffered" channel, it will block until someone reads from it. Since you are not reading until they finish writing to the channel, you have a deadlock there.
Closing the channel while they are blocked breaks the deadlock, but gives an error since they now can't write to a closed channel.
Solution involves:
Creating a buffered channel so that they can write without blocking.
Using a sync.WaitGroup so that you wait for the goroutines to finish before closing the channel.
Reading from the channel at the end, when all is done.
See here, with comments:
package main
import (
"fmt"
"time"
"sync"
)
func addCount(num int, counter chan<- int, wg *sync.WaitGroup) {
// clear one from the sync group
defer wg.Done()
time.Sleep(time.Second * 2)
counter <- num * 2
}
func main() {
int_slice := []int{2, 4}
// make the slice buffered using the slice size, so that they can write without blocking
counter := make(chan int, len(int_slice))
var wg sync.WaitGroup
for _, item := range int_slice {
// add one to the sync group, to mark we should wait for one more
wg.Add(1)
go addCount(item, counter, &wg)
}
// wait for all goroutines to end
wg.Wait()
// close the channel so that we not longer expect writes to it
close(counter)
// read remaining values in the channel
for item := range counter {
fmt.Println(item)
}
}

Answer 2

For the sake of having examples here's a slightly modified version of what #eugenioy submitted. It allows for the use of an unbuffered channel and the reading of values as they come in instead of at the end like a regular for loop.
package main
import (
"fmt"
"sync"
"time"
)
func addCount(num int, counter chan<- int, wg *sync.WaitGroup) {
// clear one from the sync group
defer wg.Done()
// not needed, unless you wanted to slow down the output
time.Sleep(time.Second * 2)
counter <- num * 2
}
func main() {
// variable names don't have underscores in Go
intSlice := []int{2, 4}
counter := make(chan int)
var wg sync.WaitGroup
for _, item := range intSlice {
// add one to the sync group, to mark we should wait for one more
wg.Add(1)
go addCount(item, counter, &wg)
}
// by wrapping wait and close in a go routine I can start reading the channel before its done, I also don't need to know the size of the
// slice
go func() {
wg.Wait()
close(counter)
}()
for item := range counter {
fmt.Println(item)
}
}

Answer 3

package main
import (
"fmt"
"time"
)
func addCount(num int, counter chan <- int) {
time.Sleep(time.Second * 2)
counter <- num * 2
}
func main() {
counter := make(chan int)
int_slice := []int{2, 4}
for _, item := range int_slice {
go addCount(item, counter)
fmt.Println(<-counter)
}
}