Deadlock after attempting to print values of channel using 'range'

Here is my code at Go Playground

package main

import (
	"fmt"
)

func sum_up(my_int int, cs chan int) {
	my_sum := 0
	for i := 0; i < my_int; i++ {
		my_sum += i
	}
	cs <- my_sum
}

func main() {

	my_channel := make(chan int)
	for i := 2; i < 5; i++ { 
		go sum_up(i, my_channel)
	}
	
	
	for ele := range my_channel {
		fmt.Println(ele)
	}  

    //fatal error: all goroutines are asleep - deadlock!

	fmt.Println("Done")

}

Which results in:

1
3
6
fatal error: all goroutines are asleep - deadlock!

And I don't understand what causes the error. My understanding is that in my function sum_up I am adding new values to my_channel. Why does the problem occur after I try to print out the values? Since I see 1,3,6 are printed it means that all goroutines have successfully finished.

Moreover, if the block that attempts to print the values of the channel

	for ele := range my_channel {
		fmt.Println(ele)
	}

Is removed, then I don't get the error. So it is including the block that causes the error, but why?

A for-range on a empty channel will block until there are elements to read from the channel or until the channel is closed.

Here is a version that uses sync.WaitGroup to account for how many goroutines remain active. After all goroutines have finished, the channel is closed and the for-range loop exists.

https://play.golang.org/p/ZnLYxLMNdF

package main

import (
	"fmt"
	"sync"
)

func sum_up(my_int int, cs chan int, wg *sync.WaitGroup) {
	my_sum := 0
	for i := 0; i < my_int; i++ {
		my_sum += i
	}
	cs <- my_sum
	wg.Done()
}

func main() {
	wg := &sync.WaitGroup{}
	my_channel := make(chan int)
	for i := 2; i < 5; i++ {
		wg.Add(1)
		go sum_up(i, my_channel, wg)
	}

	// Run a goroutine that will monitor how many sum_up are running.
	go func(cs chan int, wg *sync.WaitGroup) {
		wg.Wait()
		close(cs)
	}(my_channel, wg)

	for ele := range my_channel {
		fmt.Println(ele)
	}

	//fatal error: all goroutines are asleep - deadlock!

	fmt.Println("Done")

}

When you use range on a channel, it will wait for values forever or until the channel is closed. It's deadlocking because when the last value is written to my_channel, it will wait forever for a value that will never come.

Here's a slightly modified variant that shows how to cleanly leave the range: https://play.golang.org/p/YDlM8EcRnx

for range chan exit when chan receive close signal. You must close(my_channel) somewhere, or loop will wait forever.