Deadlock with Multiple goroutines with multiple channels

Question

I am working on a sample program to print sum of odd and sum of even number between 1 to 100 using goroutine with multiple channels.

you can find my code

here

output

sum of even number = 2550
sum of odd number = 2500
fatal error: all goroutines are asleep - deadlock!

goroutine 1 [chan receive]:
main.print(0x434100, 0x11db7c)
    /tmp/sandbox052575152/main.go:18 +0xc0
main.main()
    /tmp/sandbox052575152/main.go:14 +0x120

The code works but with deadlock. I am not sure what is wrong in my code

Answer 1

We can iterate through values sent over a channel. To break such iteration channel needs to be closed explicitly. Otherwise range would block forever in the same way as for nil channel. In your code you did't close the sum(for print function sumValues channel) channel. That's why following function will be blocked for forever.

func print(sumValues <-chan string ){
    for val := range sumValues {
        fmt.Println(val)
    }
}

So you have to close the sum channel in the doSum function after all the go routine in the doSum function are complete (otherwise sum channel might be closed before go routines are complete). You can use sync.WaitGroup to do that. See the updated doSum function below:

func doSum(sum chan<- string, oddChan <-chan int, evenChan <-chan int) {
    var waitGroup sync.WaitGroup

    waitGroup.Add(2) // Must wait for 2 calls to 'done' before moving on

    go func(sum chan<- string) {
        s1 := 0
        for val := range oddChan {
            s1 += val
        }
        sum <- fmt.Sprint("sum of odd number = ", s1)
        waitGroup.Done()
    }(sum)

    go func(sum chan<- string) {
        s1 := 0
        for val := range evenChan {
            s1 += val
        }
        sum <- fmt.Sprint("sum of even number = ", s1)
        waitGroup.Done()
    }(sum)

    // Waiting for all goroutines to exit
    waitGroup.Wait()

    // all goroutines are complete now close the sum channel
    close(sum)
}