I am currently learning golang. I wanted to check how golang channels work. I have created a program where two goroutines will print odd and even numbers alternatively. Even though the program is printing correctly, it is showing a deadlock error at the end. It is not clear from error message what is causing this issue. https://go.dev/play/p/Eg8vH845cNL

>Solution :

Just in case you don’t end up linking your code, here it is for future readers

package main

import "sync"

func main() {
    even := make(chan bool)
    odd := make(chan bool)
    go func() {
        defer close(odd)
        for i := 0; i <= 10; i += 2 {
            <-even
            print("Even ====>")
            println(i)
            odd <- true
        }
    }()
    var wait sync.WaitGroup
    wait.Add(1)
    go func() {
        for i := 1; i <= 10; i += 2 {
            _, ok := <-odd
            if !ok {
                wait.Done()
                return
            }
            print("Odd ====>")
            println(i)
            even <- true
        }
    }()
    even <- true
    wait.Wait()
}

The deadlock is likely quite easy to figure that it occurs here odd <- true as a result of a write into an unbuffered (or filled up) channel.
On the last iteration of your second goroutine having this loop for i := 1; i <= 10; i += 2 {, the channel odd has been written into already by the first goroutine but the second goroutine can’t read it because the iteration condition no longer matches because i + 2 = 11 at that particular iteration and 11 > 10 which causes the iteration to stop and the channel to stay filled.

Now, the other goroutine trying to write into the odd channel gets blocked as a result and stays blocked forever but since the runtime can easily detect this…it yields a deadlock pretty quickly afterward.