How can I stop the goroutine based on the returned value from that goroutine

Issue

Like here I created a go playground sample: sGgxEh40ev, but cannot get it work.

quit := make(chan bool)
res := make(chan int)

go func() {
    idx := 0
    for {
        select {
        case <-quit:
            fmt.Println("Detected quit signal!")
            return
        default:
            fmt.Println("goroutine is doing stuff..")
            res <- idx
            idx++
        }
    }

}()

for r := range res {
    if r == 6 {
        quit <- true
    }
    fmt.Println("I received: ", r)
}

Output:

goroutine is doing stuff..
goroutine is doing stuff..
I received:  0
I received:  1
goroutine is doing stuff..
goroutine is doing stuff..
I received:  2
I received:  3
goroutine is doing stuff..
goroutine is doing stuff..
I received:  4
I received:  5
goroutine is doing stuff..
goroutine is doing stuff..
fatal error: all goroutines are asleep - deadlock!

Is this possible? Where am I wrong

Solution

The problem is that in the goroutine you use a select to check if it should abort, but you use the default branch to do the work otherwise.

The default branch is executed if no communications (listed in case branches) can proceed. So in each iteration quit channel is checked, but if it cannot be received from (no need to quit yet), default branch is executed, which unconditionally tries to send a value on res. Now if the main goroutine is not ready to receive from it, this will be a deadlock. And this is exactly what happens when the sent value is 6, because then the main goroutine tries to send a value on quit, but if the worker goroutine is in the default branch trying to send on res, then both goroutines try to send a value, and none is trying to receive! Both channels are unbuffered, so this is a deadlock.

In the worker goroutine you must send the value on res using a proper case branch, and not in the default branch:

select {
case <-quit:
    fmt.Println("Detected quit signal!")
    return
case res <- idx:
    fmt.Println("goroutine is doing stuff..")
    idx++
}

And in the main goroutine you must break out from the for loop so the main goroutine can end and so the program can end as well:

if r == 6 {
    quit <- true
    break
}

Output this time (try it on the Go Playground):

goroutine is doing stuff..
I received:  0
I received:  1
goroutine is doing stuff..
goroutine is doing stuff..
I received:  2
I received:  3
goroutine is doing stuff..
goroutine is doing stuff..
I received:  4
I received:  5
goroutine is doing stuff..
goroutine is doing stuff..

Answered By – icza

Answer Checked By – Terry (GoLangFix Volunteer)

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