How can I use 'time.After' and 'default' in Golang?

Question

I'm trying to understand a simple code of Golang routines:

package main

import (
    "fmt"
    "time"
)

func sleep(seconds int, endSignal chan<- bool) {
    time.Sleep(time.Duration(seconds) * time.Second)
    endSignal <- true
}

func main() {
    endSignal := make(chan bool, 1)
    go sleep(3, endSignal)
    var end bool

    for !end {
        select {
        case end = <-endSignal:
            fmt.Println("The end!")
        case <-time.After(5 * time.Second):
            fmt.Println("There's no more time to this. Exiting!")
            end = true
        }
    }

}

That's fine, but Why I cannot use a simple default in this "select" block? Something like this:

for !end {
    select {
    case end = <-endSignal:
        fmt.Println("The end.")
    case <-time.After(4 * time.Second):
        fmt.Println("There's no more time to this. Exiting!")
        end = true
    default:
        fmt.Println("No end signal received.")
    }
}

It gets this output:

❯ go run goroutines-timeout.go
No end signal received!
No end signal received!
No end signal received!
No end signal received!
...
The end!

And I can't understand why.

Mr_Pink · Accepted Answer · 2016-08-29 18:33:00Z

46

Each time you execute time.After(4 * time.Second) you create a new timer channel. There's no way the select statement can remember the channel it selected on in the previous iteration. You've also taken what was an asynchronous operation and turned it into a busy loop, defeating the purpose of the select statement.

All you need is a a simple select around the two channels you're interested in. It doesn't need to loop at all.

select {
case <-endSignal:
    fmt.Println("The end!")
case <-time.After(4 * time.Second):
    fmt.Println("There's no more time to this. Exiting!")
}

https://play.golang.org/p/jb4EE8e6cw

If you really want to poll multiple times, make the timer outside of the for loop, so that the same one is checked each iteration

timeout := time.After(5 * time.Second)
pollInt := time.Second

for {
    select {
    case <-endSignal:
        fmt.Println("The end!")
        return
    case <-timeout:
        fmt.Println("There's no more time to this. Exiting!")
        return
    default:
        fmt.Println("still waiting")
    }
    time.Sleep(pollInt)
}

edited Aug 29, 2016 at 18:33

answered Aug 29, 2016 at 18:18

Mr_Pink

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3 Comments

FObersteiner Over a year ago

regarding the last part (for/select), is that really the way to go? I mean, time.Sleep is blocking, so any "endSignal" arriving during that period will not be processed immediately for example.

Mr_Pink Over a year ago

@MrFuppes: that is correct, if you want to act on endSignal immediately, you would use time.After like in the first example.

Veger Over a year ago

Or you add a second timer that fires every pollInt and add it to the select instead of the default statement. Then the select waits until it is 'time to poll' and can abort immediately on endSignal

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