How to Implement Timeouts Correctly in Go Goroutines with Channels?

Using Timeouts with Channels in Go

Goroutines and channels provide a powerful mechanism for concurrent programming in Go. However, handling timeouts in goroutines can be tricky.

In a scenario where you want to check the reachability of a list of URLs using goroutines and a timeout, there could be situations where the timeout is not getting executed even if some of the URLs are not reachable.

Let's analyze the provided code:

func check(u string) bool {
    time.Sleep(4 * time.Second)
    return true
}

func IsReachable(urls []string) bool {
    ch := make(chan bool, 1)
    for _, url := range urls {
        go func(u string) {
            select {
            case ch <- check(u):
            case <-time.After(time.Second):
                ch <- false
            }
        }(url)
    }
    return <-ch
}

The issue lies within the check function. When you use time.Sleep in a goroutine, it pauses the current goroutine, which in this case is the goroutine running the check function. While the check function is paused, the select statement in the outer goroutine will still attempt to run.

In this case, both branches of the select statement (checking for the result or the timeout) will be runnable after 4 seconds when check returns. However, since both branches are runnable, the runtime can choose to execute either one, which may result in always returning true.

To solve this, you need to create a new goroutine for each check function, as shown in the corrected code below:

func check(u string, checked chan<- bool) {
    time.Sleep(4 * time.Second)
    checked <- true
}

func IsReachable(urls []string) bool {
    ch := make(chan bool, 1)
    for _, url := range urls {
        go func(u string) {
            checked := make(chan bool)
            go check(u, checked)
            select {
            case ret := <-checked:
                ch <- ret
            case <-time.After(time.Second):
                ch <- false
            }
        }(url)
    }
    return <-ch
}

In this case, the check function is executed in a separate goroutine, ensuring that it does not pause the outer goroutine and that the timeout can be correctly executed.

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