Go using timeouts with channels

Question

I am using goroutines/channels to check if list of urls are reachable. Here is my code. This seems to always return true. Why is the timeout case not getting executed? The goal is to return false even if one of the urls is not reachable

import "fmt"
import "time"

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
}
func main() {
    fmt.Println(IsReachable([]string{"url1"}))
}

Answer 1

check(u) will sleep in the current goroutine, i.e. the one that's running func. The select statement is only run properly once it returns, and by that time, both branches are runnable and the runtime can pick whichever one it pleases.

You can solve it by running check inside yet another goroutine:

package main

import "fmt"
import "time"

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(1 * time.Second):
                ch <- false
            }
        }(url)
    }
    return <-ch
}
func main() {
    fmt.Println(IsReachable([]string{"url1"}))
}

It seems you want to check reachability of a set of URLs, and return true if one of them is available. If the timeout is long compared to the time it takes to spin up a goroutine, you could simplify this by having just one timeout for all URLs together. But we need to make sure that the channel is large enough to hold the answers from all checks, or the ones that don't "win" will block forever:

package main

import "fmt"
import "time"

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

func IsReachable(urls []string) bool {
    ch := make(chan bool, len(urls))
    for _, url := range urls {
        go check(url, ch)
    }
    time.AfterFunc(time.Second, func() { ch <- false })
    return <-ch
}
func main() {
    fmt.Println(IsReachable([]string{"url1", "url2"}))
}

Answer 2

The reason this always returns true is you are calling check(u) within your select statement. You need to call it within a go routine and then use a select to either wait for the result or timeout.

In case you want to check the reachability of multiple URLs in parallel you need to restructure your code.

First create a function which checks the reachability of one URL:

func IsReachable(url string) bool {
    ch := make(chan bool, 1)
    go func() { ch <- check(url) }()
    select {
    case reachable := <-ch:
        return reachable
    case <-time.After(time.Second):
        // call timed out
        return false
    }
}

Then call this function from a loop:

urls := []string{"url1", "url2", "url3"}
for _, url := range urls {
    go func() { fmt.Println(IsReachable(url)) }()
}

Play

Answer 3

change the line

ch := make(chan bool, 1)

to

ch := make(chan bool)

You did open a asynchronous (= non blocking) channel, but you need a blocking channel to get it work.