Familiarizing myself with concurrency, so started writing a simple ping cli with concurrent calls (let’s ignore that I’m not really measuring pings).
Problem is, I can’t close the channel properly for the range loop while also waiting for all the goroutines to finish.
If I want to concurrently call the ping function, I can’t synchronize it with waitgroups, because I will reach the wg.Wait() line before all the goroutines finish.
Is there a way to keep the ping calls concurrent and close the channel after they’re done, so the range loop can exit?
func main() {
domain := flag.String("domain", "google.com", "the domain u want to ping")
flag.Parse()
sum := 0
ch := make(chan int)
go start_pings(*domain, ch)
for elapsed := range ch {
fmt.Println("Part time: " + strconv.Itoa(elapsed))
sum += elapsed
}
avg := sum / 3
fmt.Println("Average: " + strconv.Itoa(avg))
}
func start_pings(domain string, ch chan int) {
var wg sync.WaitGroup
for i := 0; i < 3; i++ {
wg.Add(1)
go ping(domain, ch, wg)
}
wg.Wait()
close(ch)
}
func ping(domain string, ch chan int, wg sync.WaitGroup) {
url := "http://" + domain
start := time.Now()
fmt.Println("Start pinging " + url + "...")
resp, err := http.Get(url)
elapsed := time.Now().Sub(start)
if err != nil {
fmt.Println(err)
return
}
defer resp.Body.Close()
ch <- int(elapsed)
wg.Done()
}
>Solution :
You must not copy a sync.WaitGroup! It’s doc explicitly states:
A WaitGroup must not be copied after first use.
Pass a pointer to it: wg *sync.WaitGroup. And call wg.Done() deferred! You have other return statements which will cause wg.Done() to be skipped!
func start_pings(domain string, ch chan int) {
var wg sync.WaitGroup
for i := 0; i < 3; i++ {
wg.Add(1)
go ping(domain, ch, &wg)
}
wg.Wait()
close(ch)
}
func ping(domain string, ch chan int, wg *sync.WaitGroup) {
defer wg.Done()
url := "http://" + domain
start := time.Now()
fmt.Println("Start pinging " + url + "...")
resp, err := http.Get(url)
elapsed := time.Since(start)
if err != nil {
fmt.Println(err)
return
}
defer resp.Body.Close()
ch <- int(elapsed)
}
Today’s IDE’s (and golinter) warns about such obvious misuses. To avoid such mistakes, declare wg to be a pointer in the first place:
func start_pings(domain string, ch chan int) {
wg := &sync.WaitGroup{} // Pointer!
for i := 0; i < 3; i++ {
wg.Add(1)
go ping(domain, ch, wg)
}
wg.Wait()
close(ch)
}
This leaves less room for errors and misuses.