遍历通道时关闭通道的最佳时间

Question

I am playing around with Golang and I created this little app to make several concurrent api calls using goroutines.

While the app works, after the calls complete, the app gets stuck, which makes sense because it cannot exit the range c loop because the channel is not closed.

I am not sure where to better close the channel in this pattern.

package main

import "fmt"
import "net/http"

func main() {
    links := []string{
        "https://github.com/fabpot",
        "https://github.com/andrew",
        "https://github.com/taylorotwell",
        "https://github.com/egoist",
        "https://github.com/HugoGiraudel",
    }

    checkUrls(links)
}

func checkUrls(urls []string) {
    c := make(chan string)

    for _, link := range urls {
        go checkUrl(link, c)
    }

    for msg := range c {
        fmt.Println(msg)
    }

    close(c) //this won't get hit
}

func checkUrl(url string, c chan string) {
    _, err := http.Get(url)

    if err != nil {
        c <- "We could not reach:" + url
    } else {
        c <- "Success reaching the website:" + url
    }
} 

Answer 1

You close a channel when there are no more values to send, so in this case it's when all checkUrl goroutines have completed.

var wg sync.WaitGroup

func checkUrls(urls []string) {
    c := make(chan string)

    for _, link := range urls {
        wg.Add(1)
        go checkUrl(link, c)
    }

    go func() {
        wg.Wait()
        close(c)
    }()

    for msg := range c {
        fmt.Println(msg)
    }
}

func checkUrl(url string, c chan string) {
    defer wg.Done()
    _, err := http.Get(url)

    if err != nil {
        c <- "We could not reach:" + url
    } else {
        c <- "Success reaching the website:" + url
    }
}

(Note that the error from http.Get is only going to reflect connection and protocol errors. It is not going to contain http server errors if you're expecting those too, which you must be seeing how you're checking for paths and not just hosts.)