for-loopgochannelbuffered

For loop with buffered channel


I'm experimenting with Go channels and have an issue where the simple program below does not terminate.

Essentially I want to make some async HTTP get requests and then wait till they are all finished. I'm using a buffered channel but I'm not sure this is the idiomatic way.

func GetPrice(quotes chan string) {
    client := &http.Client{}
    req, _ := http.NewRequest("GET", "https://some/api", nil)
    req.Header.Set("Accept", "application/json")
    res, err := client.Do(req)
    if err != nil {
        panic(err)
    }
    defer res.Body.Close()
    body, err := ioutil.ReadAll(res.Body)
    quotes <- string(body)
}

func main() {
    const max = 3
    quotes := make(chan string, max)
    for i := 0; i < max; i++ {
        go GetPrice(quotes)
    }

    for n := range quotes {
        fmt.Printf("\n%s", n)
    }
}

The program successfully prints 3(max) items

{"price":"1.00"}
{"price":"2.00"}
{"price":"3.00"}

but then blocks and never exits.


Solution

  • sync.WaitGroup can be used here to wait for all goroutines and then closing the quotes channel:

    func getPrice(quotes chan<- string, onExit func()) {
        go func() {
            defer onExit()
    
            req, _ := http.NewRequest("GET", "https://some/api", nil)
            req.Header.Set("Accept", "application/json")
    
            client := &http.Client{}
            res, err := client.Do(req)
            if err != nil {
                panic(err) // should be handled properly
            }
            defer res.Body.Close()
    
            body, err := ioutil.ReadAll(res.Body)
            quotes <- string(body)
        }()
    }
    
    func main() {
        const max = 3
        var wg sync.WaitGroup
    
        quotes := make(chan string, max)
        for i := 0; i < max; i++ {
            wg.Add(1)
            getPrice(quotes, func() { wg.Done() })
        }
    
        go func() {
            defer close(quotes)
            wg.Wait()
        }()
    
        for n := range quotes {
            fmt.Printf("\n%s", n)
        }
    }