Close http connection programmatically after X ms in golang

Question

I'm doing X parallel http requests and when one of them does not respond in X ms (imagine is 100ms) or less I want to cut this connection. The code I wrote does not seem to work so, how can I cut the connection and get the response as nil?

This is my sample code:

cx, cancel := context.WithCancel(context.Background())
ch := make(chan *HttpResponse)
var responses []*HttpResponse

timeout := 1.000 //1ms for testing purposes
var client = &http.Client{
    Timeout: 1 * time.Second,
}

startTime := time.Now()
for _, url := range urls {
    go func(url string) {
        fmt.Printf("Fetching %s \n", url)
        req, _ := http.NewRequest("POST", url, bytes.NewReader(request)) //request is json string
        req.WithContext(cx)
        resp, err := client.Do(req)
        ch <- &HttpResponse{url, resp, err}
        var timeElapsed = time.Since(startTime)
        msec := timeElapsed.Seconds() * float64(time.Second/time.Millisecond)
        if msec >= timeout {
            cancel()
        }
        if err != nil && resp != nil && resp.StatusCode == http.StatusOK {
            resp.Body.Close()
        }
    }(url)
}

for {
    select {
    case r := <-ch:
        fmt.Printf("%s was fetched\n", r.Url)
        if r.Err != nil {
            fmt.Println("with an error", r.Err)
        }
        responses = append(responses, r)
        if len(responses) == len(*feeds) {
            return responses
        }
    case <-time.After(100):
        //Do something
    }
}

leaf bebop · Accepted Answer · 2018-02-03 14:22:26Z

1

Your code waits until a requests finishes (and get a resposne or an error), and then calculate the time passed, and if it was longer than the time expect, your code would cancel all the requests.

    req, _ := http.NewRequest("POST", url, bytes.NewReader(request)) //request is json string
    req.WithContext(cx) //Here you use a common cx, which all requests share.
    resp, err := client.Do(req) //Here the request is being sent and you wait it until done.
    ch <- &HttpResponse{url, resp, err}
    var timeElapsed = time.Since(startTime)
    msec := timeElapsed.Seconds() * float64(time.Second/time.Millisecond)
    if msec >= timeout {
        cancel() //here you cancel all the requests.
    }

The fix is to utilize the context package right.

    req, _ := http.NewRequest("POST", url, bytes.NewReader(request)) //request is json string
    ctx,cancel := context.WithTimeout(request.Context(),time.Duration(timeout)*time.Millisecond)
    resp,err:=client.Do(req.WithContext(ctx))
    defer cancel()

With that, you will get a nil resp (and an error) and get the connection cut when time out.

edited Feb 3, 2018 at 14:22

answered Feb 3, 2018 at 12:15

leaf bebop

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7 Comments

Mr_Pink Over a year ago

you forgot defer cancel(), though go vet should catch that.

xmarston Over a year ago

Thanks @leafbebop, it seems to work, now in the console I see context deadline exceeded but it does not continue with the code (it is a web server app). It keeps loading...

leaf bebop Over a year ago

@xmarston I need other parts of code to know what's the problem.

xmarston Over a year ago

@leafbebop nothing, I wasn't checking for a nil response so the code was silently failing. I corrected this behavior and now works like a charm. Thanks for the solution!!

Mr_Pink Over a year ago

@xmarston: you should never have to check for a nil response if you’re properly handling the error.

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