How to check if a request was cancelled

[Glen Huang]

I have this simple code in which I try to check if the request was cancelled. But surprisingly, it prints false instead of true in go 1.9.

I wonder what's the correct way to check that?

package main


import (
    "context"
    "log"
    "net/http"
)


func main() {
    r, _ := http.NewRequest("GET", "http://example.com", nil)
    ctx := context.Background()
    ctx, cancel := context.WithCancel(ctx)
    r = r.WithContext(ctx)
    ch := make(chan bool)
    go func() {
        _, err := http.DefaultClient.Do(r)
        log.Println(err == context.Canceled)
        ch <- true
    }()
    cancel()
    <-ch
}

[Ain]

I noticed that OP has also posted this question to SO:

https://stackoverflow.com/questions/47179024/how-to-check-if-a-request-was-cancelled

ain

[Glen Huang]

Yes. Thanks for point it out, and maybe I should've mentioned that.

After posing that on SO, I have a gut feeling that it should be the intuitive way to check that. I was hoping a go author could see the question here and maybe offer some rationale why that's not the case.

Sorry if it's considered spamming.

[Jakob Borg]

Errors from net/http are wrapped in an url.Error to contain information about the request that failed. You can unwrap it with an assertion or type switch;

_, err := http.DefaultClient.Do(r)
if urlErr, ok := err.(*url.Error); ok {
log.Println(urlErr.Err == context.Canceled) // “true", in your example
}

//jb

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[Jan Mercl]

On Wed, Nov 8, 2017 at 1:12 PM Glen Huang <hey...@gmail.com> wrote:

see https://play.golang.org/p/wMJ5PX1x9H

--

-j

[Glen Huang]

Really neat! Didn't know it was wrapped. Thank you.

[Glen Huang]

Yep, that's a good way too. Thanks. Ain also posted it on SO.

[Jakob Borg]

(expanding the code from the link)

_, err := http.DefaultClient.Do(r)

log.Println(err, ctx.Err() == context.Canceled)

Note that, to be pedantic, this only tells you that the context has been cancelled - not that that was the error returned by the HTTP request. The HTTP request may have succeeded, or failed for another reason, before the context was cancelled. Whether this matters is of course up to the application.

//jb

[Glen Huang]

In my case the client is the sole consumer of the context, so it should be fine.

But I agree, checking error directly is more bullet-proof. I should probably form a habit adopting it.