how to close/abort a Golang http.Client POST prematurely

Question

I'm using http.Client for the client-side implementation of a long-poll:

resp, err := client.Post(url, "application/json", bytes.NewBuffer(jsonPostBytes))
if err != nil {
    panic(err)
}
defer resp.Body.Close()

var results []*ResponseMessage
err = json.NewDecoder(resp.Body).Decode(&results)  // code blocks here on long-poll

Is there a standard way to pre-empt/cancel the request from the client-side?

I imagine that calling resp.Body.Close() would do it, but I'd have to call that from another goroutine, as the client is normally already blocked in reading the response of the long-poll.

I know that there is a way to set a timeout via http.Transport, but my app logic need to do the cancellation based on a user action, not just a timeout.

Answer 1

Nope, client.Post is a handy wrapper for 90% of use-cases where request cancellation is not needed.

Probably it will be enough simply to reimplement your client to get access to underlying Transport object, which has CancelRequest() function.

Just a quick example:

package main

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

func main() {
    req, _ := http.NewRequest("GET", "http://google.com", nil)
    tr := &http.Transport{} // TODO: copy defaults from http.DefaultTransport
    client := &http.Client{Transport: tr}
    c := make(chan error, 1)
    go func() {
        resp, err := client.Do(req)
        // handle response ...
        _ = resp
        c <- err
    }()

    // Simulating user cancel request channel
    user := make(chan struct{}, 0)
    go func() {
        time.Sleep(100 * time.Millisecond)
        user <- struct{}{}
    }()

    for {
        select {
        case <-user:
            log.Println("Cancelling request")
            tr.CancelRequest(req)
        case err := <-c:
            log.Println("Client finished:", err)
            return
        }
    }
}