How Can I Cancel a Long-Polling HTTP Request in Go?

Cancelling a Long-Polling Request in Golang

When implementing a client-side long-poll using http.Client, it is often desirable to have the ability to cancel the request prematurely. This could be achieved by manually calling resp.Body.Close() from a separate goroutine, but it is not a standard and elegant solution.

Cancellation Using Request Context

A more idiomatic way to cancel an HTTP request in Golang is by using the request context. By passing a context with a deadline or a cancel function, the request can be cancelled from the client side. The following code demonstrates how to use the request context to cancel a long-polling request:

import (
    "context"
    "net/http"
)

func main() {
    // Create a new HTTP client with a custom transport.
    client := &http.Client{
        Transport: &http.Transport{
            // Set a timeout for the request.
            Dial: (&net.Dialer{Timeout: 5 * time.Second}).Dial,
            TLSHandshakeTimeout: 5 * time.Second,
        },
    }

    // Create a new request with a context that can be cancelled.
    ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)

    req, err := http.NewRequest("POST", "http://example.com", nil)
    if err != nil {
        panic(err)
    }
    req = req.WithContext(ctx)

    // Start the request in a goroutine and cancel it after 5 seconds.
    go func() {
        _, err := client.Do(req)
        if err != nil {
            // Handle the canceled error.
        }
    }()

    time.Sleep(5 * time.Second)
    cancel()
}

When the cancel() function is called, the request will be cancelled and the Dial and TLSHandshakeTimeout specified in the transport will be respected.

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