How to Check if an HTTP Request Was Cancelled in Go?

How to Check if a Request Was Cancelled

Context and Cancellation

In Go, contexts provide a mechanism for controlling and cancelling operations. They allow for propagating cancellation signals through goroutines and HTTP requests.

The Problem

When using HTTP requests with a context, it's crucial to handle cancellation correctly. In Go 1.9, attempting to check if a request was cancelled using err == context.Canceled may result in incorrect results.

Solution

In Go 1.13 :

The preferred way to check for cancellation is to use the new errors.Is function:

ctx, cancel := context.WithCancel(context.Background())
cancel()

r, _ := http.NewRequestWithContext(ctx, "GET", "http://example.com", nil)

_, err := http.DefaultClient.Do(r)
log.Println(errors.Is(err, context.Canceled)) // Prints true

errors.Is checks the error chain and returns true if any error in the chain matches the provided context.Canceled error.

In Go 1.9-1.12:

For earlier versions of Go, you can use the following workaround:

type canceledErr struct {
    error
}

func (e *canceledErr) Cancelled() bool {
    return e.Error() == "context canceled"
}

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

This workaround creates a custom error type canceledErr that embeds the wrapped error and provides a Cancelled() method to check for context cancellation.

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