httpclient/http.go
func ctxhttpDo(ctx context.Context, client *http.Client, req *http.Request) (*http.Response, error) {
	if client == nil {
		client = http.DefaultClient
	}
	resp, err := client.Do(req.WithContext(ctx))
	// If we got an error, and the context has been canceled,
	// the context's error is probably more useful.
	if err != nil {
		select {
		case <-ctx.Done():
			err = ctx.Err()
		default:
		}
	}
	return resp, err
}
func ctxhttpDo(ctx context.Context, client *http.Client, req *http.Request) (*http.Response, error) {
	if client == nil {
		client = http.DefaultClient
	}
	resp, err := client.Do(req.WithContext(ctx))
	// If we got an error, and the context has been canceled,
	// the context's error is probably more useful.
	if err != nil {
		select {
		case <-ctx.Done():
			err = ctx.Err()
		default:
		}
	}
	return resp, err
}
// Do sends an HTTP request with the provided http.Client and returns
// an HTTP response.
//
// If the client is nil, http.DefaultClient is used.
//
// The provided ctx must be non-nil. If it is canceled or times out,
// ctx.Err() will be returned.
//
// Borrowed originally from "https://github.com/golang/net/blob/master/context/ctxhttp/ctxhttp.go"