func init() {
	// We only want to pay for this cost during testing.
	// When not under test, these values are always nil
	// and never assigned to.
	testHookMu = new(sync.Mutex)

	testHookClientDoResult = func(res *Response, err error) {
		if err != nil {
			if _, ok := err.(*url.Error); !ok {
				panic(fmt.Sprintf("unexpected Client.Do error of type %T; want *url.Error", err))
			}
		} else {
			if res == nil {

func (c *Client) Do(req *Request) (*Response, error) {
	return c.do(req)
}

var testHookClientDoResult func(retres *Response, reterr error)

func (c *Client) do(req *Request) (retres *Response, reterr error) {
	if testHookClientDoResult != nil {
		defer func() { testHookClientDoResult(retres, reterr) }()
	}
	if req.URL == nil {
		req.closeBody()
		return nil, &url.Error{