Appending form value to GET/POST Requests in Go

I would like to define a http.Client that automatically appends a form value to all GET/POST requests.

I naively tried implementing http.RoundTripper as copy/pasted from another library uses this technique to modify headers for every request.

type Transport struct {
	// Transport is the HTTP transport to use when making requests.
	// It will default to http.DefaultTransport if nil.
	// (It should never be an oauth.Transport.)
	Transport http.RoundTripper
}

// Client returns an *http.Client that makes OAuth-authenticated requests.
func (t *Transport) Client() *http.Client {
	return &http.Client{Transport: t}
}

func (t *Transport) transport() http.RoundTripper {
	if t.Transport != nil {
		return t.Transport
	}
	return http.DefaultTransport
}

func (t *Transport) RoundTrip(req *http.Request) (*http.Response, error) {
	// To set the Authorization header, we must make a copy of the Request
	// so that we don't modify the Request we were given.
	// This is required by the specification of http.RoundTripper.
	req = cloneRequest(req)
 >>	req.Form.Set("foo", bar)

	// Make the HTTP request.
	return t.transport().RoundTrip(req)
}

// cloneRequest returns a clone of the provided *http.Request.
// The clone is a shallow copy of the struct and its Header map.
func cloneRequest(r *http.Request) *http.Request {
	// shallow copy of the struct
	r2 := new(http.Request)
	*r2 = *r
	// deep copy of the Header
	r2.Header = make(http.Header)
	for k, s := range r.Header {
		r2.Header[k] = s
	}
	return r2
}

This however does not work. The req.Form values map doesn't seem to exist at this stage, so I get the panic:
panic: runtime error: assignment to entry in nil map

I tried adding this to the (t *Transport) RoundTrip, but no luck:

err := req.ParseForm()
misc.PanicIf(err)

I've no idea what I'm doing, any tips?

EDIT: There is no point in trying to copy req.Form values in cloneRequest method, since r.Form is empty map anyway.

Form, PostForm, and ParseForm() are only used when receiving a request. When sending a request, the Transport expects the data to be properly encoded.

You have the right idea by wrapping RoundTrip, but you have to handle the encoded data yourself.

if req.URL.RawQuery == "" {
	req.URL.RawQuery = "foo=bar"
} else {
	req.URL.RawQuery = req.URL.RawQuery + "&" + "foo=bar"
}

or alternatively:

form, _ = url.ParseQuery(req.URL.RawQuery)
form.Add("boo", "far")
req.URL.RawQuery = form.Encode()

You could also choose to check the url.Values beforehand, if you want to avoid duplicating keys. Checking the Content-Type header for multipart/form-data or application/x-www-form-urlencoded to avoid interaction with other types of queries may be a good idea too.