在Go中将表单值附加到GET / POST请求

Question

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.

Answer 1

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.