3条腿的Oauth2，Google App Engine Golang

i need some help trying to implement a 3 legged Oauth2 to google on google app engine using golang.

I have been following this workflow https://developers.google.com/accounts/docs/CrossClientAuth

I'm also using the github.com/golang/oauth2 package for oauth2 golang authentication.

in the google documentation it says that i need to get a code from the front-end client send it to the backend and exchange the code to a valid Token.

android:

//service account id
private final String scopes = "audience:server:client_id:XXX-XXX"

mGoogleApiClient = new GoogleApiClient.Builder(this).addConnectionCallbacks(this).addOnConnectionFailedListener(this).addApi(Plus.API).addScope(Plus.SCOPE_PLUS_LOGIN).build();

        findViewById(R.id.log).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                LongOperation longOperation = new LongOperation();
                //calling the async to perform the service call
                longOperation.execute(scopes); 

            }
        });  

private class LongOperation extends AsyncTask<String, Void, String> {
        @Override
        protected String doInBackground(String... params) {

            try {
                code = GoogleAuthUtil.getToken(MainActivity.this,Plus.AccountApi.getAccountName(mGoogleApiClient),params[0]);
            } catch (GoogleAuthException e) {
                e.printStackTrace();
                Log.d("ok",String.valueOf(e));
            } catch (IOException e) {
                e.printStackTrace();
                Log.d("ok",String.valueOf(e));
            }
            Log.d("ok",String.valueOf(code));

            return null;
        }
    }

NOW : backend ..

i have created a GET endpoint (for now .. it will be a POST) in order to pass the code that I'm getting in the android async trying to exchange it to a valid token by calling google Oauth2 supplying it with ServiceAccounts id using JSON. i follow this documentation http://godoc.org/github.com/golang/oauth2/google ->> ServiceAccountJSON.

package Oauth2

import (

    "github.com/golang/oauth2"
    "net/http"
    "github.com/golang/oauth2/google"

    "appengine/urlfetch"
    "appengine"
)

func ExchangeCode(r *http.Request,code string){

    c := appengine.NewContext(r);

//setting endpoints for calling the get supplying android client code , and oauth2callback
    oauth2.Endpoint("https://XXX.appspot.com/oauth2","https://XXX.appspot.com/oauth2callback");

    f, err := oauth2.New(
        google.ServiceAccountJSONKey("static/XXX.json"), //reading the json file that was         supplyde from the google console
        oauth2.Scope("https://www.googleapis.com/auth/userinfo.profile"),
        )

    if err != nil {
        c.Infof(err.Error());
    }

    f.AuthCodeURL("tiqitiq", "offline", "auto"); //u := *f.opts.AuthURL

    c.Infof("flow v%",f);

    client := urlfetch.Client(c);

    t,err := f.NewTransportFromCode(code);
    if err != nil {
        c.Errorf(err.Error());
    }

    client.Transport = t;
        c.Infof(err.Error());
        return
    }


    c.Infof("resp:= %v",resp);
}

I don't quite seem to get it right ..

from what i understand i need to call this function and supply params to get the right Token call. but i get nil pointer exception on runtime trying to use this package..

func : f.AuthCodeURL("tiqitiq", "offline", "auto"); 
Oauth2 package exception : u := *f.opts.AuthURL;

further more i understand that i need to use the t,err := f.NewTransportFromCode(code); function to transport the code got from the android app to the google Oauth2 servers and get a callback giving me the Token and refresh-token-url .. this line also brakes..

this all happening on google app engine using the golang .. please help I'm so stuck ..