oauth2: fixes tokenRefresher.Token() ignores new refresh_token

Fixes bug documented in Issue #84 (https://github.com/golang/oauth2/issues/84#issuecomment-72711375).

During a refresh request, a new refresh token MAY be returned by the authorization server.  When this occurs, tokenRefesher.Token() fails to capture the new refresh token leaving it with an invalid refresh token for future calls.

Change-Id: I33b18fdbb750549174865f75eddf85b9725cf281
Reviewed-on: https://go-review.googlesource.com/4151
Reviewed-by: Andrew Gerrand <adg@golang.org>

oauth2.go

@@ -229,35 +229,46 @@ func (c *Config) Client(ctx Context, t *Token) *http.Client {
 //
 // Most users will use Config.Client instead.
 func (c *Config) TokenSource(ctx Context, t *Token) TokenSource {
-	nwn := &reuseTokenSource{t: t}
-	nwn.new = tokenRefresher{
-		ctx:      ctx,
-		conf:     c,
-		oldToken: &nwn.t,
+	tkr := &tokenRefresher{
+		ctx:  ctx,
+		conf: c,
+	}
+	if t != nil {
+		tkr.refreshToken = t.RefreshToken
+	}
+	return &reuseTokenSource{
+		t:   t,
+		new: tkr,
 	}
-	return nwn
 }
 
 // tokenRefresher is a TokenSource that makes "grant_type"=="refresh_token"
 // HTTP requests to renew a token using a RefreshToken.
 type tokenRefresher struct {
-	ctx      Context // used to get HTTP requests
-	conf     *Config
-	oldToken **Token // pointer to old *Token w/ RefreshToken
+	ctx          Context // used to get HTTP requests
+	conf         *Config
+	refreshToken string
 }
 
-func (tf tokenRefresher) Token() (*Token, error) {
-	t := *tf.oldToken
-	if t == nil {
-		return nil, errors.New("oauth2: attempted use of nil Token")
-	}
-	if t.RefreshToken == "" {
+func (tf *tokenRefresher) Token() (*Token, error) {
+	if tf.refreshToken == "" {
 		return nil, errors.New("oauth2: token expired and refresh token is not set")
 	}
-	return retrieveToken(tf.ctx, tf.conf, url.Values{
+
+	tk, err := retrieveToken(tf.ctx, tf.conf, url.Values{
 		"grant_type":    {"refresh_token"},
-		"refresh_token": {t.RefreshToken},
+		"refresh_token": {tf.refreshToken},
 	})
+
+	if err != nil {
+		return nil, err
+	}
+	if tk.RefreshToken != tf.refreshToken {
+		// possible race condition avoided because tokenRefresher
+		// should be protected by reuseTokenSource.mu
+		tf.refreshToken = tk.RefreshToken
+	}
+	return tk, err
 }
 
 // reuseTokenSource is a TokenSource that holds a single token in memory