func doTokenRoundTrip()

in internal/token.go [231:285]
54 lines of code
9 McCabe index (conditional complexity)

func doTokenRoundTrip(ctx context.Context, req *http.Request) (*Token, error) {
	r, err := ctxhttp.Do(ctx, ContextClient(ctx), req)
	if err != nil {
		return nil, err
	}
	body, err := ioutil.ReadAll(io.LimitReader(r.Body, 1<<20))
	r.Body.Close()
	if err != nil {
		return nil, fmt.Errorf("oauth2: cannot fetch token: %v", err)
	}
	if code := r.StatusCode; code < 200 || code > 299 {
		return nil, &RetrieveError{
			Response: r,
			Body:     body,
		}
	}

	var token *Token
	content, _, _ := mime.ParseMediaType(r.Header.Get("Content-Type"))
	switch content {
	case "application/x-www-form-urlencoded", "text/plain":
		vals, err := url.ParseQuery(string(body))
		if err != nil {
			return nil, err
		}
		token = &Token{
			AccessToken:  vals.Get("access_token"),
			TokenType:    vals.Get("token_type"),
			RefreshToken: vals.Get("refresh_token"),
			Raw:          vals,
		}
		e := vals.Get("expires_in")
		expires, _ := strconv.Atoi(e)
		if expires != 0 {
			token.Expiry = time.Now().Add(time.Duration(expires) * time.Second)
		}
	default:
		var tj tokenJSON
		if err = json.Unmarshal(body, &tj); err != nil {
			return nil, err
		}
		token = &Token{
			AccessToken:  tj.AccessToken,
			TokenType:    tj.TokenType,
			RefreshToken: tj.RefreshToken,
			Expiry:       tj.expiry(),
			Raw:          make(map[string]interface{}),
		}
		json.Unmarshal(body, &token.Raw) // no error checks for optional fields
	}
	if token.AccessToken == "" {
		return nil, errors.New("oauth2: server response missing access_token")
	}
	return token, nil
}