oauthutil: support backend-specific errorHandler

This allows the backend to pass a errorHandler function to the doConfig
function. The webserver will pass the current request as a parameter to
the function.
The function can then examine all paramters and build the AuthError
struct which contains name, code, description of the error. A link to
the docs can be added to the HelpURL field.
oauthutil then takes care of formatting for the HTML response page. The
error details are also returned as an error in the server.err channel
and will be logged to the commandline.

lib/oauthutil/oauthutil.go

@@ -262,16 +262,22 @@ func NewClient(name string, oauthConfig *oauth2.Config) (*http.Client, *TokenSou
 //
 // It may run an internal webserver to receive the results
 func Config(id, name string, config *oauth2.Config, opts ...oauth2.AuthCodeOption) error {
-	return doConfig(id, name, config, true, opts)
+	return doConfig(id, name, nil, config, true, opts)
 }
 
 // ConfigNoOffline does the same as Config but does not pass the
 // "access_type=offline" parameter.
 func ConfigNoOffline(id, name string, config *oauth2.Config, opts ...oauth2.AuthCodeOption) error {
-	return doConfig(id, name, config, false, opts)
+	return doConfig(id, name, nil, config, false, opts)
 }
 
-func doConfig(id, name string, oauthConfig *oauth2.Config, offline bool, opts []oauth2.AuthCodeOption) error {
+// ConfigErrorCheck does the same as Config, but allows the backend to pass a error handling function
+// This function gets called with the request made to rclone as a parameter if no code was found
+func ConfigErrorCheck(id, name string, errorHandler func(*http.Request) AuthError, config *oauth2.Config, opts ...oauth2.AuthCodeOption) error {
+	return doConfig(id, name, errorHandler, config, true, opts)
+}
+
+func doConfig(id, name string, errorHandler func(*http.Request) AuthError, oauthConfig *oauth2.Config, offline bool, opts []oauth2.AuthCodeOption) error {
 	oauthConfig, changed := overrideCredentials(name, oauthConfig)
 	automatic := config.FileGet(name, config.ConfigAutomatic) != ""
 
@@ -350,12 +356,14 @@ func doConfig(id, name string, oauthConfig *oauth2.Config, offline bool, opts []
 
 	// Prepare webserver
 	server := authServer{
-		state:       state,
-		bindAddress: bindAddress,
-		authURL:     authURL,
+		state:        state,
+		bindAddress:  bindAddress,
+		authURL:      authURL,
+		errorHandler: errorHandler,
 	}
 	if useWebServer {
 		server.code = make(chan string, 1)
+		server.err = make(chan error, 1)
 		go server.Start()
 		defer server.Stop()
 		authURL = "http://" + bindAddress + "/auth"
@@ -371,9 +379,13 @@ func doConfig(id, name string, oauthConfig *oauth2.Config, offline bool, opts []
 		// Read the code, and exchange it for a token.
 		fmt.Printf("Waiting for code...\n")
 		authCode = <-server.code
+		authError := <-server.err
 		if authCode != "" {
 			fmt.Printf("Got code\n")
 		} else {
+			if authError != nil {
+				return authError
+			}
 			return errors.New("failed to get code")
 		}
 	} else {
@@ -399,12 +411,22 @@ func doConfig(id, name string, oauthConfig *oauth2.Config, offline bool, opts []
 
 // Local web server for collecting auth
 type authServer struct {
-	state       string
-	listener    net.Listener
-	bindAddress string
-	code        chan string
-	authURL     string
-	server      *http.Server
+	state        string
+	listener     net.Listener
+	bindAddress  string
+	code         chan string
+	err          chan error
+	authURL      string
+	server       *http.Server
+	errorHandler func(*http.Request) AuthError
+}
+
+// AuthError gets returned by the backend's errorHandler function
+type AuthError struct {
+	Name        string
+	Description string
+	Code        string
+	HelpURL     string
 }
 
 // startWebServer runs an internal web server to receive config details
@@ -428,6 +450,7 @@ func (s *authServer) Start() {
 		w.Header().Set("Content-Type", "text/html")
 		fs.Debugf(nil, "Received request on auth server")
 		code := req.FormValue("code")
+		var err error
 		if code != "" {
 			state := req.FormValue("state")
 			if state != s.state {
@@ -443,11 +466,20 @@ func (s *authServer) Start() {
 			}
 		} else {
 			fs.Debugf(nil, "No code found on request")
+			httpResponse := "<h1>Failed!</h1>\nNo code found returned by remote server."
+			if s.errorHandler != nil {
+				authError := s.errorHandler(req)
+				errorDesc := fmt.Sprintf("Error: %s\nCode: %s\nDescription: %s\nHelp: %s",
+					authError.Name, authError.Code, authError.Description, authError.HelpURL)
+				httpResponse += "\n<p>" + strings.Replace(errorDesc, "\n", "<br>", -1) + "</p>"
+				err = errors.New(errorDesc)
+			}
 			w.WriteHeader(500)
-			_, _ = fmt.Fprintf(w, "<h1>Failed!</h1>\nNo code found returned by remote server.")
+			_, _ = fmt.Fprintf(w, httpResponse)
 		}
 		if s.code != nil {
 			s.code <- code
+			s.err <- err
 		}
 	})