Allow appservices to register without a login type (#529)

* Allow appservices to register without a logintype

Signed-off-by: Andrew Morgan <andrewm@matrix.org>

* Reduce cyclomatic complexity on Register function

* Fix some grammar

src/github.com/matrix-org/dendrite/clientapi/routing/register.go

@@ -415,6 +415,22 @@ func validateApplicationService(
 	return matchedApplicationService.ID, nil
 }
 
+// authTypeIsValid checks the registration authentication type of the request
+// and returns true or false depending on whether the auth type is valid
+func authTypeIsValid(authType *authtypes.LoginType, req *http.Request) bool {
+	// If no auth type is specified by the client, send back the list of available flows
+	if *authType == "" && req.URL.Query().Get("access_token") != "" {
+		// Assume this is an application service registering a user if an empty login
+		// type was provided alongside an access token
+		*authType = authtypes.LoginTypeApplicationService
+	} else if *authType == "" {
+		// Not an access token, and no login type. Send back the flows
+		return false
+	}
+
+	return true
+}
+
 // Register processes a /register request.
 // http://matrix.org/speculator/spec/HEAD/client_server/unstable.html#post-matrix-client-unstable-register
 func Register(
@@ -454,8 +470,9 @@ func Register(
 		r.Username = strconv.FormatInt(id, 10)
 	}
 
-	// If no auth type is specified by the client, send back the list of available flows
+	// Check r.Auth.Type is correct for the client requesting (handles application
-	if r.Auth.Type == "" {
+	// services requesting without an auth type)
+	if !authTypeIsValid(&r.Auth.Type, req) {
 		return util.JSONResponse{
 			Code: http.StatusUnauthorized,
 			JSON: newUserInteractiveResponse(sessionID,
@@ -475,7 +492,7 @@ func Register(
 
 	// Make sure normal user isn't registering under an exclusive application
 	// service namespace. Skip this check if no app services are registered.
-	if r.Auth.Type != "m.login.application_service" &&
+	if r.Auth.Type != authtypes.LoginTypeApplicationService &&
 		len(cfg.Derived.ApplicationServices) != 0 &&
 		UsernameMatchesExclusiveNamespaces(cfg, r.Username) {
 		return util.JSONResponse{