Do not check if a username is exclusive if the request is for an appservice in /register (#1652)

* Do not check if a username is exclusive if the request is for an appservice in /register

* remove useless comment

* Move statements

* fix broken test

* Also fix the senderLocalpart problem

* Check domain name is ours

* Handle accessTokenErr

* Return unauthorised instead of forbidden

Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>

clientapi/routing/register.go

@@ -328,7 +328,22 @@ func UserIDIsWithinApplicationServiceNamespace(
 	userID string,
 	appservice *config.ApplicationService,
 ) bool {
+
+	var local, domain, err = gomatrixserverlib.SplitID('@', userID)
+	if err != nil {
+		// Not a valid userID
+		return false
+	}
+
+	if domain != cfg.Matrix.ServerName {
+		return false
+	}
+
 	if appservice != nil {
+		if appservice.SenderLocalpart == local {
+			return true
+		}
+
 		// Loop through given application service's namespaces and see if any match
 		for _, namespace := range appservice.NamespaceMap["users"] {
 			// AS namespaces are checked for validity in config
@@ -341,6 +356,9 @@ func UserIDIsWithinApplicationServiceNamespace(
 
 	// Loop through all known application service's namespaces and see if any match
 	for _, knownAppService := range cfg.Derived.ApplicationServices {
+		if knownAppService.SenderLocalpart == local {
+			return true
+		}
 		for _, namespace := range knownAppService.NamespaceMap["users"] {
 			// AS namespaces are checked for validity in config
 			if namespace.RegexpObject.MatchString(userID) {
@@ -488,17 +506,6 @@ func Register(
 		return *resErr
 	}
 
-	// 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 != authtypes.LoginTypeApplicationService &&
-		len(cfg.Derived.ApplicationServices) != 0 &&
-		UsernameMatchesExclusiveNamespaces(cfg, r.Username) {
-		return util.JSONResponse{
-			Code: http.StatusBadRequest,
-			JSON: jsonerror.ASExclusive("This username is reserved by an application service."),
-		}
-	}
-
 	logger := util.GetLogger(req.Context())
 	logger.WithFields(log.Fields{
 		"username":   r.Username,
@@ -581,11 +588,39 @@ func handleRegistrationFlow(
 	// TODO: Handle mapping registrationRequest parameters into session parameters
 
 	// TODO: email / msisdn auth types.
+	accessToken, accessTokenErr := auth.ExtractAccessToken(req)
+	if accessTokenErr != nil {
+		return util.JSONResponse{
+			Code: http.StatusUnauthorized,
+			JSON: jsonerror.MissingToken(accessTokenErr.Error()),
+		}
+	}
+
+	// Appservices are special and are not affected by disabled
+	// registration or user exclusivity.
+	if r.Auth.Type == authtypes.LoginTypeApplicationService ||
+		(r.Auth.Type == "" && accessTokenErr == nil) {
+		return handleApplicationServiceRegistration(
+			accessToken, accessTokenErr, req, r, cfg, userAPI,
+		)
+	}
 
 	if cfg.RegistrationDisabled && r.Auth.Type != authtypes.LoginTypeSharedSecret {
 		return util.MessageResponse(http.StatusForbidden, "Registration has been disabled")
 	}
 
+	// Make sure normal user isn't registering under an exclusive application
+	// service namespace. Skip this check if no app services are registered.
+	// If an access token is provided, ignore this check this is an appservice
+	// request and we will validate in validateApplicationService
+	if len(cfg.Derived.ApplicationServices) != 0 &&
+		UsernameMatchesExclusiveNamespaces(cfg, r.Username) {
+		return util.JSONResponse{
+			Code: http.StatusBadRequest,
+			JSON: jsonerror.ASExclusive("This username is reserved by an application service."),
+		}
+	}
+
 	switch r.Auth.Type {
 	case authtypes.LoginTypeRecaptcha:
 		// Check given captcha response
@@ -611,36 +646,15 @@ func handleRegistrationFlow(
 		// Add SharedSecret to the list of completed registration stages
 		AddCompletedSessionStage(sessionID, authtypes.LoginTypeSharedSecret)
 
-	case "":
-		// Extract the access token from the request, if there's one to extract
-		// (which we can know by checking whether the error is nil or not).
-		accessToken, err := auth.ExtractAccessToken(req)
-
-		// A missing auth type can mean either the registration is performed by
-		// an AS or the request is made as the first step of a registration
-		// using the User-Interactive Authentication API. This can be determined
-		// by whether the request contains an access token.
-		if err == nil {
-			return handleApplicationServiceRegistration(
-				accessToken, err, req, r, cfg, userAPI,
-			)
-		}
-
-	case authtypes.LoginTypeApplicationService:
-		// Extract the access token from the request.
-		accessToken, err := auth.ExtractAccessToken(req)
-		// Let the AS registration handler handle the process from here. We
-		// don't need a condition on that call since the registration is clearly
-		// stated as being AS-related.
-		return handleApplicationServiceRegistration(
-			accessToken, err, req, r, cfg, userAPI,
-		)
-
 	case authtypes.LoginTypeDummy:
 		// there is nothing to do
 		// Add Dummy to the list of completed registration stages
 		AddCompletedSessionStage(sessionID, authtypes.LoginTypeDummy)
 
+	case "":
+		// An empty auth type means that we want to fetch the available
+		// flows. It can also mean that we want to register as an appservice
+		// but that is handed above.
 	default:
 		return util.JSONResponse{
 			Code: http.StatusNotImplemented,