MM-42201: Enforce group is not the same as username. (#19660)

* MM-42201: Enforce group is not the same as username.

* MM-42201: Adjust test for change to translation id.

app/user.go

@@ -219,6 +219,11 @@ func (a *App) CreateGuest(c *request.Context, user *model.User) (*model.User, *m
 }
 
 func (a *App) createUserOrGuest(c *request.Context, user *model.User, guest bool) (*model.User, *model.AppError) {
+	if err := a.isUniqueToGroupNames(user.Username); err != nil {
+		err.Where = "createUserOrGuest"
+		return nil, err
+	}
+
 	ruser, nErr := a.ch.srv.userService.CreateUser(user, users.UserCreateOptions{Guest: guest})
 	if nErr != nil {
 		var appErr *model.AppError
@@ -1053,6 +1058,21 @@ func (a *App) sendUpdatedUserEvent(user model.User) {
 	a.Publish(sourceUserMessage)
 }
 
+func (a *App) isUniqueToGroupNames(val string) *model.AppError {
+	if val == "" {
+		return nil
+	}
+	var notFoundErr *store.ErrNotFound
+	group, err := a.Srv().Store.Group().GetByName(val, model.GroupSearchOpts{})
+	if err != nil && !errors.As(err, &notFoundErr) {
+		return model.NewAppError("", "app.user.get_by_name_failure", nil, err.Error(), http.StatusInternalServerError)
+	}
+	if group != nil {
+		return model.NewAppError("", "app.user.group_name_conflict", nil, "", http.StatusBadRequest)
+	}
+	return nil
+}
+
 func (a *App) UpdateUser(user *model.User, sendNotifications bool) (*model.User, *model.AppError) {
 	prev, err := a.ch.srv.userService.GetUser(user.Id)
 	if err != nil {
@@ -1065,6 +1085,13 @@ func (a *App) UpdateUser(user *model.User, sendNotifications bool) (*model.User,
 		}
 	}
 
+	if user.Username != prev.Username {
+		if err := a.isUniqueToGroupNames(user.Username); err != nil {
+			err.Where = "UpdateUser"
+			return nil, err
+		}
+	}
+
 	var newEmail string
 	if user.Email != prev.Email {
 		if !users.CheckUserDomain(user, *a.Config().TeamSettings.RestrictCreationToDomains) {