fix(api/custom-templates): prevent name collisions on update (#4250)

api/http/handler/customtemplates/customtemplate_update.go

@@ -57,6 +57,17 @@ func (handler *Handler) customTemplateUpdate(w http.ResponseWriter, r *http.Requ
 		return &httperror.HandlerError{http.StatusBadRequest, "Invalid request payload", err}
 	}
 
+	customTemplates, err := handler.DataStore.CustomTemplate().CustomTemplates()
+	if err != nil {
+		return &httperror.HandlerError{http.StatusInternalServerError, "Unable to retrieve custom templates from the database", err}
+	}
+
+	for _, existingTemplate := range customTemplates {
+		if existingTemplate.ID != portainer.CustomTemplateID(customTemplateID) && existingTemplate.Title == payload.Title {
+			return &httperror.HandlerError{http.StatusInternalServerError, "Template name must be unique", errors.New("Template name must be unique")}
+		}
+	}
+
 	customTemplate, err := handler.DataStore.CustomTemplate().CustomTemplate(portainer.CustomTemplateID(customTemplateID))
 	if err == bolterrors.ErrObjectNotFound {
 		return &httperror.HandlerError{http.StatusNotFound, "Unable to find a custom template with the specified identifier inside the database", err}