Prevented creation of circular fall-back language paths. Allowed for update to no fall-back language.

src/Umbraco.Web.UI.Client/src/views/languages/edit.controller.js

@@ -112,6 +112,11 @@
             if (formHelper.submitForm({ scope: $scope })) {
                 vm.page.saveButtonState = "busy";
 
+                // Handle selection of no fall-back language (should pass null)
+                if (!vm.language.fallbackLanguage.id) {
+                    vm.language.fallbackLanguage = null;
+                }
+
                 // We need to attach the ISO code to the fall-back language to pass
                 // server-side validation.
                 if (vm.language.fallbackLanguage) {

src/Umbraco.Web/Editors/LanguageController.cs

@@ -148,6 +148,13 @@ namespace Umbraco.Web.Editors
             found.Mandatory = language.Mandatory;
             found.IsDefaultVariantLanguage = language.IsDefaultVariantLanguage;
             AssociateFallbackLanguage(language, found);
+
+            if (UpdatedFallbackLanguageCreatesCircularPath(found))
+            {
+                ModelState.AddModelError("FallbackLanguage", "The selected fall back language '" + found.FallbackLanguage.CultureName + "' would create a circular path.");
+                throw new HttpResponseException(Request.CreateValidationErrorResponse(ModelState));
+            }
+
             Services.LocalizationService.Save(found);
             return Mapper.Map<Language>(found);
         }
@@ -156,6 +163,7 @@ namespace Umbraco.Web.Editors
         {
             if (submittedLanguage.FallbackLanguage == null)
             {
+                languageToCreateOrUpdate.FallbackLanguage = null;
                 return;
             }
 
@@ -166,5 +174,27 @@ namespace Umbraco.Web.Editors
                     CultureName = fallbackLanguageCulture.DisplayName
                 };
         }
+
+        private bool UpdatedFallbackLanguageCreatesCircularPath(ILanguage language)
+        {
+            if (language.FallbackLanguage == null)
+            {
+                return false;
+            }
+
+            var languages = Services.LocalizationService.GetAllLanguages().ToArray();
+            var fallbackLanguage = language.FallbackLanguage;
+            while (fallbackLanguage != null)
+            {
+                if (fallbackLanguage.Id == language.Id)
+                {
+                    return true;
+                }
+
+                fallbackLanguage = languages.Single(x => x.Id == fallbackLanguage.Id).FallbackLanguage;
+            }
+
+            return false;
+        }
     }
 }