Workaround: keep the cache we used to have but clear all cache for membergroups when a member group changes

src/Umbraco.Core/Persistence/Repositories/MemberGroupRepository.cs

@@ -131,17 +131,18 @@ namespace Umbraco.Core.Persistence.Repositories
 
         public IMemberGroup GetByName(string name)
         {
-            // cannot use name in the cache key, 'cos when clearing the cache for a given
-            // member group that has been renamed, we clear the new name but not the old
-            // name - it just cannot work - and we cannot use the dirty props 'cos it won't
-            // work on distributed cache - and besides, we end up caching both by name and
-            // by id
-            // disabling the cache entirely for now - if we want to cache we'd need to
-            // implement some sort of cross-reference name/id cache
-
-            var qry = new Query<IMemberGroup>().Where(group => group.Name.Equals(name));
-            var result = GetByQuery(qry);
-            return result.FirstOrDefault();
+            return IsolatedCache.GetCacheItem<IMemberGroup>(
+                string.Format("{0}.{1}", typeof(IMemberGroup).FullName, name),
+                () =>
+                {
+                    var qry = new Query<IMemberGroup>().Where(group => group.Name.Equals(name));
+                    var result = GetByQuery(qry);
+                    return result.FirstOrDefault();
+                },
+                //cache for 5 mins since that is the default in the RuntimeCacheProvider
+                TimeSpan.FromMinutes(5),
+                //sliding is true
+                true);
         }
 
         public IMemberGroup CreateIfNotExists(string roleName)

src/Umbraco.Web/Cache/MemberGroupCacheRefresher.cs

@@ -1,6 +1,6 @@
 using System;
 using System.Linq;
-using System.Web.Script.Serialization;
+using System.Web;
 using Newtonsoft.Json;
 using Umbraco.Core;
 using Umbraco.Core.Cache;
@@ -83,35 +83,28 @@ namespace Umbraco.Web.Cache
 
         public override void Refresh(string jsonPayload)
         {
-            ClearCache(DeserializeFromJsonPayload(jsonPayload));
+            ClearCache();
             base.Refresh(jsonPayload);
         }
 
         public override void Refresh(int id)
         {
-            ClearCache(FromMemberGroup(ApplicationContext.Current.Services.MemberGroupService.GetById(id)));
+            ClearCache();
             base.Refresh(id);
         }
 
         public override void Remove(int id)
         {
-            ClearCache(FromMemberGroup(ApplicationContext.Current.Services.MemberGroupService.GetById(id)));
+            ClearCache();
             base.Remove(id);
         }
 
-        private void ClearCache(params JsonPayload[] payloads)
+        private void ClearCache()
         {
-            if (payloads == null) return;
-
-            var memberGroupCache = ApplicationContext.Current.ApplicationCache.IsolatedRuntimeCache.GetCache<IMemberGroup>();
-            payloads.ForEach(payload =>
-            {
-                if (payload != null && memberGroupCache)
-                {
-                    memberGroupCache.Result.ClearCacheItem(RepositoryBase.GetCacheIdKey<IMemberGroup>(payload.Id));
-                }
-            });
-
+            // Since we cache by group name, it could be problematic when renaming to
+            // previously existing names - see http://issues.umbraco.org/issue/U4-10846.
+            // To work around this, just clear all the cache items
+            ApplicationContext.Current.ApplicationCache.IsolatedRuntimeCache.ClearCache<IMemberGroup>();
         }
     }
 }