adds tests for caching exceptions and only caching one time and fixes HttpRequestCacheProvider to work with them.

src/Umbraco.Core/Cache/HttpRequestCacheProvider.cs

@@ -2,6 +2,7 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading;
 using System.Web;
 
 namespace Umbraco.Core.Cache
@@ -105,7 +106,7 @@ namespace Umbraco.Core.Cache
                 
                 if (result == null || GetSafeLazyValue(result, true) == null) // get non-created as NonCreatedValue & exceptions as null
                 {
-                    result = new Lazy<object>(getCacheItem);
+                    result = new Lazy<object>(getCacheItem, LazyThreadSafetyMode.PublicationOnly);
                     ContextItems[cacheKey] = result;
                 }
             }

src/Umbraco.Tests/Cache/CacheProviderTests.cs

@@ -1,4 +1,5 @@
-using System.Linq;
+using System;
+using System.Linq;
 using System.Web.UI;
 using NUnit.Framework;
 using Umbraco.Core.Cache;
@@ -23,6 +24,59 @@ namespace Umbraco.Tests.Cache
             Provider.ClearAllCache();
         }
 
+        [Test]
+        public void Does_Not_Cache_Exceptions()
+        {
+            var counter = 0;
+
+            object result;
+            try
+            {
+                result = Provider.GetCacheItem("Blah", () =>
+                    {
+                        counter++;
+                        throw new Exception("Do not cache this");
+                    });
+            }
+            catch (Exception){}
+
+            try
+            {
+                result = Provider.GetCacheItem("Blah", () =>
+                {
+                    counter++;
+                    throw new Exception("Do not cache this");
+                });
+            }
+            catch (Exception){}
+
+            Assert.Greater(counter, 1);
+
+        }
+
+        [Test]
+        public void Ensures_Delegate_Result_Is_Cached_Once()
+        {
+            var counter = 0;
+
+            object result;
+            
+            result = Provider.GetCacheItem("Blah", () =>
+            {
+                counter++;
+                return "";
+            });
+
+            result = Provider.GetCacheItem("Blah", () =>
+            {
+                counter++;
+                return "";
+            });
+
+            Assert.AreEqual(counter, 1);
+
+        }
+
         [Test]
         public void Can_Get_By_Search()
         {