Explicit exception on cache reentry

src/Umbraco.Core/Cache/DictionaryCacheProviderBase.cs

@@ -61,8 +61,15 @@ namespace Umbraco.Core.Cache
             if (lazy.Value is ExceptionHolder) return null;
 
             // we have a value and execution has not thrown so returning
-            // here does not throw
-            return lazy.Value;
+            // here does not throw - unless we're re-entering, take care of it
+            try
+            {
+                return lazy.Value;
+            }
+            catch (InvalidOperationException e)
+            {
+                throw new InvalidOperationException("The method that computes a value for the cache has tried to read that value from the cache.", e);
+            }
         }
 
         internal class ExceptionHolder

src/Umbraco.Tests/Cache/CacheProviderTests.cs

@@ -24,6 +24,30 @@ namespace Umbraco.Tests.Cache
             Provider.ClearAllCache();
         } 
 
+        [Test]
+        public void Throws_On_Reentry()
+        {
+            // don't run for StaticCacheProvider - not making sense
+            if (GetType() == typeof (StaticCacheProviderTests))
+                Assert.Ignore("Do not run for StaticCacheProvider.");
+
+            Exception exception = null;
+            var result = Provider.GetCacheItem("blah", () =>
+            {
+                try
+                {
+                    var result2 = Provider.GetCacheItem("blah");
+                }
+                catch (Exception e)
+                {
+                    exception = e;
+                }
+                return "value";
+            });
+            Assert.IsNotNull(exception);
+            Assert.IsAssignableFrom<InvalidOperationException>(exception);
+        }
+
         [Test]
         public void Does_Not_Cache_Exceptions()
         {