U4-6597 - fix issue with caches that would cache exceptions

src/Umbraco.Core/Cache/HttpRuntimeCacheProvider.cs

@@ -129,7 +129,7 @@ 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 = GetSafeLazy(getCacheItem);
                     var absolute = isSliding ? System.Web.Caching.Cache.NoAbsoluteExpiration : (timeout == null ? System.Web.Caching.Cache.NoAbsoluteExpiration : DateTime.Now.Add(timeout.Value));
                     var sliding = isSliding == false ? System.Web.Caching.Cache.NoSlidingExpiration : (timeout ?? System.Web.Caching.Cache.NoSlidingExpiration);
 
@@ -138,10 +138,17 @@ namespace Umbraco.Core.Cache
                 }
             }
 
-            // this may throw if getCacheItem throws, but this is the only place where
-            // it would throw as everywhere else we use GetLazySaveValue() to hide exceptions
-            // and pretend exceptions were never inserted into cache to begin with.
-            return result.Value;
+            // using GetSafeLazy and GetSafeLazyValue ensures that we don't cache
+            // exceptions (but try again and again) and silently eat them - however at
+            // some point we have to report them - so need to re-throw here
+
+            // this does not throw anymore
+            //return result.Value;
+
+            value = result.Value; // will not throw (safe lazy)
+            var eh = value as ExceptionHolder;
+            if (eh != null) throw eh.Exception; // throw once!
+            return value;
         }
 
         public object GetCacheItem(string cacheKey, Func<object> getCacheItem, TimeSpan? timeout, bool isSliding = false, CacheItemPriority priority = CacheItemPriority.Normal, CacheItemRemovedCallback removedCallback = null, string[] dependentFiles = null)
@@ -173,7 +180,7 @@ namespace Umbraco.Core.Cache
             // NOTE - here also we must insert a Lazy<object> but we can evaluate it right now
             // and make sure we don't store a null value.
 
-            var result = new Lazy<object>(getCacheItem);
+            var result = GetSafeLazy(getCacheItem);
             var value = result.Value; // force evaluation now - this may throw if cacheItem throws, and then nothing goes into cache
             if (value == null) return; // do not store null values (backward compat)