Lru OnRemove locks value instead of item (#442)

* handle null

* lfu test

---------

Author: bitfaster

BitFaster.Caching.UnitTests/Lfu/ConcurrentLfuTests.cs

@@ -672,6 +672,16 @@ public void WhenRemovedInWriteBuffer()
         public void WhenItemDoesNotExistTryUpdateIsFalse()
         {
             cache.TryUpdate(1, 2).Should().BeFalse();
+        }
+
+        [Fact]
+        public void WhenAddingNullValueCanBeAddedAndRemoved()
+        {
+            // use foreground so that any null ref exceptions will surface
+            var lfu = new ConcurrentLfu<int, string>(1, 20, new ForegroundScheduler(), EqualityComparer<int>.Default);
+            lfu.GetOrAdd(1, _ => null).Should().BeNull();
+            lfu.AddOrUpdate(1, null);
+            lfu.TryRemove(1).Should().BeTrue();
         }
 
         [Fact]

BitFaster.Caching.UnitTests/Lru/ConcurrentLruTests.cs

@@ -574,6 +574,13 @@ public void WhenValueExpiresItIsDisposed()
             disposableValueFactory.Items[6].IsDisposed.Should().BeFalse();
         }
 
+        [Fact]
+        public void WhenAddingNullValueCanBeAddedAndRemoved()
+        {
+            lru.GetOrAdd(1, _ => null).Should().BeNull();
+            lru.AddOrUpdate(1, null);
+            lru.TryRemove(1).Should().BeTrue();
+        }
 
         [Fact]
         public void WhenValueEvictedItemRemovedEventIsFired()

BitFaster.Caching/Lru/ConcurrentLruCore.cs

@@ -359,7 +359,7 @@ private void OnRemove(K key, I item)
             this.telemetryPolicy.OnItemRemoved(key, item.Value, ItemRemovedReason.Removed);
 
             // serialize dispose (common case dispose not thread safe)
-            lock (item.Value)
+            lock (item)
             {
                 Disposer<V>.Dispose(item.Value);
             }