soak lfu add + fix detached nodes

BitFaster.Caching.UnitTests/Lfu/ConcurrentLfuSoakTests.cs

@@ -108,6 +108,29 @@ await Threaded.RunAsync(4, async () => {
             scheduler.Dispose();
             await scheduler.Completion;
 
+            RunIntegrityCheck(lfu);
+        }
+
+        [Theory]
+        [Repeat(iterations)]
+        public async Task WhenConcurrentGetAndUpdateCacheEndsInConsistentState(int iteration)
+        {
+            var scheduler = new BackgroundThreadScheduler();
+            var lfu = new ConcurrentLfuBuilder<int, string>().WithCapacity(9).WithScheduler(scheduler).Build() as ConcurrentLfu<int, string>;
+
+            await Threaded.Run(4, () => {
+                for (int i = 0; i < 100000; i++)
+                {
+                    lfu.TryUpdate(i + 1, i.ToString());
+                    lfu.GetOrAdd(i + 1, i => i.ToString());
+                }
+            });
+
+            this.output.WriteLine($"iteration {iteration} keys={string.Join(" ", lfu.Keys)}");
+
+            scheduler.Dispose();
+            await scheduler.Completion;
+
             RunIntegrityCheck(lfu);
         }
 

BitFaster.Caching/Lfu/ConcurrentLfu.cs

@@ -691,7 +691,13 @@ private void OnWrite(LfuNode<K, V> node)
         }
 
         private void PromoteProbation(LfuNode<K, V> node)
-        {
+        {
+            if (node.list == null)
+            {
+                // Ignore stale accesses for an entry that is no longer present
+                return;
+            }
+
             this.probationLru.Remove(node);
             this.protectedLru.AddLast(node);
             node.Position = Position.Protected;