Fix #13 KeyNotFoundException by using ConcurrentDictionary.GetOrAdd i…

…nstead of indexer

src/JSPool/JsPool.cs

@@ -33,7 +33,7 @@ public class JsPool<TPooled, TOriginal> : IJsPool<TPooled> where TPooled : Poole
 		/// <summary>
 		/// Metadata for the engines. Total number of engines that have been created is reflected in its Count.
 		/// </summary>
-		protected readonly IDictionary<TPooled, EngineMetadata> _metadata = new ConcurrentDictionary<TPooled, EngineMetadata>();
+		protected readonly ConcurrentDictionary<TPooled, EngineMetadata> _metadata = new ConcurrentDictionary<TPooled, EngineMetadata>();
 		/// <summary>
 		/// Factory method used to create engines.
 		/// </summary>
@@ -179,8 +179,9 @@ public virtual TPooled GetEngine(TimeSpan? timeout = null)
 		/// <param name="engine"></param>
 		private TPooled TakeEngine(TPooled engine)
 		{
-			var metadata = _metadata[engine];
-			metadata.InUse = true;
+		    var metadata = _metadata.GetOrAdd(engine, _ => new EngineMetadata());
+
+            metadata.InUse = true;
 			metadata.UsageCount++;
 			return engine;
 		}
@@ -247,7 +248,7 @@ public virtual void DisposeEngine(TPooled engine, bool repopulateEngines = true)
 			{
 				((IDisposable)engine.InnerEngine).Dispose();
 			}
-			_metadata.Remove(engine);
+			_metadata.TryRemove(engine, out var _);
 
 			if (repopulateEngines)
 			{