Fix leaking hocon objects due to multiple fallback injection in Actor…

…Materializer (#4672)

* Fix leaking hocon objects due to multiple fallback injection

* Have `Config` walk the fallback object Hocon graph to determine if it already contain the fallback values

src/core/Akka.Streams/ActorMaterializer.cs

@@ -74,7 +74,9 @@ public static ActorMaterializer Create(IActorRefFactory context, ActorMaterializ
         {
             var haveShutDown = new AtomicBoolean();
             var system = ActorSystemOf(context);
+
             system.Settings.InjectTopLevelFallback(DefaultConfig());
+
             settings = settings ?? ActorMaterializerSettings.Create(system);
 
             return new ActorMaterializerImpl(

src/core/Akka/Actor/Settings.cs

@@ -44,6 +44,9 @@ private void RebuildConfig()
         /// <param name="config">TBD</param>
         public void InjectTopLevelFallback(Config config)
         {
+            if (Config.Contains(config)) 
+                return;
+
             _fallbackConfig = config.SafeWithFallback(_fallbackConfig);
             RebuildConfig();
         }

src/core/Akka/Configuration/Config.cs

@@ -7,6 +7,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using Akka.Configuration.Hocon;
 using Akka.Util.Internal;
 
@@ -461,6 +462,9 @@ public virtual Config WithFallback(Config fallback)
             if (IsEmpty)
                 return fallback;
 
+            if (Contains(fallback))
+                return this;
+
             var mergedRoot = fallback.Root.GetObject().MergeImmutable(Root.GetObject());
             var newRoot = new HoconValue();
             newRoot.AppendValue(mergedRoot);
@@ -542,6 +546,42 @@ public virtual IEnumerable<KeyValuePair<string, HoconValue>> AsEnumerable()
         /// A static "Empty" configuration we can use instead of <c>null</c> in some key areas.
         /// </summary>
         public static readonly Config Empty = ConfigurationFactory.Empty;
+
+        internal bool Contains(Config other)
+            => Contains(other.Root.GetObject().Items, "");
+
+        private bool Contains(Dictionary<string, HoconValue> other, string path)
+        {
+            foreach (var kvp in other)
+            {
+                var currentPath = path == "" ? kvp.Key : $"{path}.\"{kvp.Key}\"";
+                if (!HasPath(currentPath))
+                    return false;
+
+                var value = kvp.Value;
+                if (value.IsObject())
+                {
+                    if (!Contains(value.GetObject().Items, currentPath))
+                        return false;
+                }
+                else if (value.IsArray())
+                {
+                    var list = GetStringList(currentPath);
+                    foreach (var str in value.GetArray().Select(v => v.GetString()))
+                    {
+                        if (!list.Contains(str))
+                            return false;
+                    }
+                }
+                else
+                {
+                    if (value.GetString() != GetString(currentPath))
+                        return false;
+                }
+            }
+
+            return true;
+        }
     }
 
     /// <summary>

src/core/Akka/Configuration/Hocon/HoconValue.cs

@@ -169,7 +169,7 @@ private string ConcatString()
         /// <returns>The element at the given key.</returns>
         public HoconValue GetChildObject(string key)
         {
-            return GetObject().GetKey(key);
+            return GetObject()?.GetKey(key);
         }
 
         /// <summary>
@@ -343,9 +343,9 @@ public IList<string> GetStringList()
         /// <returns>A list of values represented by this <see cref="HoconValue"/>.</returns>
         public IList<HoconValue> GetArray()
         {
-            IEnumerable<HoconValue> x = from arr in Values
-                where arr.IsArray()
-                from e in arr.GetArray()
+            IEnumerable<HoconValue> x = from element in Values
+                where element.IsArray()
+                from e in element.GetArray()
                 select e;
 
             return x.ToList();
@@ -359,7 +359,7 @@ from e in arr.GetArray()
         /// </returns>
         public bool IsArray()
         {
-            return GetArray() != null;
+            return GetArray().Count != 0;
         }
 
         /// <summary>