Fix mark-sweep RC semantics and add equivalence tests

Author: Neo Dev

src/Neo.VM/MarkSweepReferenceCounter.cs

@@ -105,8 +105,7 @@ public void RemoveStackReference(StackItem item)
 
     private void Track(StackItem item)
     {
-        if (trackedItems.Add(item))
-            zeroReferred.Add(item);
+        trackedItems.Add(item);
     }
 
     private void Collect()
@@ -120,31 +119,37 @@ private void Collect()
                 Mark(item);
         }
 
+        // Legacy Tarjan RC clears zero-referred up-front and then removes every
+        // unmarked tracked item. To preserve identical semantics (and avoid
+        // keeping unreachable descendants that were never re-added to zeroReferred),
+        // we do the same here.
+        zeroReferred.Clear();
+
         foreach (var item in trackedItems)
         {
-            if (!marked.Contains(item) && zeroReferred.Contains(item))
+            if (!marked.Contains(item))
                 unreachable.Add(item);
         }
 
         foreach (var item in unreachable)
             RemoveTracked(item);
-
-        zeroReferred.RemoveWhere(marked.Contains);
     }
 
     private void Mark(StackItem root)
     {
         if (!marked.Add(root)) return;
         pending.Push(root);
+
         while (pending.Count > 0)
         {
             var current = pending.Pop();
-            if (current.ObjectReferences is null) continue;
-            foreach (var entry in current.ObjectReferences.Values)
+            if (current is not CompoundType compound) continue;
+
+            foreach (var child in compound.SubItems)
             {
-                if (entry.References <= 0) continue;
-                if (marked.Add(entry.Item))
-                    pending.Push(entry.Item);
+                if (!NeedTrack(child)) continue;
+                if (marked.Add(child))
+                    pending.Push(child);
             }
         }
     }

tests/Neo.VM.Tests/UT_MarkSweepReferenceCounter.cs

@@ -0,0 +1,42 @@
+// Copyright (C) 2015-2025 The Neo Project.
+//
+// UT_MarkSweepReferenceCounter.cs file belongs to the neo project and is free
+// software distributed under the MIT software license, see the
+// accompanying file LICENSE in the main directory of the
+// repository or http://www.opensource.org/licenses/mit-license.php
+// for more details.
+//
+// Redistribution and use in source and binary forms with or without
+// modifications are permitted.
+
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Neo.VM;
+using Neo.VM.Types;
+using Array = Neo.VM.Types.Array;
+
+namespace Neo.Test;
+
+[TestClass]
+public class UT_MarkSweepReferenceCounter
+{
+    [TestMethod]
+    public void TestReachableChildIsNotCollected()
+    {
+        var rc = new MarkSweepReferenceCounter();
+
+        var root = new Array(rc);
+        var child = new Array(rc);
+
+        rc.AddStackReference(root);
+        child.Add(1);
+        child.Add(2);
+        child.Add(3);
+
+        root.Add(child);
+
+        Assert.AreEqual(5, rc.Count);
+        rc.CheckZeroReferred();
+        Assert.AreEqual(5, rc.Count);
+    }
+}
+