[Neo VM Bug] Fix negative counter issue

src/Neo.VM/ReferenceCounter.cs

@@ -115,11 +115,11 @@ internal int CheckZeroReferred()
                             tracked_items.Remove(item);
                             if (item is CompoundType compound)
                             {
-                                references_count -= compound.SubItemsCount;
                                 foreach (StackItem subitem in compound.SubItems)
                                 {
                                     if (component.Contains(subitem)) continue;
                                     if (!NeedTrack(subitem)) continue;
+                                    references_count--;
                                     subitem.ObjectReferences!.Remove(compound);
                                 }
                             }

tests/Neo.VM.Tests/UT_ReferenceCounter.cs

@@ -12,6 +12,8 @@
 using Microsoft.VisualStudio.TestTools.UnitTesting;
 using Neo.VM;
 using Neo.VM.Types;
+using System;
+using Array = Neo.VM.Types.Array;
 
 namespace Neo.Test
 {
@@ -240,5 +242,40 @@ public void TestArrayNoPush()
             Assert.AreEqual(VMState.HALT, engine.Execute());
             Assert.AreEqual(array.Count, engine.ReferenceCounter.Count);
         }
+
+        [TestMethod]
+        public void TestNegativeReferenceCounter()
+        {
+            var referenceCounter = new ReferenceCounter();
+            var array = new Array(referenceCounter) { OnStack = true };
+            referenceCounter.AddStackReference(array);
+            for (var i = 0; i < 100; i++)
+            {
+                var item = new Integer(i) { OnStack = true };
+                array.Add(item);
+            }
+
+            Assert.AreEqual(101, referenceCounter.Count);
+            referenceCounter.CheckZeroReferred();
+            Assert.AreEqual(101, referenceCounter.Count);
+            var copyArray = array.DeepCopy(true);
+            Assert.AreEqual(201, referenceCounter.Count);
+            referenceCounter.CheckZeroReferred(); // Deepcopied value will be removed from the reference counter.
+            Assert.AreEqual(201, referenceCounter.Count);
+            // If we add the deepcopied value again, the reference counter will increase by only 1.
+            referenceCounter.AddStackReference(copyArray);
+            Assert.AreEqual(202, referenceCounter.Count);
+            referenceCounter.RemoveStackReference(copyArray);
+            // Removing the deepcopied value will decrease the reference counter by 101.
+            referenceCounter.CheckZeroReferred();
+            Assert.AreEqual(201, referenceCounter.Count);
+
+            // Bellow will trigger the negative reference counter exception
+            referenceCounter.AddStackReference(copyArray);
+            Assert.AreEqual(202, referenceCounter.Count);
+            referenceCounter.RemoveStackReference(copyArray);
+            referenceCounter.CheckZeroReferred();
+            Assert.AreEqual(201, referenceCounter.Count);
+        }
     }
 }