fix: NetworkList changes pending during client synchronization are applied twice

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -22,6 +22,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 ### Fixed
 
+- Issue where a client synchronized while a `NetworkList<T>` still had unsent changes applied those changes twice, resulting in duplicate entries and index-based events landing on the wrong elements on that client.
 - Issue where moving a `NetworkObject` into another scene made the clients that did not observe it log "Trying to synchronize NetworkObjectId but it was not spawned". The scene migration is now only sent to the clients that observe the `NetworkObject`. (#4185)
 - Issue where a `NetworkObject` that was moved into another scene while hidden from a client spawned in that client's active scene when it was shown with `NetworkShow`, instead of the scene it is in on the server. (#4185)
 - Issue where mixed authority nested `NetworkTransform` instances could stop child/nested instances from updating because an authoritative `NetworkTransform` (root or otherwise) would remove the `NetworkObject` from the update group, preventing non-authoritative instances from updating their state on the authority side. (#4169)

com.unity.netcode.gameobjects/Runtime/NetworkVariable/Collections/NetworkList.cs

@@ -16,6 +16,10 @@ public class NetworkList<T> : NetworkVariableBase where T : unmanaged, IEquatabl
         private NativeList<T> m_List = new NativeList<T>(64, Allocator.Persistent);
         private NativeList<NetworkListEvent<T>> m_DirtyEvents = new NativeList<NetworkListEvent<T>>(64, Allocator.Persistent);
 
+        // The list as it was before the first local change that is still pending in m_DirtyEvents
+        private NativeList<T> m_LastSentList = new NativeList<T>(64, Allocator.Persistent);
+        private bool m_HasLastSentList;
+
         /// <summary>
         /// Delegate type for list changed event
         /// </summary>
@@ -67,6 +71,7 @@ public override void ResetDirty()
             {
                 m_DirtyEvents.Clear();
             }
+            m_HasLastSentList = false;
         }
 
         /// <inheritdoc cref="NetworkVariable{T}.IsDirty"/>
@@ -149,10 +154,42 @@ private T ReadValue(FastBufferReader reader)
         /// <inheritdoc cref="NetworkVariable{T}.WriteField"/>
         public override void WriteField(FastBufferWriter writer)
         {
-            writer.WriteValueSafe((ushort)m_List.Length);
-            for (int i = 0; i < m_List.Length; i++)
+            WriteList(writer, m_List);
+        }
+
+        internal override void WriteFieldSynchronization(FastBufferWriter writer)
+        {
+            // The pending events are sent to the synchronizing client with the next delta, so it must
+            // receive the list they apply to. Otherwise it would apply them twice.
+            if (m_HasLastSentList && m_DirtyEvents.Length > 0)
+            {
+                WriteList(writer, m_LastSentList);
+            }
+            else
+            {
+                base.WriteFieldSynchronization(writer);
+            }
+        }
+
+        private static void WriteList(FastBufferWriter writer, NativeList<T> list)
+        {
+            writer.WriteValueSafe((ushort)list.Length);
+            for (int i = 0; i < list.Length; i++)
             {
-                NetworkVariableSerialization<T>.Serializer.Write(writer, ref m_List.ElementAt(i));
+                NetworkVariableSerialization<T>.Serializer.Write(writer, ref list.ElementAt(i));
+            }
+        }
+
+        /// <summary>
+        /// Invoked before a local change. Keeps a copy of the list as of the last sent delta
+        /// so <see cref="WriteFieldSynchronization"/> can send it while changes are pending.
+        /// </summary>
+        private void CaptureLastSentList()
+        {
+            if (m_DirtyEvents.Length == 0)
+            {
+                m_LastSentList.CopyFrom(m_List);
+                m_HasLastSentList = true;
             }
         }
 
@@ -439,6 +476,7 @@ public void Add(T item)
                 return;
             }
 
+            CaptureLastSentList();
             m_List.Add(item);
 
             var listEvent = new NetworkListEvent<T>()
@@ -466,6 +504,7 @@ public void Clear()
                 return;
             }
 
+            CaptureLastSentList();
             m_List.Clear();
 
             var listEvent = new NetworkListEvent<T>()
@@ -510,6 +549,7 @@ public bool Remove(T item)
                 return false;
             }
 
+            CaptureLastSentList();
             m_List.RemoveAt(index);
             var listEvent = new NetworkListEvent<T>()
             {
@@ -553,6 +593,7 @@ public void Insert(int index, T item)
                 return;
             }
 
+            CaptureLastSentList();
             if (index < m_List.Length)
             {
                 m_List.InsertRangeWithBeginEnd(index, index + 1);
@@ -589,6 +630,7 @@ public void RemoveAt(int index)
             }
 
             var value = m_List[index];
+            CaptureLastSentList();
             m_List.RemoveAt(index);
 
             var listEvent = new NetworkListEvent<T>()
@@ -631,6 +673,7 @@ public void Set(int index, T value, bool forceUpdate = false)
                 return;
             }
 
+            CaptureLastSentList();
             m_List[index] = value;
 
             var listEvent = new NetworkListEvent<T>()
@@ -706,6 +749,11 @@ public override void Dispose()
                 m_DirtyEvents.Dispose();
             }
 
+            if (m_LastSentList.IsCreated)
+            {
+                m_LastSentList.Dispose();
+            }
+
             base.Dispose();
         }
     }

com.unity.netcode.gameobjects/Tests/Runtime/NetworkVariable/NetworkListTests.cs

@@ -80,6 +80,52 @@ private bool OnVerifyData(StringBuilder errorLog)
             return true;
         }
 
+        /// <summary>
+        /// Validates that a client synchronized while the list still has unsent changes
+        /// does not apply those changes twice (snapshot + next delta).
+        /// </summary>
+        [UnityTest]
+        public IEnumerator LateJoinWithPendingChanges()
+        {
+            var authority = GetAuthorityNetworkManager();
+            var instantiatedObject = SpawnObject(m_ListObjectPrefab, authority).GetComponent<NetworkObject>();
+            m_TestObjectId = instantiatedObject.NetworkObjectId;
+
+            yield return WaitForSpawnedOnAllOrTimeOut(instantiatedObject);
+            AssertOnTimeout("[Setup] Failed to spawn list object");
+
+            var list = instantiatedObject.GetComponent<NetworkListTest>().TheList;
+            m_ExpectedValues.Clear();
+            for (int i = 0; i < 5; i++)
+            {
+                m_ExpectedValues.Add(i);
+                list.Add(i);
+            }
+
+            yield return WaitForConditionOrTimeOut(OnVerifyData);
+            AssertOnTimeout("[Setup] Failed to add items to list");
+
+            // Hold back deltas so these changes are still pending when the new client synchronizes
+            list.SetUpdateTraits(new NetworkVariableUpdateTraits { MinSecondsBetweenUpdates = 3600 });
+            list.Add(100);
+            m_ExpectedValues.Add(100);
+            list.Insert(0, 101);
+            m_ExpectedValues.Insert(0, 101);
+            list.RemoveAt(2);
+            m_ExpectedValues.RemoveAt(2);
+            list[1] = 102;
+            m_ExpectedValues[1] = 102;
+            list.Remove(3);
+            m_ExpectedValues.Remove(3);
+
+            yield return CreateAndStartNewClient();
+            Assert.IsTrue(list.IsDirty(), "[Setup] Changes should still be pending after the new client synchronized");
+
+            list.SetUpdateTraits(default);
+            yield return WaitForConditionOrTimeOut(OnVerifyData);
+            AssertOnTimeout("Late joined client did not match the authority's list");
+        }
+
         [UnityTest]
         public IEnumerator ValidateNetworkListSynchronization()
         {