fix: cast messagesize as uint (credit: kaen / #1592)

com.unity.netcode.gameobjects/CHANGELOG.md

@@ -16,6 +16,7 @@ Additional documentation and release notes are available at [Multiplayer Documen
 
 - Disallowed async keyword in RPCs (#1681)
 - Fixed an issue where Alpha release versions of Unity (version 20202.2.0a5 and later) will not compile due to the UNet Transport no longer existing (#1678)
+- Fixed messages larger than 64k being written with incorrectly truncated message size in header (#1686) (credit: @kaen)
 - Fixed overloading RPC methods causing collisions and failing on IL2CPP targets. (#1694)
 - Fixed spawn flow to propagate `IsSceneObject` down to children NetworkObjects, decouple implicit relationship between object spawning & `IsSceneObject` flag (#1685)
 

com.unity.netcode.gameobjects/Runtime/Messaging/MessagingSystem.cs

@@ -377,7 +377,7 @@ internal unsafe int SendPreSerializedMessage<TMessageType>(in FastBufferWriter t
 
             var header = new MessageHeader
             {
-                MessageSize = (ushort)tmpSerializer.Length,
+                MessageSize = (uint)tmpSerializer.Length,
                 MessageType = m_MessageTypes[typeof(TMessageType)],
             };
             BytePacker.WriteValueBitPacked(headerSerializer, header.MessageType);

com.unity.netcode.gameobjects/Tests/Runtime/NetworkObject/NetworkObjectSpawnManyObjectsTests.cs

@@ -0,0 +1,59 @@
+using System.Collections;
+using NUnit.Framework;
+using UnityEngine;
+using UnityEngine.TestTools;
+
+namespace Unity.Netcode.RuntimeTests
+{
+    public class NetworkObjectSpawnManyObjectsTests
+    {
+        // "many" in this case means enough to exceed a ushort_max message size written in the header
+        // 1500 is not a magic number except that it's big enough to trigger a failure
+        private const int k_SpawnedObjects = 1500;
+
+        [UnityTest]
+        // When this test fails it does so without an exception and will wait the default ~6 minutes
+        [Timeout(10000)]
+        public IEnumerator WhenManyObjectsAreSpawnedAtOnce_AllAreReceived()
+        {
+            MultiInstanceHelpers.Create(1, out NetworkManager server, out NetworkManager[] clients);
+
+            // create prefab
+            var gameObject = new GameObject("TestObject");
+            var networkObject = gameObject.AddComponent<NetworkObject>();
+            MultiInstanceHelpers.MakeNetworkObjectTestPrefab(networkObject);
+
+            server.NetworkConfig.NetworkPrefabs.Add(new NetworkPrefab()
+            {
+                Prefab = gameObject
+            });
+
+            for (int i = 0; i < clients.Length; i++)
+            {
+                clients[i].NetworkConfig.NetworkPrefabs.Add(new NetworkPrefab()
+                {
+                    Prefab = gameObject
+                });
+            }
+
+            MultiInstanceHelpers.Start(false, server, clients);
+
+            for (int i = 0; i < k_SpawnedObjects; i++)
+            {
+                NetworkObject serverObject = Object.Instantiate(gameObject).GetComponent<NetworkObject>();
+                serverObject.NetworkManagerOwner = server;
+                serverObject.Spawn();
+            }
+
+            // wait for connection on client side
+            yield return MultiInstanceHelpers.Run(MultiInstanceHelpers.WaitForClientsConnected(clients, null, 10));
+
+            // wait for connection on server side
+            yield return MultiInstanceHelpers.Run(MultiInstanceHelpers.WaitForClientConnectedToServer(server, null, 10));
+
+            // ensure all objects are replicated
+            Assert.AreEqual(k_SpawnedObjects, clients[0].SpawnManager.SpawnedObjectsList.Count);
+            MultiInstanceHelpers.Destroy();
+        }
+    }
+}