Spawning prefab from in-scene NetworkObject results in "an object with that id is already in the spawned list" warning and "Size mismatch" warning

[zachstronaut]

Description

I additively load a scene (and I am using Netcode's scene management). In that scene file there is a GameObject at the root with a NetworkObject and this script:

using UnityEngine;
using Unity.Netcode;
using UnityEngine.SceneManagement;

public class WarnTest : NetworkBehaviour
{
 public NetworkObject spawnPrefab;

 protected override void OnNetworkPostSpawn() {
 if (NetworkManager.IsServer)
 {
 NetworkObject childNetObj = GameObject.Instantiate(spawnPrefab, transform.position, transform.rotation);
 SceneManager.MoveGameObjectToScene(childNetObj.gameObject, gameObject.scene);
 childNetObj.Spawn(destroyWithScene: true);
 childNetObj.TrySetParent(transform);
 }
 }
}

The prefab I spawn is the simplest case... just a prefab with a NetworkObject. This prefab is in a manually created network prefab list that I have set in my NetworkManager.

This results in two warnings:

[Netcode] Trying to spawn a NetworkObject with a NetworkObjectId of 109 but an object with that id is already in the spawned list. This should not happen!

[Netcode] [null][Deserialize][NetworkBehaviourSynchronization][Size mismatch] Expected: 122 Currently At: 121!

(Please note that the second warning will cause a NullException without a patch fix applied for #4071)

Am I forgetting some rule about parenting or about spawning? What is the issue here? I'm not seeing my mistake in the docs.

Environment

• OS: Windows
• Unity Version: 6000.3.16f1
• Netcode Version: 2.12.0 (although certainly not a new problem to this version)
• Netcode Topology: Client-Server

[NoelStephensUnity]

@zachstronaut

The only thing I could see that might be going on with those messages is the order in which you are migrating the instance into the scene and then spawning it. You might try spawning it and then migrating it into the scene you want (make sure scene migration synchronization is enabled on the prefab's NetworkObject).

If that does not resolve the issue, then the fixes in the recently landed PRs #4086 and/or #4073 --might-- be needed to "fully" resolve your issue (this will be v2.13.1). We found a few additional issues with scene migration that could be contributing to that issue over-all.

Those two PRs landed late today and so you could also try replacing your manifest.json entry for com.unity.netcode.gameobjects with this:

"com.unity.netcode.gameobjects": "https://github.com/Unity-Technologies/com.unity.netcode.gameobjects.git?path=com.unity.netcode.gameobjects#develop-2.0.0",

Let me know if any of the above resolves the issue and/or if you try v2.13.1 it could provide logs that would yield better insights as to what the issue could be.

[zachstronaut]

@NoelStephensUnity I haven't tried that new version yet, but I can tell you that putting MoveGameObjectToScene after the Spawn works as long as I don't also TrySetParent ... if you call TrySetParent too then the warnings come back.

Incidentally, wouldn't setting the parent already change the scene in this case? You do still get the warnings if you only TrySetParent and skip the MoveGameObjectToScene, though.

[zachstronaut]

@NoelStephensUnity I'm not able to test my actual project against develop-2.0.0 because of a new NullReferenceException during a ServerRPC we call from inside a OnClientConnectedCallback that wasn't happening before:

NullReferenceException: Object reference not set to an instance of an object
Unity.Netcode.NetworkBehaviour.__endSendServerRpc (Unity.Netcode.FastBufferWriter& bufferWriter, System.UInt32 rpcMethodId, Unity.Netcode.ServerRpcParams serverRpcParams, Unity.Netcode.RpcDelivery rpcDelivery) (at ./Packages/com.unity.netcode.gameobjects/Runtime/Core/NetworkBehaviour.cs:143)

Looks like m_NetworkManager is null, but also this error is thrown on the host/server so I'm not sure how IsServer isn't true? How is this line even executing on the host?

Perhaps you've got a regression in develop-2.0.0 around something here.

[NoelStephensUnity]

I am putting together a replication project and am missing the context of what is invoking the ServerRpc.
Is this being invoked on something else, the GameObject at the root that spawns the child, or the child you are instantiating, spawning, and moving into a different scene?

Also, is this ServerRpc being invoked by the host/server itself or a client joining?