Infinite hang when reloading a scene contains a manually spawned NetworkObject with a SpriteLibrary and a SpriteResolver

[acavailhez]

Description

Infinite hang when reloading a scene contains a manually spawned NetworkObject with a SpriteLibrary and a SpriteResolver

Reproduce Steps

1. Create an empty scene
2. Add a NetworkManager, with UnityTransport
3. Create a new prefab with NetworkObject, SpriteRenderer, Animator, SpriteLibrary, SpriteResolver (following basically this tutorial: https://www.youtube.com/watch?v=4xPHqEEa-V0)
4. Create an empty object TestCrash with the following code, and link the prefab from (3) in the inspector

public class TestCrash : NetworkBehaviour
{
    [SerializeField] public GameObject prefab = null!;
    private bool _alreadyCalled;

    private void Start()
    {
        var networkManager = NetworkManager.Singleton;
        var unityTransport = networkManager.GetComponent<UnityTransport>();

        // Start local server
        unityTransport.UseWebSockets = false; // if applicable
        unityTransport.SetConnectionData("0.0.0.0", 7777);
        networkManager!.NetworkConfig.NetworkTransport = unityTransport;
        networkManager.StartHost();

        NetworkManager.Singleton.SpawnManager.InstantiateAndSpawn(
            prefab!.GetComponent<NetworkObject>(),
            OwnerClientId,
            destroyWithScene: true,
            position: Vector3.zero
        );
    }

    private void Update()
    {
        if (_alreadyCalled) return;
        if (Time.timeSinceLevelLoad < 3f) return;
        _alreadyCalled = true;
       NetworkManager.Singleton.SceneManager.LoadScene(SceneManager.GetActiveScene().name, LoadSceneMode.Single);
    }
}

TestCrash is some code that

• starts a local server
• spawns the prefab
• waits 3 seconds
• reloads the scene

Actual Outcome

After a few attempts, the scene never reloads and the Unity editor hangs forever (has to be force quitted)

Expected Outcome

The scene reloads every 3s

Environment

• OS: Windows 11
• Unity Version: 6000.4.7f1
• Netcode Version: 2.12.0

Additional Context

• This bug did not happen with 2.12.0 and Unity 6000.3.11f1 (started happening after upgrading to 6000.4.7f1)
• The bug doesn't happen without destroyWithScene: true,
• Removing the SpriteResolver/SpriteLibrary stops the bug from happening
• The NetworkObject must be spawned with InstantiateAndSpawn, if the prefab is added to the scene, the bug doesn't happen

Here is a dump of the Main Thread stack when the editor is hanging. The CPU is at 0% at that point. Maybe a lock contention?

<unknown> 0x00007ff973d63cb4
<unknown> 0x00007ff973c30543
<unknown> 0x00007ff97107e558
UnityClassic::Baselib_SystemFutex_Wait(int * __ptr64,int,unsigned int) 0x00007ff8886af7f6
ProfilerMutexLock(Mutex & __ptr64,char const * __ptr64,profiling::Marker & __ptr64,char const * __ptr64,int) 0x00007ff88b1a4450
PersistentManager::Lock(PersistentManager::LockFlags,profiling::Marker * __ptr64) __ptr64 0x00007ff88b1a1b83
PersistentManager::GetPathName(EntityId) __ptr64 0x00007ff88b1968bb
GetGUIDFromEntityId(EntityId) 0x00007ff888e786fe
AssetChangedNotifier::UnsubscribeFromAll(EntityId) __ptr64 0x00007ff88a209aee
SpriteRenderer::MainThreadCleanup(void) __ptr64 0x00007ff88b631e42
delete_object_internal_step1(Object * __ptr64) 0x00007ff88b8cd380
CommitBatchDelete(BatchDelete & __ptr64) 0x00007ff88b5286d2
DestroyObjectHighLevel_Internal(Object * __ptr64,bool) 0x00007ff88b532e31
DestroyObjectHighLevel(Object * __ptr64,bool) 0x00007ff88b532ba8
DelayedDestroyCallback(Object * __ptr64,void * __ptr64) 0x00007ff88b7963cb
DelayedCallManager::Update(int) __ptr64 0x00007ff88b797ac8
<unknown> 0x00007ff88b4f9d6b
ExecutePlayerLoop(NativePlayerLoopSystem * __ptr64) 0x00007ff88b517f5b
ExecutePlayerLoop(NativePlayerLoopSystem * __ptr64) 0x00007ff88b51811a
PlayerLoop(void) 0x00007ff88b51d105
EditorPlayerLoop::Execute(void) __ptr64 0x00007ff889ab957a
PlayerLoopController::InternalUpdateScene(bool,bool) __ptr64 0x00007ff889ab2a9c
PlayerLoopController::UpdateSceneIfNeededFromMainLoop(void) __ptr64 0x00007ff889ab4dad
Application::TickTimer(void) __ptr64 0x00007ff889acc4c3
<unknown> 0x00007ff8894bfb2c
UnityMain 0x00007ff8894c8693
<unknown> 0x00007ff6ae972f2a
<unknown> 0x00007ff97360e957
<unknown> 0x00007ff973caad6c

[acavailhez]

Actually it seems like SpriteLibrary/SpriteResolver are not needed for the bug to happen, simply replacing the sprite triggers the bug:

var go = NetworkManager.Singleton.SpawnManager.InstantiateAndSpawn(
        prefab!.GetComponent<NetworkObject>(),
        OwnerClientId,
        destroyWithScene: true,
        position: Vector3.zero
    );
go.GetComponent<SpriteRenderer>.sprite = [other sprite]

[NoelStephensUnity]

Hi @acavailhez,

Recent post above:

Is this a typo?
go.GetComponent<SpriteRenderer>.sprite = [other sprite]

It should be this:
go.GetComponent<SpriteRenderer>().sprite = [other sprite]

Issue posted:

Issue-1: Destroy with scene on a spawned instance just prior to performing a LoadSceneMode.Single scene loading event

It looks like you are spawning a prefab instance in the currently active scene (where everything will be instantiated) and then immediately doing a scene loading event using LoadSceneMode.Single which will destroy anything in the currently active scene upon finishing the scene loading event (i.e. your spawned object is getting destroyed).

Issue-2: For integrated scene management, use the NetworkSceneManager completed events or OnSceneEvent

The supported way to know a netcode synchronized scene event is finished is to use the NetworkSceneManager events and not the UnityEngine.SceneManagement.SceneManager driven Time.timeSinceLevelLoad as that can lead to scenarios where UnityEngine.SceneManagement.SceneManager is finished but NetworkSceneManager is not.

Issue-3: Using a NetworkBehaviour to start the NetworkManager

We do not recommend using a NetworkBehaviour to start the NetworkManager and while it might work under certain conditions, it coudl also lead to unexpected results/issues. The recommend component to use for starting NetworkManager is to use a MonoBehaviour and I personally would recommend using the pattern of placing that on the same GameObject as the NetworkManager.

Resolution fix

Here is a quick modification of your script which should work.

using Unity.Multiplayer.PlayMode;
using Unity.Netcode;
using Unity.Netcode.Transports.UTP;
using UnityEngine;
using UnityEngine.SceneManagement;

/// <summary>
/// Add to your NetworkManager.
/// </summary>
/// <remarks>
/// Don't use a NetworkBehaviour to start NetworkManager as NetworkBehaviours
/// depend upon NetworkManager.
/// </remarks>
public class TestCrash : MonoBehaviour
{
    public bool DestroyInstanceWithScene = false;

    [SerializeField] public GameObject PrefabToSpawn = null!;
    private NetworkManager m_NetworkManager;

    /// <summary>
    /// Requires: Multiplayer Playmode Tool
    /// Will start a host or client based on the editor window instance type.
    /// </summary>
    /// <returns>true or false as to whether the NetworkManager started.</returns>
    private bool QuickStart()
    {
        var unityTransport = m_NetworkManager.GetComponent<UnityTransport>();

        // This should be pre-configured but ok to do this as well.
        unityTransport.UseWebSockets = false; // if applicable
        unityTransport.SetConnectionData("0.0.0.0", 7777);
        m_NetworkManager.NetworkConfig.NetworkTransport = unityTransport;

        // Main editor view always starts as host
        if (CurrentPlayer.IsMainEditor)
        {
            return m_NetworkManager.StartHost();

        }
        else // MPPM virtual clients start as a client
        {
            return m_NetworkManager.StartClient();
        }
    }

    private void Start()
    {
        m_NetworkManager = GetComponent<NetworkManager>();
        if (m_NetworkManager == null)
        {
            Debug.LogError($"No {nameof(NetworkManager)} found! Add this component to the same {nameof(GameObject)} as your {nameof(NetworkManager)}!");
            return;
        }
        if (!QuickStart())
        {
            Debug.Log($"Failed to start {nameof(NetworkManager)} on {gameObject.name}!");
            return;
        }

        // If not destroying with the scene, then it is ok to spawn in the current scene prior to a scene loading
        // event of type  LoadSceneMode.Single (i.e. everything in the current active scene will be destroyed that
        // is marked to be destroyed).
        if (!DestroyInstanceWithScene)
        {
            // Ok to spawn an instance that doesn't destroy with a scene.
            SpawnPrefabInstance();
        }

        // Use the completed scene events to know everyone has finished loading and synchronizing the scene.
        m_NetworkManager.SceneManager.OnLoadEventCompleted += OnLoadEventCompleted;
        m_NetworkManager.SceneManager.LoadScene(SceneManager.GetActiveScene().name, LoadSceneMode.Single);
    }

    private void SpawnPrefabInstance()
    {
        if (PrefabToSpawn == null)
        {
            Debug.Log($"Will not spawn prefab instance as it is not set!");
            return;
        }

        NetworkObject.InstantiateAndSpawn(PrefabToSpawn, m_NetworkManager, destroyWithScene: DestroyInstanceWithScene, position: Vector3.zero);

    }

    private void OnLoadEventCompleted(string sceneName, LoadSceneMode loadSceneMode, System.Collections.Generic.List<ulong> clientsCompleted, System.Collections.Generic.List<ulong> clientsTimedOut)
    {
        Debug.Log($"[{sceneName}] Loaded via {loadSceneMode} scene mode.");

        // If you are destroying with the scene and immediately loading a scene using LoadSceneMode.Single,
        // then you need to spawn after the scene load to avoid having the newly spawned instance getting destroyed.
        if (DestroyInstanceWithScene)
        {
            SpawnPrefabInstance();
        }
    }
}

Let me know if this resolves your issues?