NetworkList "EventType.Full" synchronization always sends an empty list on client-side when spawning since commit #24388c4c636544871bfe40d1fccf616751665ade causing next "EventType.Value" to throw an Exception trying to set a value at a given index in an empty internal list.

[WhippetsAintDogs]

Description

The fix to prevent duplicate application (#1917) by @jeffreyrainy introduces a bug where when a client gets the spawn call of an object, the server marks dirty the NetworkList's base.IsDirty() causing the NetworkList's next WriteDelta to enter the if sending the EventType.Full which is using the modified WriteField that sends value from m_ListAtLastReset instead of m_List. Since m_ListAtLastReset is empty upon spawning the NetworkList's NetworkObject because no ResetDirty has happened yet, the client will receive an empty list and further EventType.Value will throw by trying to set the value in an empty m_List.

Environment

• OS: Windows 10
• Unity Version: 2020.3.16f1
• Netcode Version: develop branch, commit #24388c4c636544871bfe40d1fccf616751665ade

[jeffreyrainy]

Hi Marc-Antoine Fortier,

I just tried reproducing this, both at head revision and at the commit specified above. I was unable to.

I've put below the script I used. I put this script both on the PlayerObject and on a scene-placed object (with NetworkObject components). My logs show the host writing to the NetworkList and the clients (tried 2 and 3) receiving the updated lists.

using Unity.Netcode;
using UnityEngine;

public class PlayerWithList : NetworkBehaviour
{
    private NetworkList<int> m_MyNetworkList;

    private void Awake()
    {
        Debug.Log("Awake");
        m_MyNetworkList = new NetworkList<int>();
        m_MyNetworkList.OnListChanged += Changed;
    }

    private void Changed(NetworkListEvent<int> someEvent)
    {
        Debug.Log($"Object {name} Type {someEvent.Type} Index {someEvent.Index} Value {someEvent.Value}");
    }

    // Start is called before the first frame update
    public override void OnNetworkSpawn()
    {
        Debug.Log("OnNetworkSpawn");
    }

    private float m_LastTime = 0.0f;

    private void Update()
    {
        if (IsServer && Time.unscaledTime > m_LastTime + 1.0f)
        {
            m_LastTime = Time.unscaledTime;
            m_MyNetworkList.Add((int)m_LastTime);

            if (m_MyNetworkList.Count > 20)
            {
                m_MyNetworkList.Clear();
            }
            Debug.Log($"List has {m_MyNetworkList.Count} elements");
        }
    }
}

Would you be able to verify if this works for you? Ideally, if the issue repro'es locally, you could provide us with such a small, contained, component. Then I'd drop it on my scene locally and see if I get to repro your issue.

Thanks!
Jeff.

[TheCaveOfWonders]

I think i have the same issue
#1981

I also noticed the exact same net code steps that OP mentions while debugging the net code.
Workaround is posted in the issue i linked above.

[akoolenbourke]

I think I have just discovered it too as my lists are going across to standalone clients empty. Note, they work fine on a host client.

I am using a custom struct that implements INetworkSerializable and IEquatable.

Other NetworkLists I have seem to work find - even ones with custom structs.

[akoolenbourke]

Just to add, I have found a reasonably workaround. After setting the list on the server, I can call ResetDirty(). This effectively will copy a modified list into m_ListAtLastReset and so synching will work.

[JesseOlmer]

Hey folks,

Can you please verify this still occurs on 1.0.0-pre.10. If it does, can you please share a minimal repro with us? A few of us have tried (on multiple versions with multiple types) and have so far been unable to hit this issue.