Client cannot join host when player prefab dynamically adds a parent to its transform

[LPLafontaineB]

Describe the bug
When a player prefab contains a script that dynamically adds a parent to its transform, the client cannot join a game hosted by another client.

To Reproduce
Steps to reproduce the behavior:

1. Set up basic networking game with at least one gameobject in a scene that is not the player.
2. Add a script to the player prefab that adds parenting to its transform via gameObject.transform.SetParent() in the Start() method.
3. Launch one instance of the game as Host.
4. Launch another instance and try to join as Client.

Actual outcome
The client cannot join and an exception is raised.

Expected outcome
The client can join.

Environment

• OS: Windows 10
• Unity Version: 2021.2.0b12
• Netcode Commit: 2225a010

Stack trace
OverflowException: Attempted to read without first calling TryBeginRead()
Unity.Netcode.NetworkObject+SceneObject.Deserialize (Unity.Netcode.FastBufferReader& reader) (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/Core/NetworkObject.cs:1016)
Unity.Netcode.SceneEventData.SynchronizeSceneNetworkObjects (Unity.Netcode.NetworkManager networkManager) (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/SceneManagement/SceneEventData.cs:693)
Unity.Netcode.NetworkSceneManager.HandleClientSceneEvent () (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/SceneManagement/NetworkSceneManager.cs:1385)
Unity.Netcode.NetworkSceneManager.ClientLoadedSynchronization (System.UInt32 sceneIndex, System.Int32 sceneHandle) (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/SceneManagement/NetworkSceneManager.cs:1353)
Unity.Netcode.NetworkSceneManager.OnClientBeginSync () (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/SceneManagement/NetworkSceneManager.cs:1288)
Unity.Netcode.NetworkSceneManager.HandleClientSceneEvent () (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/SceneManagement/NetworkSceneManager.cs:1378)
Unity.Netcode.NetworkSceneManager.HandleSceneEvent (System.UInt64 clientId, Unity.Netcode.FastBufferReader& reader) (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/SceneManagement/NetworkSceneManager.cs:1541)
Unity.Netcode.SceneEventMessage.Receive (Unity.Netcode.FastBufferReader& reader, Unity.Netcode.NetworkContext context) (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/Messaging/Messages/SceneEventMessage.cs:16)
Unity.Netcode.MessagingSystem.HandleMessage (Unity.Netcode.MessageHeader& header, Unity.Netcode.FastBufferReader& reader, System.UInt64 senderId, System.Single timestamp) (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/Messaging/MessagingSystem.cs:301)
UnityEngine.Debug:LogException(Exception)
Unity.Netcode.MessagingSystem:HandleMessage(MessageHeader&, FastBufferReader&, UInt64, Single) (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/Messaging/MessagingSystem.cs:305)
Unity.Netcode.MessagingSystem:ProcessIncomingMessageQueue() (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/Messaging/MessagingSystem.cs:321)
Unity.Netcode.NetworkManager:OnNetworkEarlyUpdate() (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/Core/NetworkManager.cs:1096)
Unity.Netcode.NetworkManager:NetworkUpdate(NetworkUpdateStage) (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/Core/NetworkManager.cs:1067)
Unity.Netcode.NetworkUpdateLoop:RunNetworkUpdateStage(NetworkUpdateStage) (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/Core/NetworkUpdateLoop.cs:149)
Unity.Netcode.<>c:b__0_0() (at C:/Users/louisphilippe.lb/Documents/com.unity.netcode.gameobjects/com.unity.netcode.gameobjects/Runtime/Core/NetworkUpdateLoop.cs:172)

[will-mearns]

This is in our backlog as MTT-1270

[NoelStephensUnity]

1. Does the in-scene placed GameObject have a NetworkObject assigned to it? I only say this because the requirement for parenting a NetworkObject is that the parent must have a NetworkObject component as well.
2. When you want to parent a NetworkObject, you will want to do this after it has spawned. It is best to do this when or after the in-scene placed NetworkObject has been spawned. Otherwise, you could run into a scenario where you are trying to parent to something that does not exist.

If you want to do this when a player has first connected and all NetworkObjects (in-scene placed and already dynamically spawned by the server-host) have been fully synchronized with the client then I would recommend using the NetworkManager.SceneManager.OnSceneEvent to trap for the C2S_SyncComplete event.

Here is an example script that I would recommend using for what you are trying to accomplish:

using Unity.Netcode;

public class ParentPlayerToInSceneNetworkObject : NetworkBehaviour
{   
    public override void OnNetworkSpawn()
    {
        if (IsServer)
        {
            // Server subscribes to the NetworkSceneManager.OnSceneEvent event
            NetworkManager.SceneManager.OnSceneEvent += SceneManager_OnSceneEvent;

            // Server player is parented under this NetworkObject
            SetPlayerParent(NetworkManager.LocalClientId);
        }
    }

    private void SetPlayerParent(ulong clientId)
    {
        if (IsSpawned && IsServer)
        {
            // As long as the client (player) is in the connected clients list
            if (NetworkManager.ConnectedClients.ContainsKey(clientId))
            {
                // Set the player as a child of this in-scene placed NetworkObject 
                NetworkManager.ConnectedClients[clientId].PlayerObject.transform.parent = transform;
            }
        }
    }

    private void SceneManager_OnSceneEvent(SceneEvent sceneEvent)
    {
        // OnSceneEvent is very useful for many things
        switch (sceneEvent.SceneEventType)
        {
            // The C2S_SyncComplete event tells the server that a client-player has:
            // 1.) Connected and Spawned
            // 2.) Loaded all scenes that were loaded on the server at the time of connecting
            // 3.) Synchronized (instantiated and spawned) all NetworkObjects in the network session
            case SceneEventData.SceneEventTypes.C2S_SyncComplete:
                {
                    // As long as we are not the server-player
                    if (sceneEvent.ClientId != NetworkManager.LocalClientId)
                    {
                        // Set the newly joined and synchronized client-player as a child of this in-scene placed NetworkObject
                        SetPlayerParent(sceneEvent.ClientId);
                    }
                    break;
                }
        }
    }
}

You should place this script on your in-scene placed NetworkObject (i.e. the 1 GameObject) and do the parenting from it in order to avoid any timing issues of when it is spawned or the like. It only runs the script on the server-host side since parenting is server authoritative. I added comments in the script to further explain what is happening.

Remove any parenting code you might have had from your player prefab before using the above script.

Here is a screenshot of the in-scene placed GameObject with both a NetworkObject and ParentPlayerToInSceneNetworkObject component:

Here is a screenshot of the end result, where the host and the connected client are both children of the in-scene placed NetworkObject:

If this resolves your issue please let me know and I will close out this issue.
If this does not resolve your issue, please provide further details on what the above solution does not include that you are needing to accomplish.

Cheers,

Noel

[LPLafontaineB]

I just tested it and it resolved my issue, thanks! It was simply a question of timing

[NoelStephensUnity]

Glad it worked out. I will make sure to contact the docs group to include additional documentation on this.