MultiplayerSynchronizer does not sync properties on scene root

[jgillich]

Godot version

136f84f

System information

Fedora Silverblue 36, Ryzen 2400G

Issue description

MultiplayerSynchronizer does not sync properties if they are located on the root node of a scene. Instead they remain at their initial values. I have tested this with CharacterBody2D and CharacterBody3D nodes, not sure if it would be the same with a simple Node at the scene root.

Steps to reproduce

Get the reproduction project: https://github.com/jgillich/gotodot-mp-repro (run with at least version 0df0a07, not alpha9)

1. Run 3 instances:

godot --server
godot --client
godot --client

2. In one of the client windows, use A/D or Left/Right to move your character
3. Observe how the position in the other client window does not update
4. Repeat the steps above but add --use-child to the run arguments
5. Observe how the position updates in all clients when a character is moved

There is potentially another bug, if you uncomment the create_player line in world.gd so the server is also a player, and then try to join with a client, you get a bunch of errors. In the bomber example the server is also a player, but I haven't managed to figure out what the difference is.

Minimal reproduction project

https://github.com/jgillich/gotodot-mp-repro

[jgillich]

Did some more investigation, if we change root_path it to literally anything other than the default .., like ../Camera2D, the example starts working. What exactly is the purpose of root_path?

[Faless]

What exactly is the purpose of root_path?

It's the starting path to use for referencing properties.

[danhealy]

It also works if you make a standard node the parent of CharacterBody2D, i.e. if .. is not the root of the scene

[Faless]

This is fixed after #62961 with the following small change to the gdscript code (adding await get_tree().process_frame):

diff --git a/world.gd b/world.gd
index 32e01b3..f481f3d 100644
--- a/world.gd
+++ b/world.gd
@@ -16,7 +16,8 @@ func _enter_tree():
 
                print('server listening on localhost 12100')
                multiplayer.set_multiplayer_peer(peer)
-               #create_player(multiplayer.get_unique_id())
+               await get_tree().process_frame
+               create_player(multiplayer.get_unique_id())
        else:
                peer.create_client("localhost", 12100)
                multiplayer.set_multiplayer_peer(peer)

Spawning on a child during the _ready() phase of the parent has known issues, and we should probably document that while try to make an internal fix.

I'll leave this issue open for now since it has nice keywords for search, but I should probably open a dedicated issue soon.

[jgillich]

One more thing I've noticed while testing is that MPSynchronizer bypasses property setters. Is this expected or a bug?

[Faless]

Is this expected or a bug?

That's likely a bug, it's supposed to call setter/getters.
I think I have an idea why it's happening, we should track that in a separate issue.
Thanks for reporting it :)

[jgillich]

@Faless I made #63308

Haven't bothered to make a reproduction project since you already have an idea, lmk if you'd like one