MultiplayerSpawner doesn't work with UIDs

[Summersay415]

Tested versions

Reproducible in: 4.3dev5, 4.2.2, latest master [4a01602]

System information

Godot v4.2.2.stable - Ubuntu 23.10 23.10 - X11 - GLES3 (Compatibility) - Mesa Intel(R) Graphics (ADL GT2) () - 12th Gen Intel(R) Core(TM) i5-1235U (12 Threads)

Issue description

MultiplayerSpawner doesn't work if auto spawn path is UID. It prints some errors in console:

E 0:00:08:0516   get_node: Node not found: "World/Players/1/MultiplayerSynchronizer" (relative to "/root").
  <Ошибка C++>   Method/function failed. Returning: nullptr
  <Исходный код C++>scene/main/node.cpp:1651 @ get_node()
E 0:00:08:0516   process_simplify_path: Parameter "node" is null.
  <Исходный код C++>modules/multiplayer/scene_cache_interface.cpp:110 @ process_simplify_path()
E 0:00:08:0516   get_node: Node not found: "World/Players/1/Inputs/InputsSync" (relative to "/root").
  <Ошибка C++>   Method/function failed. Returning: nullptr
  <Исходный код C++>scene/main/node.cpp:1651 @ get_node()
E 0:00:08:0516   process_simplify_path: Parameter "node" is null.
  <Исходный код C++>modules/multiplayer/scene_cache_interface.cpp:110 @ process_simplify_path()
E 0:00:08:0516   get_node: Node not found: "World/Players/168206040/MultiplayerSynchronizer" (relative to "/root").
  <Ошибка C++>   Method/function failed. Returning: nullptr
  <Исходный код C++>scene/main/node.cpp:1651 @ get_node()
E 0:00:08:0516   process_simplify_path: Parameter "node" is null.
  <Исходный код C++>modules/multiplayer/scene_cache_interface.cpp:110 @ process_simplify_path()

Also, after each engine restart UID is erased from Auto Spawn List with error in console:

modules/multiplayer/multiplayer_spawner.cpp:103 - Condition "!FileAccess::exists(p_path)" is true.

Can be worked around with ResourceUID.get_id_path(ResourceUID.text_to_id()), but only if spawnable scenes added in code.

Steps to reproduce

1. Take multiplayer project with MultiplayerSpawner which uses Auto Spawn List (Multiplayer Bomber in MRP)
2. Change paths of Auto Spawn List scenes to UIDs
3. Run game as usual
4. Request spawn
   Notice no scenes spawned on client, instead errors printed.

Minimal reproduction project (MRP)

bomber.zip

1. Open world.tscn
2. in PlayerSpawner add UID of player.tscn
3. Run the game, create server and connect from client
4. Start game
5. Observe result

[AThousandShips]

File access can't open UIDs like this, that's limited to the resource management system, so this is a limitation in that side of things, unsure how to proceed though

Note that UIDs are not guaranteed to be valid in the other client, as you might be communicating with a different version of the game, including having a separate server build

[Summersay415]

This should be documented then?

[AThousandShips]

Depending on what path we take yes, the team for multiplayer should evaluate if this is a feature we should add, or document that it's not supported

[AThousandShips]

It seems to be an error in this case, the loading itself uses ResourceLoader so the check should be replaced by ResourceLoader::exists instead, as it works otherwise

[OwenAEdwards]

First time contributor, here's my pull request #91084