await signal yields before its listeners get executed, if they were registered via anonymous function

[AlexanderFarkas]

Godot version

v4.0.1.stable.official [cacf499]

System information

Mac OS 13.0.1

Issue description

Following tutortial https://docs.godotengine.org/en/stable/getting_started/first_2d_game/06.heads_up_display.html

Wanted to connect signal via code instead of editor. But it breaks the logic - signal callback is triggered after await Timer.timeout

"Dodge the\nCreeps!" doesn't appear on the screen

Steps to reproduce

class_name HUD
extends CanvasLayer

signal start_game

# Called when the node enters the scene tree for the first time.
func _ready():
	
	$MessageTimer.timeout.connect(func(): 
		$Message.hide()
		print("Hide")
	)
	$StartButton.pressed.connect(func():
		$StartButton.hide()
		start_game.emit()
	)


func show_message(text):
	$Message.text = text
	$Message.show()
	$MessageTimer.start()

func show_game_over():
	show_message("Game Over")
	# Wait until the MessageTimer has counted down.
	await $MessageTimer.timeout

	$Message.text = "Dodge the\nCreeps!"
	$Message.show()
	print("Show")
	# Make a one-shot timer and wait for it to finish.
	await get_tree().create_timer(1.0).timeout
	$StartButton.show()
	
func update_score(score):
	$ScoreLabel.text = str(score)
	
# Called every frame. 'delta' is the elapsed time since the previous frame.
func _process(delta):
	pass

Console output:

Hide
Show
Hide

Minimal reproduction project

Dodge_the_Creeps.zip

[AlexanderFarkas]

If I replace this code:

$MessageTimer.timeout.connect(func(): 
		$Message.hide()
		print("Hide")
	)

with this:

$MessageTimer.timeout.connect(_timeout)

...

func _timeout():
	$Message.hide()
	print("Hide")

It starts working as expected
Output:

Hide
Hide
Show

[AlexanderFarkas]

That happens due to how Godot handles connections.
object.cpp:1287 - VMap doesn't support order of insertion.

gscript_vm.cpp:2367 - VM connects callback for resumption. And get_base_comparator for lamda functions is low relative to methods.

So await returns before lambda functions have chance to execute.
object.cpp:1028 - If insertion order will be preserved, await-produced one-shot callback would always be the last one.

Hope, it helps @Calinou

[AlexanderFarkas]

If maintaining insertion order is cumbersome, I would just allow _signal_callback's connection to have lowest possible value. So, "await connection" will always be executed last.

So, this:

Error err = sig.connect(Callable(gdfs.ptr(), "_signal_callback").bind(retvalue), Object::CONNECT_ONE_SHOT);

Will become this:

Error err = sig.connect(Callable(gdfs.ptr(), "_signal_callback").bind(retvalue), Object::CONNECT_ONE_SHOT,  priority=NEGATIVE_INFINITY);

[AlexanderFarkas]

Actually, insertion order is not a fix. You always want await connections to execute last, no matter when they were added. Lowest priority should be the fix, I believe.

[AlexanderFarkas]

@Calinou I want to PR, could we discuss how it should be implemented? I wanted to add new slot_map, but there are lots of things going on around slot_map. Adding late_slot_map doesn't seem suitable.

CONNECT_DEFERRED also doesn't seem to solve the problem, since other deffered connections could be added.

Adding new CallableCustom for await-resume exclusively doesn't work, since comparison functions don't work for different derived classes (they just compare functions???)

The only thing I understand - await-resume functions should always be executed last. There should be either separate vector for them or way to sort them last.