Undefined behavior when set_script on itself

[ghost]

Godot/OS version:

92ece2e | Antergos x86_64

Issue description:

Adding these scripts after set_script

• print("some string")
• set_process(true)

Expected: old script stops execution when set_script on itself / Not crash / some docs about it
Happened: print prints Null, set_process crashes the game

Steps to reproduce:

1. Create script
   A.gd

extends Node
func _ready():
    set_script(preload("res://B.gd"))
    print(0)
    set_process(true)

B.gd

extends Node
func _process(dt):
    print(1)

2. Create node N attach A
3. Run

Minimal reproduction project:

SetScript.zip

[vnen]

It's natural that the current function will finish execution, or at least try to. I'm not sure why the reference is lost so soon to make it crash.

[reduz]

I am not sure how to fix this or whether it's worth fixing, up to you @vnen

[ghost]

If it couldn't be fixed (expected because it's probably not intended to be used this way) then I'd appreciate some documentation/warning messages discouraging users to do so.

[vnen]

We can add a warning (or even an error) if you try this, similar to the error you get when you use get_node() without onready. You can probably do this properly by using call_deferred().

[tom-jk]

Testing MRP on linux mint 18.3:

3.0: Error immediately, but no crash on nearly every run.
Invalid call. Nonexistent function '' in base 'Node (B.gd)'. error.
Output shows Null and No loader found for resource: res://.
Debugger->Stack Frame shows 0 - :6.
Debugger->Errors has Condition ' script.is_null() ' is true.

3.1: same as 3.0, but game crashes on most runs.

3.2, 3.2.1: crashes on every run. No output in editor but terminal outputs null, handle crash with signal 11, backtrace.

3.2.2, 3.2.3.rc3: same as 3.0 without script.is_null() error.

Backtrace from a crash in 3.2.2 (Calinou godot-3.2.2-debug-linux-gcc5.4) (crashed less than every 1/100 runs)

Running: /home/thomas/Downloads/Godot_v3.2.2-debug/godot.x11.tools.64 --path /home/thomas/Downloads/GodotIssues/16943 --remote-debug 127.0.0.1:6007 --allow_focus_steal_pid 28469 --position 208,1050
Loading resource: res://
ERROR: _load: Resource file not found: res://.
   At: core/io/resource_loader.cpp:282.
Godot Engine v3.2.2.stable.custom_build.6a9fbafcb - https://godotengine.org
OpenGL ES 3.0 Renderer: GeForce GTX 750 Ti/PCIe/SSE2
 
Null
handle_crash: Program crashed with signal 11
Dumping the backtrace. Please include this when reporting the bug on https://github.com/godotengine/godot/issues
[1] /lib/x86_64-linux-gnu/libc.so.6(+0x354c0) [0x7faa5a20a4c0] (??:0)
[2] StringName::operator String() const (/godot/./core/string_name.h:122)
[3] GDScriptFunction::call(GDScriptInstance*, Variant const**, int, Variant::CallError&, GDScriptFunction::CallState*) (/godot/modules/gdscript/gdscript_function.cpp:1594)
[4] GDScriptInstance::_ml_call_reversed(GDScript*, StringName const&, Variant const**, int) (/godot/modules/gdscript/gdscript.cpp:1268)
[5] GDScriptInstance::call_multilevel_reversed(StringName const&, Variant const**, int) (/godot/modules/gdscript/gdscript.cpp:1277)
[6] Node::_notification(int) (/godot/scene/main/node.cpp:152)
[7] Node::_notificationv(int, bool) (/godot/./scene/main/node.h:46 (discriminator 14))
[8] Object::notification(int, bool) (/godot/core/object.cpp:934)
[9] Node::_propagate_ready() (/godot/scene/main/node.cpp:197)
[10] Node::_propagate_ready() (/godot/scene/main/node.cpp:186 (discriminator 2))
[11] Node::_set_tree(SceneTree*) (/godot/scene/main/node.cpp:2574)
[12] SceneTree::init() (/godot/scene/main/scene_tree.cpp:464)
[13] OS_X11::run() (/godot/platform/x11/os_x11.cpp:3259)
[14] /home/thomas/Downloads/Godot_v3.2.2-debug/godot.x11.tools.64(main+0x116) [0x1424b7c] (/godot/platform/x11/godot_x11.cpp:57)
[15] /lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf0) [0x7faa5a1f5840] (??:0)
[16] /home/thomas/Downloads/Godot_v3.2.2-debug/godot.x11.tools.64(_start+0x29) [0x1424999] (??:?)
-- END OF BACKTRACE --
Socket error: 104
Loading resource: res://
ERROR: _load: Resource file not found: res://.
   At: core/io/resource_loader.cpp:282.
Loading resource: res://
ERROR: _load: Resource file not found: res://.
   At: core/io/resource_loader.cpp:282.

tl;dr: MRP does still crash Godot, sometimes.

---

Using call_deferred appears to work, at least in this particular test case:

A.gd

extends Node

func _ready():
	call_deferred("set_script", preload("res://B.gd"))
	call_deferred("set_process", true)
	print("A.gd: My final message.")

B.gd

extends Node

func _process(dt):
	print("B.gd: process")

produces good output (sample from 3.2.2):

--- Debugging process started ---
Godot Engine v3.2.2.stable.custom_build.6a9fbafcb - https://godotengine.org
OpenGL ES 3.0 Renderer: GeForce GTX 750 Ti/PCIe/SSE2
 
A.gd: My final message.
B.gd: process
B.gd: process
(...)
B.gd: process
B.gd: process
--- Debugging process stopped ---

Tested with 3.0, 3.1, 3.2, 3.2.1, 3.2.2 (50 runs), 3.2.3.rc3, all successful.

Small quirk: in recent versions where you don't need to set_process(true), you can omit it but A.gd must have a dummy _process function of its own instead, or B.gd's won't be called.