can't use Rigigbody2D's apply_impulse and apply_torque_impulse when in _ready() function

[Raphael2048]

Godot version:

d2f38db

OS/device including version:

Issue description:

Create a normal rigidbody2D in scene, attach a script to it, and try to call apply_impulse or apply_torque_impulse in _ready(),it has no effect.
Reason: Rigidbody2d's updating inertia is delayed to next frame, so when you call a function need mass or inertia in _ready(), it has no effect.

godot/servers/physics_2d/body_2d_sw.cpp

Lines 36 to 40 in d2f38db

	void Body2DSW::_update_inertia() {
	if (!user_inertia && get_space() && !inertia_update_list.in_list())
	get_space()->body_add_to_inertia_update_list(&inertia_update_list);
	}

RigidBody has the same problem, the difference is RigidBody's inertia is inited with identity instead of zero.
Steps to reproduce:

Minimal reproduction project:

bug2.zip

[KoBeWi]

Doesn't call_deferred solve the issue?

[Raphael2048]

If it's updated at end of frame, you can't get calculated result at once after you apply impulse.

[KoBeWi]

Deferring the calculation too would be inconvenient, I guess? I'll tag it as bug, but it might be possible that it can't be fixed and it's more a documentation issue.

[bmwalters]

I just encountered this too. My Player instances a throwing knife on a keypress and flings it using apply_impulse right after instancing it. I have to delay this impulse for 2 physics frames otherwise the knife flings, but doesn't start spinning at all.

I was clued into this by a 2 year old reddit post which links to a similarly old forum post.

[quinnyo]

I did some testing and found the following:

• The issue isn't strictly about _ready(), but the body's first frame. i.e. you get the same behaviour if you apply_impulse on the first call to _physics_process

• The body's linear velocity is affected. The torque has no effect.

• The issue seems to be that the body has zero (inverse) inertia at that point. (it hasn't been auto-calculated yet, I guess)

• If you set the inertia manually, applying torque on a body's first frame works as expected.