State restore debugging

[LeXXik]

I am currently debugging a crash that happens when a body is added/removed/destroyed from the world between state restores. As a starting point, one pattern is visible, which I would like to ask about:

World steps:

1. Add body
2. Save state
3. Remove body
4. Restore state

Now, the bodyInterface.IsAdded(bodyId) says false, even though we have restored to the state, when it was in the world. Is this correct? Is body interface aware of the restored state? The app will crash, if I try to destroy that body later. It doesn't though, if I add it again, which doesn't seem right (it should already be in the world as we restored to such state). So adding it (again?) may be causing other issues.

[jrouwe]

State saving only saves the part of the state that the simulation itself changes (position, velocity etc.). Everything you change through an API call (e.g. RemoveBody, SetFriction etc.) is not saved with the state and needs to be restored manually. So in this case you need to re-add the body yourself if you removed it. It needs to be added before calling restore state. State saving is meant to save the absolute minimum needed to sync the state over a network.

[LeXXik]

Hmm, I am curious why the current variant "seems" to work, where I add a body after restore and then remove it? "Seems" as in I don't see any asserts triggering. Is this going to have issues down the line?

[jrouwe]

It is going to cause subtle issues. If a body is not added to the physics system then certain things cannot be restored. For example: cached contact points, if a body is active, sleep timing info etc. A body that's not in the physics system cannot be activated, so you might end up having an inactive body in mid air that, when activated, suddenly remembers that it was falling at high speed. I didn't test if this is actually possible to trigger, but this is just giving an idea of the kind of things that could go wrong.

[gigablox]

What if the body is added back in and removed all before the first sim step after the restore?

Is it the Restore function that creates those rebindings on execution or are they searched for during the Step function?

[jrouwe]

What if the body is added back in and removed all before the first sim step after the restore?

Removing and readding a body will reset certain state, like the sleep timers / active state.

Is it the Restore function that creates those rebindings on execution or are they searched for during the Step function?

All logic is contained in the Restore function. Step always does the same logic regardless if you called Restore or not.