Non-deterministic behavior when colliding with another object [sample included]

[xissburg]

After many attempts and experiments I have found the situation that causes my simulation to diverge. I am simulating a vehicle with custom constraints and as long as I stay on the same triangle mesh the simulation is deterministic. When I move onto another object, be a plane, a box or another triangle mesh, the simulation diverges slightly upon first contact.

My setup for testing are two identical vehicles which are spawned at the exact same position and orientation with collisions disabled between them and the same inputs are applied on both.

I have created a sample to showcase the problem. It's a vehicle created with hinge2's and you can move it forwards/backwards with the up/down keys. Drive it off the box and try to drive it back onto the box, collide with it. At some point the vehicles should diverge.

You can find the sample in my fork https://github.com/xissburg/bullet3/blob/master/examples/NonDeterminismExample/NonDeterminismExample.cpp.

I have also recorded a video https://youtu.be/psmZfdk-Rz4.

Thanks