Resolved Wrong angle calculation with relative joints 2D

When angle -180+ meets -180- relative joint 2d goes around.
It can be seen clearly in the gif

 

Joint stores a target angular offset and use it to solve the constraint.

The problem is because the Transform.rotation is actually stored with quaternion for 3D.
Since 2d physics only handle with the single float as rotation, Unity did it by converting the quaternion rotation to euler angle and extract the z axis as 2d rotation.
The converting only occurs when transform "sync" to rigidbody, and the sync happens iftransform was changed.

The converting could make the angle of rigidbody 2d lose if the angle is out of -180 or 180 which causes this problem.

For example, there are two rigidbodies A and B with rotation 180 and 90 and a joint to constraint them with -90 angular offset, meaning (B's angle - A's angle) = -90.
Then A rotates to 181 degree by changes rotation in transform (modified in editor).
Since the transform changed, physics 2d would sync the transform to rigidbody.
Pass through converting from quanternion to euler, the 181 degree becomes -179 degree.
Now A is -179, B is 90, and the joint's target angular offset is -90 still. physics engine will calculate as B should rotate to -269 to fit constraint.

The idea is if you want to use joints which considered angular, you should not directly modified the rigidbody's transform to prevent from syncing.
Unity did something I don't know inside the physics engine to make the rigidbody somehow don't wrap into -180 ~ +180 degree if there is any joint considered of angle. But it will mess up if transform sync to rigidbody.

 

Thanks for the answer! Just tried it with AddTorque and rigidbody.rotation and everything works fine, but I think I'd got this issue in my game project where all rigidbodies controlled via rigidbody.rotation. But there are more complex chaints of joints, I'll try to localize it futher.