Crash when removing multibody

[ThierryBerger]

• Following up to Despawn a MultibodyJoint bevy_rapier#663 ; This issue seems to come from rapier, so fixing this in rapier before investigating in bevy_rapier is a better approach.

thread 'dynamics::joint::multibody_joint::test::multibody_joint_remove_and_step' panicked at crates/rapier2d/../../src/dynamics/solver/velocity_solver.rs:101:22:
called `Option::unwrap()` on a `None` value

• Maybe related: Fix occasional crashes when inserting or removing multibody links #672

⚠️ I easily make it crash by running cargo run --bin all_examples3 -> hitting a few times A (remove any 10% multibodies) with 2 scenari:

• either the simulation explodes and we have the sap axis panic. (unrelated to this PR)
• or there is a failing unwrap at https://github.com/Vrixyz/rapier/blob/d357433b2da9da12c48264ad3acd2fb6c9411b6d/src/dynamics/solver/contact_constraint/generic_one_body_constraint.rs#L67
  • should the MultibodyJointSet remove recursively the rb2mb when it detects a removal leading to empty link?
  • something like: ( ⚠️ not working either)

diff --git a/src/dynamics/joint/multibody_joint/multibody_joint_set.rs b/src/dynamics/joint/multibody_joint/multibody_joint_set.rs
index 3dd3e36..aac7538 100644
--- a/src/dynamics/joint/multibody_joint/multibody_joint_set.rs
+++ b/src/dynamics/joint/multibody_joint/multibody_joint_set.rs
@@ -250,8 +250,13 @@ impl MultibodyJointSet {
                             .connectivity_graph
                             .remove_node(isolated_graph_id.graph_id)
                         {
-                            self.rb2mb.get_mut(other.0).unwrap().graph_id =
-                                isolated_graph_id.graph_id;
+                            let other_mb_id = self.rb2mb.get_mut(other.0).unwrap();
+                            other_mb_id.graph_id = isolated_graph_id.graph_id;
+                            let other_mb_id = other_mb_id.clone();
+                            let other_mb = self.multibodies.get(other_mb_id.multibody.0).unwrap();
+                            if other_mb.num_links() == 1 {
+                                self.remove(MultibodyJointHandle(other_mb_id.multibody.0), wake_up);
+                            }
                         }
                     } else {
                         let mb_id = self.multibodies.insert(multibody);

[ThierryBerger]

Almost same content as multibody::test_multibody_remove but adds a simulation step between each removal

[ThierryBerger]

This test is added to catch at the earliest the original error of:

thread 'dynamics::joint::multibody_joint::test::multibody_joint_remove_and_step' panicked at crates/rapier2d/../../src/dynamics/solver/velocity_solver.rs:101:22:
called `Option::unwrap()` on a `None` value

⚠️ While this test pass, it's not enough because I can still make the example crash