From 14fc82f14294062a51dc7ef260610816a275b7b2 Mon Sep 17 00:00:00 2001
From: Thierry Berger <contact@thierryberger.com>
Date: Fri, 12 Jul 2024 15:06:16 +0200
Subject: [PATCH 1/4] attempt to reproduce a crash with a more complex example

---
 .../examples/multi_world_deletion3.rs         | 145 ++++++++++++++++++
 1 file changed, 145 insertions(+)
 create mode 100644 bevy_rapier3d/examples/multi_world_deletion3.rs

diff --git a/bevy_rapier3d/examples/multi_world_deletion3.rs b/bevy_rapier3d/examples/multi_world_deletion3.rs
new file mode 100644
index 00000000..6b455d80
--- /dev/null
+++ b/bevy_rapier3d/examples/multi_world_deletion3.rs
@@ -0,0 +1,145 @@
+use bevy::{input::common_conditions::input_just_pressed, prelude::*};
+use bevy_rapier3d::prelude::*;
+
+const N_WORLDS: usize = 2;
+
+fn main() {
+    App::new()
+        .insert_resource(ClearColor(Color::srgb(
+            0xF9 as f32 / 255.0,
+            0xF9 as f32 / 255.0,
+            0xFF as f32 / 255.0,
+        )))
+        .insert_resource(TimestepMode::Interpolated {
+            dt: 1f32 / 60f32,
+            time_scale: 1f32,
+            substeps: 4,
+        })
+        .add_plugins((
+            DefaultPlugins,
+            RapierPhysicsPlugin::<NoUserData>::default()
+                .with_default_world(RapierContextInitialization::NoAutomaticRapierContext),
+            RapierDebugRenderPlugin::default(),
+        ))
+        .add_systems(
+            Startup,
+            ((create_worlds, setup_physics).chain(), setup_graphics),
+        )
+        .add_systems(Update, move_platforms)
+        .add_systems(PostUpdate, remove_entities_colliding)
+        .add_systems(
+            Update,
+            change_world.run_if(input_just_pressed(KeyCode::KeyC)),
+        )
+        .run();
+}
+
+fn create_worlds(mut commands: Commands) {
+    for i in 0..N_WORLDS {
+        let mut world = commands.spawn((RapierContext::default(), WorldId(i)));
+        if i == 0 {
+            world.insert(DefaultRapierContext);
+        }
+    }
+}
+
+fn setup_graphics(mut commands: Commands) {
+    commands.spawn(Camera3dBundle {
+        transform: Transform::from_xyz(0.0, 3.0, -10.0)
+            .looking_at(Vec3::new(0.0, 0.0, 0.0), Vec3::Y),
+        ..Default::default()
+    });
+}
+
+#[derive(Component)]
+pub struct WorldId(pub usize);
+
+#[derive(Component)]
+struct Platform {
+    starting_y: f32,
+}
+
+fn move_platforms(time: Res<Time>, mut query: Query<(&mut Transform, &Platform)>) {
+    for (mut transform, platform) in query.iter_mut() {
+        transform.translation.y = platform.starting_y + -time.elapsed_seconds().sin();
+    }
+}
+
+/// Demonstrates how easy it is to move one entity to another world.
+fn change_world(
+    query_context: Query<Entity, With<DefaultRapierContext>>,
+    mut query_links: Query<(Entity, &mut RapierContextEntityLink)>,
+) {
+    let default_context = query_context.single();
+    for (e, mut link) in query_links.iter_mut() {
+        if link.0 == default_context {
+            continue;
+        }
+        link.0 = default_context;
+        println!("changing world of {} for world {}", e, link.0);
+    }
+}
+
+pub fn setup_physics(
+    context: Query<(Entity, &WorldId), With<RapierContext>>,
+    mut commands: Commands,
+) {
+    for (context_entity, id) in context.iter() {
+        let id = id.0;
+
+        let color = [
+            Hsla::hsl(220.0, 1.0, 0.3),
+            Hsla::hsl(180.0, 1.0, 0.3),
+            Hsla::hsl(260.0, 1.0, 0.7),
+        ][id % 3];
+
+        /*
+         * Ground
+         */
+        let ground_size = 5.1;
+        let ground_height = 0.1;
+
+        let starting_y = (id as f32) * -0.5 - ground_height;
+
+        let mut platforms = commands.spawn((
+            TransformBundle::from(Transform::from_xyz(0.0, starting_y, 0.0)),
+            Collider::cuboid(ground_size, ground_height, ground_size),
+            ColliderDebugColor(color),
+            RapierContextEntityLink(context_entity),
+        ));
+        if id == 1 {
+            platforms.insert(Platform { starting_y });
+        }
+
+        /*
+         * Create the cube
+         */
+
+        commands.spawn((
+            TransformBundle::from(Transform::from_xyz(0.0, 1.0 + id as f32 * 5.0, 0.0)),
+            RigidBody::Dynamic,
+            Collider::cuboid(0.5, 0.5, 0.5),
+            ColliderDebugColor(color),
+            RapierContextEntityLink(context_entity),
+            ActiveEvents::COLLISION_EVENTS,
+        ));
+    }
+}
+
+/* A system that displays the events. */
+fn remove_entities_colliding(
+    mut commands: Commands,
+    mut collision_events: EventReader<CollisionEvent>,
+) {
+    for collision_event in collision_events.read() {
+        match collision_event {
+            CollisionEvent::Started(e1, e2, _) => {
+                commands.entity(*e1).despawn_recursive();
+                commands.entity(*e2).despawn_recursive();
+            }
+            CollisionEvent::Stopped(_, _, _) => {
+                // Ignored
+            }
+        }
+    }
+}

From 3631e17d5318e72c8eb1789a06fd4202e5eed455 Mon Sep 17 00:00:00 2001
From: Thierry Berger <contact@thierryberger.com>
Date: Fri, 12 Jul 2024 15:53:57 +0200
Subject: [PATCH 2/4] real reproduction!

---
 .../examples/multi_world_deletion3.rs         | 145 ------------------
 .../examples/multi_world_interpolated3.rs     |  99 ++++++++++++
 2 files changed, 99 insertions(+), 145 deletions(-)
 delete mode 100644 bevy_rapier3d/examples/multi_world_deletion3.rs
 create mode 100644 bevy_rapier3d/examples/multi_world_interpolated3.rs

diff --git a/bevy_rapier3d/examples/multi_world_deletion3.rs b/bevy_rapier3d/examples/multi_world_deletion3.rs
deleted file mode 100644
index 6b455d80..00000000
--- a/bevy_rapier3d/examples/multi_world_deletion3.rs
+++ /dev/null
@@ -1,145 +0,0 @@
-use bevy::{input::common_conditions::input_just_pressed, prelude::*};
-use bevy_rapier3d::prelude::*;
-
-const N_WORLDS: usize = 2;
-
-fn main() {
-    App::new()
-        .insert_resource(ClearColor(Color::srgb(
-            0xF9 as f32 / 255.0,
-            0xF9 as f32 / 255.0,
-            0xFF as f32 / 255.0,
-        )))
-        .insert_resource(TimestepMode::Interpolated {
-            dt: 1f32 / 60f32,
-            time_scale: 1f32,
-            substeps: 4,
-        })
-        .add_plugins((
-            DefaultPlugins,
-            RapierPhysicsPlugin::<NoUserData>::default()
-                .with_default_world(RapierContextInitialization::NoAutomaticRapierContext),
-            RapierDebugRenderPlugin::default(),
-        ))
-        .add_systems(
-            Startup,
-            ((create_worlds, setup_physics).chain(), setup_graphics),
-        )
-        .add_systems(Update, move_platforms)
-        .add_systems(PostUpdate, remove_entities_colliding)
-        .add_systems(
-            Update,
-            change_world.run_if(input_just_pressed(KeyCode::KeyC)),
-        )
-        .run();
-}
-
-fn create_worlds(mut commands: Commands) {
-    for i in 0..N_WORLDS {
-        let mut world = commands.spawn((RapierContext::default(), WorldId(i)));
-        if i == 0 {
-            world.insert(DefaultRapierContext);
-        }
-    }
-}
-
-fn setup_graphics(mut commands: Commands) {
-    commands.spawn(Camera3dBundle {
-        transform: Transform::from_xyz(0.0, 3.0, -10.0)
-            .looking_at(Vec3::new(0.0, 0.0, 0.0), Vec3::Y),
-        ..Default::default()
-    });
-}
-
-#[derive(Component)]
-pub struct WorldId(pub usize);
-
-#[derive(Component)]
-struct Platform {
-    starting_y: f32,
-}
-
-fn move_platforms(time: Res<Time>, mut query: Query<(&mut Transform, &Platform)>) {
-    for (mut transform, platform) in query.iter_mut() {
-        transform.translation.y = platform.starting_y + -time.elapsed_seconds().sin();
-    }
-}
-
-/// Demonstrates how easy it is to move one entity to another world.
-fn change_world(
-    query_context: Query<Entity, With<DefaultRapierContext>>,
-    mut query_links: Query<(Entity, &mut RapierContextEntityLink)>,
-) {
-    let default_context = query_context.single();
-    for (e, mut link) in query_links.iter_mut() {
-        if link.0 == default_context {
-            continue;
-        }
-        link.0 = default_context;
-        println!("changing world of {} for world {}", e, link.0);
-    }
-}
-
-pub fn setup_physics(
-    context: Query<(Entity, &WorldId), With<RapierContext>>,
-    mut commands: Commands,
-) {
-    for (context_entity, id) in context.iter() {
-        let id = id.0;
-
-        let color = [
-            Hsla::hsl(220.0, 1.0, 0.3),
-            Hsla::hsl(180.0, 1.0, 0.3),
-            Hsla::hsl(260.0, 1.0, 0.7),
-        ][id % 3];
-
-        /*
-         * Ground
-         */
-        let ground_size = 5.1;
-        let ground_height = 0.1;
-
-        let starting_y = (id as f32) * -0.5 - ground_height;
-
-        let mut platforms = commands.spawn((
-            TransformBundle::from(Transform::from_xyz(0.0, starting_y, 0.0)),
-            Collider::cuboid(ground_size, ground_height, ground_size),
-            ColliderDebugColor(color),
-            RapierContextEntityLink(context_entity),
-        ));
-        if id == 1 {
-            platforms.insert(Platform { starting_y });
-        }
-
-        /*
-         * Create the cube
-         */
-
-        commands.spawn((
-            TransformBundle::from(Transform::from_xyz(0.0, 1.0 + id as f32 * 5.0, 0.0)),
-            RigidBody::Dynamic,
-            Collider::cuboid(0.5, 0.5, 0.5),
-            ColliderDebugColor(color),
-            RapierContextEntityLink(context_entity),
-            ActiveEvents::COLLISION_EVENTS,
-        ));
-    }
-}
-
-/* A system that displays the events. */
-fn remove_entities_colliding(
-    mut commands: Commands,
-    mut collision_events: EventReader<CollisionEvent>,
-) {
-    for collision_event in collision_events.read() {
-        match collision_event {
-            CollisionEvent::Started(e1, e2, _) => {
-                commands.entity(*e1).despawn_recursive();
-                commands.entity(*e2).despawn_recursive();
-            }
-            CollisionEvent::Stopped(_, _, _) => {
-                // Ignored
-            }
-        }
-    }
-}
diff --git a/bevy_rapier3d/examples/multi_world_interpolated3.rs b/bevy_rapier3d/examples/multi_world_interpolated3.rs
new file mode 100644
index 00000000..839199fa
--- /dev/null
+++ b/bevy_rapier3d/examples/multi_world_interpolated3.rs
@@ -0,0 +1,99 @@
+use bevy::prelude::*;
+use bevy_rapier3d::prelude::*;
+
+const N_WORLDS: usize = 2;
+
+fn main() {
+    App::new()
+        .insert_resource(ClearColor(Color::srgb(
+            0xF9 as f32 / 255.0,
+            0xF9 as f32 / 255.0,
+            0xFF as f32 / 255.0,
+        )))
+        .add_plugins((
+            DefaultPlugins,
+            RapierPhysicsPlugin::<NoUserData>::default()
+                .with_default_world(RapierContextInitialization::NoAutomaticRapierContext),
+            RapierDebugRenderPlugin::default(),
+        ))
+        .insert_resource(TimestepMode::Interpolated {
+            dt: 1.0 / 60.0,
+            time_scale: 1.0,
+            substeps: 2,
+        })
+        .add_systems(
+            Startup,
+            ((create_worlds, setup_physics).chain(), setup_graphics),
+        )
+        .add_systems(Update, change_world)
+        .run();
+}
+
+fn create_worlds(mut commands: Commands) {
+    for i in 0..N_WORLDS {
+        let mut world = commands.spawn((RapierContext::default(), WorldId(i)));
+        if i == 0 {
+            world.insert(DefaultRapierContext);
+        }
+    }
+}
+
+fn setup_graphics(mut commands: Commands) {
+    commands.spawn(Camera3dBundle {
+        transform: Transform::from_xyz(0.0, 3.0, -10.0)
+            .looking_at(Vec3::new(0.0, 0.0, 0.0), Vec3::Y),
+        ..Default::default()
+    });
+}
+
+#[derive(Component)]
+pub struct WorldId(pub usize);
+
+fn change_world(
+    query_context: Query<Entity, With<DefaultRapierContext>>,
+    q_other_ctx: Query<Entity, (With<RapierContext>, Without<DefaultRapierContext>)>,
+    mut query_links: Query<&mut RapierContextEntityLink>,
+) {
+    let default_context = query_context.single();
+    for mut link in query_links.iter_mut() {
+        if link.0 == default_context {
+            link.0 = q_other_ctx.single();
+            continue;
+        }
+        link.0 = default_context;
+    }
+}
+
+pub fn setup_physics(mut commands: Commands) {
+    /*
+     * Create the cube
+     */
+
+    let color = [
+        Hsla::hsl(220.0, 1.0, 0.3),
+        Hsla::hsl(180.0, 1.0, 0.3),
+        Hsla::hsl(260.0, 1.0, 0.7),
+    ][0 % 3];
+
+    commands.spawn((
+        TransformBundle::from(Transform::from_xyz(0.0, 0.0, 0.0)),
+        RigidBody::Dynamic,
+        Collider::cuboid(0.5, 0.5, 0.5),
+        ColliderDebugColor(color),
+        ActiveEvents::all(),
+    ));
+
+    /*
+     * Ground
+     */
+    let color = Hsla::hsl(260.0, 1.0, 0.7);
+    let ground_size = 5.1;
+    let ground_height = 0.1;
+    let starting_y = -0.5 - ground_height;
+
+    commands.spawn((
+        TransformBundle::from(Transform::from_xyz(0.0, starting_y, 0.0)),
+        Collider::cuboid(ground_size, ground_height, ground_size),
+        ColliderDebugColor(color),
+    ));
+}

From ece7d00690aa9fdb8376f4276a231f2bebfa43a7 Mon Sep 17 00:00:00 2001
From: Thierry Berger <contact@thierryberger.com>
Date: Tue, 16 Jul 2024 14:30:27 +0200
Subject: [PATCH 3/4] ugly fix by bypassing generational index

---
 Cargo.toml                                    |   2 +-
 bevy_rapier3d/examples/multi_world3.rs        |   2 +-
 .../examples/multi_world_interpolated3.rs     |  14 +-
 src/pipeline/events.rs                        |  15 +-
 src/plugin/plugin.rs                          |  12 +-
 src/plugin/systems/collider.rs                |   1 +
 src/plugin/systems/mod.rs                     |   6 +-
 .../systems/multiple_rapier_contexts.rs       | 159 +++++++++++++++++-
 src/plugin/systems/remove.rs                  |  88 +++++++---
 9 files changed, 249 insertions(+), 50 deletions(-)

diff --git a/Cargo.toml b/Cargo.toml
index 69f7b7de..27338c04 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -14,7 +14,7 @@ codegen-units = 1
 #parry2d = { path = "../parry/crates/parry2d" }
 #parry3d = { path = "../parry/crates/parry3d" }
 #rapier2d = { path = "../rapier/crates/rapier2d" }
-#rapier3d = { path = "../rapier/crates/rapier3d" }
+rapier3d = { path = "../rapier/crates/rapier3d" }
 
 #nalgebra = { git = "https://github.com/dimforge/nalgebra", branch = "dev" }
 #parry2d = { git = "https://github.com/dimforge/parry", branch = "master" }
diff --git a/bevy_rapier3d/examples/multi_world3.rs b/bevy_rapier3d/examples/multi_world3.rs
index 65e448cb..dc5fa807 100644
--- a/bevy_rapier3d/examples/multi_world3.rs
+++ b/bevy_rapier3d/examples/multi_world3.rs
@@ -13,7 +13,7 @@ fn main() {
         .add_plugins((
             DefaultPlugins,
             RapierPhysicsPlugin::<NoUserData>::default()
-                .with_default_world(RapierContextInitialization::NoAutomaticRapierContext),
+                .with_custom_initialization(RapierContextInitialization::NoAutomaticRapierContext),
             RapierDebugRenderPlugin::default(),
         ))
         .add_systems(
diff --git a/bevy_rapier3d/examples/multi_world_interpolated3.rs b/bevy_rapier3d/examples/multi_world_interpolated3.rs
index 839199fa..0631621e 100644
--- a/bevy_rapier3d/examples/multi_world_interpolated3.rs
+++ b/bevy_rapier3d/examples/multi_world_interpolated3.rs
@@ -1,4 +1,4 @@
-use bevy::prelude::*;
+use bevy::{input::common_conditions::input_just_pressed, prelude::*};
 use bevy_rapier3d::prelude::*;
 
 const N_WORLDS: usize = 2;
@@ -13,7 +13,7 @@ fn main() {
         .add_plugins((
             DefaultPlugins,
             RapierPhysicsPlugin::<NoUserData>::default()
-                .with_default_world(RapierContextInitialization::NoAutomaticRapierContext),
+                .with_custom_initialization(RapierContextInitialization::NoAutomaticRapierContext),
             RapierDebugRenderPlugin::default(),
         ))
         .insert_resource(TimestepMode::Interpolated {
@@ -25,13 +25,16 @@ fn main() {
             Startup,
             ((create_worlds, setup_physics).chain(), setup_graphics),
         )
-        .add_systems(Update, change_world)
+        .add_systems(
+            Update,
+            change_world, //.run_if(input_just_pressed(KeyCode::KeyC)),
+        )
         .run();
 }
 
 fn create_worlds(mut commands: Commands) {
     for i in 0..N_WORLDS {
-        let mut world = commands.spawn((RapierContext::default(), WorldId(i)));
+        let mut world = commands.spawn(RapierContext::default());
         if i == 0 {
             world.insert(DefaultRapierContext);
         }
@@ -46,9 +49,6 @@ fn setup_graphics(mut commands: Commands) {
     });
 }
 
-#[derive(Component)]
-pub struct WorldId(pub usize);
-
 fn change_world(
     query_context: Query<Entity, With<DefaultRapierContext>>,
     q_other_ctx: Query<Entity, (With<RapierContext>, Without<DefaultRapierContext>)>,
diff --git a/src/pipeline/events.rs b/src/pipeline/events.rs
index 897d8b02..3f558b59 100644
--- a/src/pipeline/events.rs
+++ b/src/pipeline/events.rs
@@ -64,11 +64,18 @@ pub(crate) struct EventQueue<'a> {
 
 impl<'a> EventQueue<'a> {
     fn collider2entity(&self, colliders: &ColliderSet, handle: ColliderHandle) -> Entity {
+        let handle_index = handle.into_raw_parts().0;
         colliders
-            .get(handle)
-            .map(|co| Entity::from_bits(co.user_data as u64))
-            .or_else(|| self.deleted_colliders.get(&handle).copied())
-            .expect("Internal error: entity not found for collision event.")
+            .get_unknown_gen(handle_index)
+            .map(|co| Entity::from_bits(co.0.user_data as u64))
+            .or_else(|| self.deleted_colliders.iter().find_map(|co| if co.0.into_raw_parts().0 == handle_index {
+                Some(*co.1)
+            }else {None}))
+            .unwrap_or_else(|| {
+                println!("{:#?}", colliders);
+                dbg!(&self.deleted_colliders);
+                panic!("Internal error: entity not found for collision event with collider handle: {handle:?}.")
+            })
     }
 }
 
diff --git a/src/plugin/plugin.rs b/src/plugin/plugin.rs
index 4ca2cfd4..00c83539 100644
--- a/src/plugin/plugin.rs
+++ b/src/plugin/plugin.rs
@@ -47,10 +47,8 @@ where
         self
     }
 
-    /// Specifies a default world initialization strategy.
-    ///
-    /// The default is to initialize a [`RapierContext`] with a length unit of 1.
-    pub fn with_default_world(
+    /// Specifies an initialization strategy when the [`App`] starts.
+    pub fn with_custom_initialization(
         mut self,
         default_world_initialization: RapierContextInitialization,
     ) -> Self {
@@ -110,6 +108,7 @@ where
                 systems::on_add_entity_with_parent,
                 systems::on_change_world,
                 systems::sync_removals,
+                apply_deferred,
                 #[cfg(all(feature = "dim3", feature = "async-collider"))]
                 systems::init_async_scene_colliders,
                 #[cfg(all(feature = "dim3", feature = "async-collider"))]
@@ -206,8 +205,7 @@ where
             .register_type::<RapierConfiguration>()
             .register_type::<SimulationToRenderTime>()
             .register_type::<DefaultRapierContext>()
-            .register_type::<RapierContextInitialization>()
-            .register_type::<ColliderDebugColor>();
+            .register_type::<RapierContextInitialization>();
 
         app.insert_resource(Events::<CollisionEvent>::default())
             .insert_resource(Events::<ContactForceEvent>::default())
@@ -215,7 +213,7 @@ where
         let default_world_init = app.world().get_resource::<RapierContextInitialization>();
         if let Some(world_init) = default_world_init {
             warn!("RapierPhysicsPlugin added but a `RapierContextInitialization` resource was already existing.\
-            This might overwrite previous configuration made via `RapierPhysicsPlugin::with_default_world`\
+            This will overwrite previous configuration made via `RapierPhysicsPlugin::with_custom_initialization`\
             or `RapierPhysicsPlugin::with_length_unit`.
             The following resource will be used: {:?}", world_init);
         } else {
diff --git a/src/plugin/systems/collider.rs b/src/plugin/systems/collider.rs
index be3aa755..37552cc6 100644
--- a/src/plugin/systems/collider.rs
+++ b/src/plugin/systems/collider.rs
@@ -384,6 +384,7 @@ pub fn init_colliders(
         let Some(context_entity) = context_entity else {
             continue;
         };
+        println!("init collider {entity} for world {context_entity}");
 
         let config = config.get(context_entity).unwrap_or_else(|_| {
             panic!("Failed to retrieve `RapierConfiguration` on entity {context_entity}.")
diff --git a/src/plugin/systems/mod.rs b/src/plugin/systems/mod.rs
index f22423b2..4cd46a35 100644
--- a/src/plugin/systems/mod.rs
+++ b/src/plugin/systems/mod.rs
@@ -28,6 +28,7 @@ use bevy::prelude::*;
 /// for scene queries.
 pub fn step_simulation<Hooks>(
     mut context: Query<(
+        Entity,
         &mut RapierContext,
         &RapierConfiguration,
         &mut SimulationToRenderTime,
@@ -44,10 +45,12 @@ pub fn step_simulation<Hooks>(
 {
     let hooks_adapter = BevyPhysicsHooksAdapter::new(hooks.into_inner());
 
-    for (mut context, config, mut sim_to_render_time) in context.iter_mut() {
+    for (entity, mut context, config, mut sim_to_render_time) in context.iter_mut() {
         let context = &mut *context;
 
         if config.physics_pipeline_active {
+            println!("step {entity}");
+            //println!("step {:?}", context.colliders);
             context.step_simulation(
                 config.gravity,
                 *timestep_mode,
@@ -57,6 +60,7 @@ pub fn step_simulation<Hooks>(
                 &mut sim_to_render_time,
                 Some(&mut interpolation_query),
             );
+            println!("clear");
             context.deleted_colliders.clear();
         } else {
             context.propagate_modified_body_positions_to_colliders();
diff --git a/src/plugin/systems/multiple_rapier_contexts.rs b/src/plugin/systems/multiple_rapier_contexts.rs
index fa9944fe..4f1d60c8 100644
--- a/src/plugin/systems/multiple_rapier_contexts.rs
+++ b/src/plugin/systems/multiple_rapier_contexts.rs
@@ -27,8 +27,11 @@ pub fn on_add_entity_with_parent(
         let mut parent = Some(parent.get());
         while let Some(parent_entity) = parent {
             if let Ok(pw) = q_physics_world.get(parent_entity) {
-                commands.entity(ent).insert(*pw);
-                remove_old_physics(ent, &mut commands);
+                if q_physics_world.get(ent).map(|x| x != pw).unwrap_or(true) {
+                    info!("Removing old physx for {ent:?}!");
+                    remove_old_physics(ent, &mut commands);
+                    commands.entity(ent).insert(*pw);
+                }
                 break;
             }
             parent = q_parent.get(parent_entity).ok().map(|x| x.get());
@@ -72,7 +75,7 @@ pub fn on_change_world(
             })
             .unwrap_or(false)
         {
-            remove_old_physics(dbg!(entity), &mut commands);
+            remove_old_physics(entity, &mut commands);
             bubble_down_world_change(
                 &mut commands,
                 entity,
@@ -103,6 +106,7 @@ fn bubble_down_world_change(
         {
             return;
         }
+        println!("bubbling down for {child}");
 
         remove_old_physics(child, commands);
         commands.entity(child).insert(new_physics_world);
@@ -121,10 +125,13 @@ fn bubble_down_world_change(
 mod test {
     use crate::plugin::systems::tests::HeadlessRenderPlugin;
     use crate::plugin::{
-        NoUserData, PhysicsSet, RapierContext, RapierContextEntityLink, RapierPhysicsPlugin,
+        systems, DefaultRapierContext, NoUserData, PhysicsSet, RapierContext,
+        RapierContextEntityLink, RapierContextInitialization, RapierPhysicsPlugin, TimestepMode,
     };
     use crate::prelude::{ActiveEvents, Collider, ContactForceEventThreshold, RigidBody, Sensor};
+    use bevy::ecs::schedule::Stepping;
     use bevy::prelude::*;
+    use bevy::tasks::{IoTaskPool, TaskPoolBuilder};
     use bevy::time::{TimePlugin, TimeUpdateStrategy};
     use rapier::math::Real;
 
@@ -217,4 +224,148 @@ mod test {
                 });
         }
     }
+
+    #[test]
+    pub fn spam_change_context_interpolated() {
+        let mut app = App::new();
+        app.add_plugins((
+            HeadlessRenderPlugin,
+            MinimalPlugins,
+            TransformPlugin,
+            RapierPhysicsPlugin::<NoUserData>::default()
+                .with_custom_initialization(RapierContextInitialization::NoAutomaticRapierContext),
+        ))
+        .insert_resource(TimestepMode::Interpolated {
+            dt: 1.0 / 60.0,
+            time_scale: 1.0,
+            substeps: 2,
+        })
+        .add_systems(Startup, (create_worlds, setup_physics).chain())
+        .add_systems(Update, change_world);
+        // Simulates 60 updates per seconds
+        //app.insert_resource(TimeUpdateStrategy::ManualDuration(
+        //    std::time::Duration::from_secs_f32(1f32 / 60f32),
+        //));
+        // First update for setups, + creating default rapier context.
+
+        //app.run();
+        //return;
+
+        app.update();
+        let world = app.world_mut();
+
+        let mut query = world.query_filtered::<Entity, With<DefaultRapierContext>>();
+        let default_rapier_context = query.single(world);
+        let mut query =
+            world.query_filtered::<Entity, (With<RapierContext>, Without<DefaultRapierContext>)>();
+        let new_rapier_context = query.single(world);
+
+        let mut stepping = Stepping::new();
+        stepping
+            .add_schedule(PostUpdate)
+            .enable()
+            .set_breakpoint(PostUpdate, systems::sync_removals)
+            .set_breakpoint(PostUpdate, systems::init_rigid_bodies);
+        app.insert_resource(stepping);
+
+        let (mut context_with_entities, mut empty_context) =
+            (new_rapier_context, default_rapier_context);
+        for i in 0..1000 {
+            // swap contexts
+            (context_with_entities, empty_context) = (empty_context, context_with_entities);
+
+            let mut stepping = app.world_mut().resource_mut::<Stepping>();
+            stepping.continue_frame();
+            // Breaking before sync_removals.
+            app.update();
+            // TODO: check that there are no deletect colliders, step once or twice (apply deferred), check there are
+
+            let world = app.world_mut();
+            let mut query = world.query::<&RapierContext>();
+            let context_with_entities_data = query.get(world, context_with_entities).unwrap();
+            assert_eq!(context_with_entities_data.deleted_colliders.len(), 0);
+
+            let mut stepping = app.world_mut().resource_mut::<Stepping>();
+            stepping.continue_frame();
+            // Stepping after sync_removals.
+            app.update();
+            let world = app.world_mut();
+            let mut query = world.query::<&RapierContext>();
+            let context_with_entities_data = query.get(world, context_with_entities).unwrap();
+            if context_with_entities_data.deleted_colliders.len() != 2 {
+                println!("This will crash! (rapier context: {context_with_entities})")
+            }
+            if context_with_entities_data.deleted_colliders.len() != 2 {
+                println!("This will crash! (rapier context: {context_with_entities})")
+            }
+            println!("{:?}", context_with_entities_data.colliders);
+            let mut stepping = app.world_mut().resource_mut::<Stepping>();
+            stepping.continue_frame();
+
+            // Finishing the frame.
+            app.update();
+
+            let mut world = app.world_mut();
+            let mut query =
+                world.query_filtered::<&RapierContextEntityLink, With<MarkerChangeWorld>>();
+            for link in query.iter(&mut world) {
+                assert_eq!(link.0, context_with_entities);
+                // TODO: stepping + verify deleted colliders
+            }
+        }
+        return;
+
+        #[derive(Component)]
+        pub struct MarkerChangeWorld;
+
+        #[cfg(feature = "dim3")]
+        fn cuboid(hx: Real, hy: Real, hz: Real) -> Collider {
+            Collider::cuboid(hx, hy, hz)
+        }
+        #[cfg(feature = "dim2")]
+        fn cuboid(hx: Real, hy: Real, _hz: Real) -> Collider {
+            Collider::cuboid(hx, hy)
+        }
+        fn create_worlds(mut commands: Commands) {
+            for i in 0..2 {
+                let mut world = commands.spawn(RapierContext::default());
+                if i == 0 {
+                    world.insert(DefaultRapierContext);
+                }
+            }
+        }
+        pub fn setup_physics(mut commands: Commands) {
+            commands.spawn((
+                TransformBundle::from(Transform::from_xyz(0.0, 0.0, 0.0)),
+                RigidBody::Dynamic,
+                cuboid(0.5, 0.5, 0.5),
+                ActiveEvents::all(),
+            ));
+            /*
+             * Ground
+             */
+            let ground_size = 5.1;
+            let ground_height = 0.1;
+            let starting_y = -0.5 - ground_height;
+
+            commands.spawn((
+                TransformBundle::from(Transform::from_xyz(0.0, starting_y, 0.0)),
+                cuboid(ground_size, ground_height, ground_size),
+            ));
+        }
+        fn change_world(
+            query_context: Query<Entity, With<DefaultRapierContext>>,
+            q_other_ctx: Query<Entity, (With<RapierContext>, Without<DefaultRapierContext>)>,
+            mut query_links: Query<&mut RapierContextEntityLink>,
+        ) {
+            let default_context = query_context.single();
+            for mut link in query_links.iter_mut() {
+                if link.0 == default_context {
+                    link.0 = q_other_ctx.single();
+                    continue;
+                }
+                link.0 = default_context;
+            }
+        }
+    }
 }
diff --git a/src/plugin/systems/remove.rs b/src/plugin/systems/remove.rs
index f1ca65b0..1f78b648 100644
--- a/src/plugin/systems/remove.rs
+++ b/src/plugin/systems/remove.rs
@@ -12,6 +12,7 @@ use crate::plugin::RapierContext;
 use crate::prelude::MassModifiedEvent;
 use crate::prelude::RigidBodyDisabled;
 use crate::prelude::Sensor;
+use bevy::ecs::query::QueryData;
 use bevy::prelude::*;
 
 /// System responsible for removing from Rapier the rigid-bodies/colliders/joints which had
@@ -19,7 +20,8 @@ use bevy::prelude::*;
 /// despawn).
 pub fn sync_removals(
     mut commands: Commands,
-    mut context_accessor: RapierContextAccessMut,
+    //mut context_accessor: RapierContextAccessMut,
+    mut context_accessor: RapierContextAccessMutEntity,
     mut removed_bodies: RemovedComponents<RapierRigidBodyHandle>,
     mut removed_colliders: RemovedComponents<RapierColliderHandle>,
     mut removed_impulse_joints: RemovedComponents<RapierImpulseJointHandle>,
@@ -42,12 +44,13 @@ pub fn sync_removals(
      * Rigid-bodies removal detection.
      */
     for entity in removed_bodies.read() {
-        let Some((mut context, handle)) = find_context(&mut context_accessor, |context| {
+        let Some((mut context, handle)) = find_context_entity(&mut context_accessor, |context| {
             context.entity2body.remove(&entity)
         }) else {
+            dbg!("no context1!");
             continue;
         };
-        let context = &mut *context;
+        let context = &mut *context.1;
 
         let _ = context.last_body_transform_set.remove(&handle);
         context.bodies.remove(
@@ -61,10 +64,10 @@ pub fn sync_removals(
     }
 
     for entity in orphan_bodies.iter() {
-        if let Some((mut context, handle)) = find_context(&mut context_accessor, |context| {
+        if let Some((mut context, handle)) = find_context_entity(&mut context_accessor, |context| {
             context.entity2body.remove(&entity)
         }) {
-            let context = &mut *context;
+            let context = &mut *context.1;
             let _ = context.last_body_transform_set.remove(&handle);
             context.bodies.remove(
                 handle,
@@ -82,12 +85,15 @@ pub fn sync_removals(
      * Collider removal detection.
      */
     for entity in removed_colliders.read() {
-        let Some((mut context, handle)) = find_context(&mut context_accessor, |context| {
-            context.entity2collider.remove(&entity)
-        }) else {
+        let Some((mut context_accessed, handle)) =
+            find_context_entity(&mut context_accessor, |context| {
+                context.entity2collider.remove(&entity)
+            })
+        else {
+            dbg!("no context2!");
             continue;
         };
-        let context = &mut *context;
+        let context = &mut *context_accessed.1;
         if let Some(parent) = context.collider_parent(entity) {
             mass_modified.send(parent.into());
         }
@@ -95,14 +101,16 @@ pub fn sync_removals(
         context
             .colliders
             .remove(handle, &mut context.islands, &mut context.bodies, true);
+
+        println!("removed {handle:?} from {}", context_accessed.0);
         context.deleted_colliders.insert(handle, entity);
     }
 
     for entity in orphan_colliders.iter() {
-        if let Some((mut context, handle)) = find_context(&mut context_accessor, |context| {
+        if let Some((mut context, handle)) = find_context_entity(&mut context_accessor, |context| {
             context.entity2collider.remove(&entity)
         }) {
-            let context = &mut *context;
+            let context = &mut *context.1;
             if let Some(parent) = context.collider_parent(entity) {
                 mass_modified.send(parent.into());
             }
@@ -119,20 +127,20 @@ pub fn sync_removals(
      * Impulse joint removal detection.
      */
     for entity in removed_impulse_joints.read() {
-        let Some((mut context, handle)) = find_context(&mut context_accessor, |context| {
+        let Some((mut context, handle)) = find_context_entity(&mut context_accessor, |context| {
             context.entity2impulse_joint.remove(&entity)
         }) else {
             continue;
         };
-        let context = &mut *context;
+        let context = &mut *context.1;
         context.impulse_joints.remove(handle, true);
     }
 
     for entity in orphan_impulse_joints.iter() {
-        if let Some((mut context, handle)) = find_context(&mut context_accessor, |context| {
+        if let Some((mut context, handle)) = find_context_entity(&mut context_accessor, |context| {
             context.entity2impulse_joint.remove(&entity)
         }) {
-            let context = &mut *context;
+            let context = &mut *context.1;
             context.impulse_joints.remove(handle, true);
         }
         commands.entity(entity).remove::<RapierImpulseJointHandle>();
@@ -142,20 +150,20 @@ pub fn sync_removals(
      * Multibody joint removal detection.
      */
     for entity in removed_multibody_joints.read() {
-        let Some((mut context, handle)) = find_context(&mut context_accessor, |context| {
+        let Some((mut context, handle)) = find_context_entity(&mut context_accessor, |context| {
             context.entity2multibody_joint.remove(&entity)
         }) else {
             continue;
         };
-        let context = &mut *context;
+        let context = &mut *context.1;
         context.multibody_joints.remove(handle, true);
     }
 
     for entity in orphan_multibody_joints.iter() {
-        if let Some((mut context, handle)) = find_context(&mut context_accessor, |context| {
+        if let Some((mut context, handle)) = find_context_entity(&mut context_accessor, |context| {
             context.entity2multibody_joint.remove(&entity)
         }) {
-            let context = &mut *context;
+            let context = &mut *context.1;
             context.multibody_joints.remove(handle, true);
         }
         commands
@@ -167,30 +175,30 @@ pub fn sync_removals(
      * Marker components removal detection.
      */
     for entity in removed_sensors.read() {
-        if let Some((mut context, handle)) = find_context(&mut context_accessor, |context| {
+        if let Some((mut context, handle)) = find_context_entity(&mut context_accessor, |context| {
             context.entity2collider.get(&entity).copied()
         }) {
-            if let Some(co) = context.colliders.get_mut(handle) {
+            if let Some(co) = context.1.colliders.get_mut(handle) {
                 co.set_sensor(false);
             }
         }
     }
 
     for entity in removed_colliders_disabled.read() {
-        if let Some((mut context, handle)) = find_context(&mut context_accessor, |context| {
+        if let Some((mut context, handle)) = find_context_entity(&mut context_accessor, |context| {
             context.entity2collider.get(&entity).copied()
         }) {
-            if let Some(co) = context.colliders.get_mut(handle) {
+            if let Some(co) = context.1.colliders.get_mut(handle) {
                 co.set_enabled(true);
             }
         }
     }
 
     for entity in removed_rigid_body_disabled.read() {
-        if let Some((mut context, handle)) = find_context(&mut context_accessor, |context| {
+        if let Some((mut context, handle)) = find_context_entity(&mut context_accessor, |context| {
             context.entity2body.get(&entity).copied()
         }) {
-            if let Some(rb) = context.bodies.get_mut(handle) {
+            if let Some(rb) = context.1.bodies.get_mut(handle) {
                 rb.set_enabled(true);
             }
         }
@@ -208,3 +216,33 @@ fn find_context<'a, T>(
         .iter_mut()
         .find_map(|mut context| item_finder(&mut context).map(|handle| (context, handle)))
 }
+
+/// Utility [`SystemParam`] to easily access any [`RapierContext`] mutably
+#[derive(bevy::ecs::system::SystemParam)]
+pub struct RapierContextAccessMutEntity<'w, 's> {
+    /// Query used to retrieve a [`RapierContext`].
+    /// It's helpful to iterate over every rapier contexts,
+    /// or get a handle over a specific context, for example through:
+    /// - a marker component such as [`DefaultRapierContext`]
+    /// - a [`RapierContextEntityLink`]. See [context](RapierContextAccess::context)
+    pub rapier_context: Query<'w, 's, (Entity, &'static mut RapierContext)>,
+}
+
+fn find_context_entity<'a, T>(
+    context_accessor: &'a mut RapierContextAccessMutEntity,
+    item_finder: impl Fn(&mut RapierContext) -> Option<T>,
+) -> Option<((Entity, Mut<'a, RapierContext>), T)> {
+    context_accessor
+        .rapier_context
+        .iter_mut()
+        .find_map(|mut context| item_finder(&mut context.1).map(|handle| (context, handle)))
+}
+
+fn find_context_generic<'a, 'w, 's, P: QueryData, T>(
+    context_accessor: &'a mut Query<'w, 's, P>,
+    item_finder: impl Fn(&mut <P as bevy::ecs::query::WorldQuery>::Item<'a>) -> Option<T>,
+) -> Option<(<P as bevy::ecs::query::WorldQuery>::Item<'a>, T)> {
+    context_accessor
+        .iter_mut()
+        .find_map(|mut context| item_finder(&mut context).map(|handle| (context, handle)))
+}

From 74866d1e651c5bacbcaaa6decf1a303a9ec5c93f Mon Sep 17 00:00:00 2001
From: Thierry Berger <contact@thierryberger.com>
Date: Tue, 16 Jul 2024 16:55:42 +0200
Subject: [PATCH 4/4] more investigation, I probably have a path to a fix

---
 src/pipeline/events.rs                         | 11 +++--------
 src/plugin/context/mod.rs                      |  1 +
 src/plugin/systems/mod.rs                      |  3 ++-
 src/plugin/systems/multiple_rapier_contexts.rs |  9 ++++-----
 4 files changed, 10 insertions(+), 14 deletions(-)

diff --git a/src/pipeline/events.rs b/src/pipeline/events.rs
index 3f558b59..0223f43a 100644
--- a/src/pipeline/events.rs
+++ b/src/pipeline/events.rs
@@ -64,16 +64,11 @@ pub(crate) struct EventQueue<'a> {
 
 impl<'a> EventQueue<'a> {
     fn collider2entity(&self, colliders: &ColliderSet, handle: ColliderHandle) -> Entity {
-        let handle_index = handle.into_raw_parts().0;
         colliders
-            .get_unknown_gen(handle_index)
-            .map(|co| Entity::from_bits(co.0.user_data as u64))
-            .or_else(|| self.deleted_colliders.iter().find_map(|co| if co.0.into_raw_parts().0 == handle_index {
-                Some(*co.1)
-            }else {None}))
+            .get(handle)
+            .map(|co| Entity::from_bits(co.user_data as u64))
+            .or_else(|| self.deleted_colliders.get(&handle).copied())
             .unwrap_or_else(|| {
-                println!("{:#?}", colliders);
-                dbg!(&self.deleted_colliders);
                 panic!("Internal error: entity not found for collision event with collider handle: {handle:?}.")
             })
     }
diff --git a/src/plugin/context/mod.rs b/src/plugin/context/mod.rs
index 445ab7a9..91b031dd 100644
--- a/src/plugin/context/mod.rs
+++ b/src/plugin/context/mod.rs
@@ -289,6 +289,7 @@ impl RapierContext {
                     substep_integration_parameters.dt = dt / (substeps as Real) * time_scale;
 
                     for _ in 0..substeps {
+                        dbg!("execute step");
                         self.pipeline.step(
                             &gravity.into(),
                             &substep_integration_parameters,
diff --git a/src/plugin/systems/mod.rs b/src/plugin/systems/mod.rs
index 4cd46a35..83688c70 100644
--- a/src/plugin/systems/mod.rs
+++ b/src/plugin/systems/mod.rs
@@ -61,7 +61,8 @@ pub fn step_simulation<Hooks>(
                 Some(&mut interpolation_query),
             );
             println!("clear");
-            context.deleted_colliders.clear();
+            // That's the offending line ! This should be within `step_simulation` only if we actually step.
+            // context.deleted_colliders.clear();
         } else {
             context.propagate_modified_body_positions_to_colliders();
         }
diff --git a/src/plugin/systems/multiple_rapier_contexts.rs b/src/plugin/systems/multiple_rapier_contexts.rs
index 4f1d60c8..8df41432 100644
--- a/src/plugin/systems/multiple_rapier_contexts.rs
+++ b/src/plugin/systems/multiple_rapier_contexts.rs
@@ -236,7 +236,7 @@ mod test {
                 .with_custom_initialization(RapierContextInitialization::NoAutomaticRapierContext),
         ))
         .insert_resource(TimestepMode::Interpolated {
-            dt: 1.0 / 60.0,
+            dt: 1.0 / 600.0,
             time_scale: 1.0,
             substeps: 2,
         })
@@ -283,7 +283,7 @@ mod test {
             let world = app.world_mut();
             let mut query = world.query::<&RapierContext>();
             let context_with_entities_data = query.get(world, context_with_entities).unwrap();
-            assert_eq!(context_with_entities_data.deleted_colliders.len(), 0);
+            //assert_eq!(context_with_entities_data.deleted_colliders.len(), 0);
 
             let mut stepping = app.world_mut().resource_mut::<Stepping>();
             stepping.continue_frame();
@@ -292,13 +292,12 @@ mod test {
             let world = app.world_mut();
             let mut query = world.query::<&RapierContext>();
             let context_with_entities_data = query.get(world, context_with_entities).unwrap();
-            if context_with_entities_data.deleted_colliders.len() != 2 {
+            /*if context_with_entities_data.deleted_colliders.len() != 2 {
                 println!("This will crash! (rapier context: {context_with_entities})")
-            }
+            }*/
             if context_with_entities_data.deleted_colliders.len() != 2 {
                 println!("This will crash! (rapier context: {context_with_entities})")
             }
-            println!("{:?}", context_with_entities_data.colliders);
             let mut stepping = app.world_mut().resource_mut::<Stepping>();
             stepping.continue_frame();