From 6fb6b05d8f4c191108c7a40438c48eb110d6ab65 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Fran=C3=A7ois?= Date: Tue, 5 Jul 2022 18:44:54 +0000 Subject: [PATCH] add a more helpful error to help debug panicking command on despawned entity (#5198) # Objective - Help users fix issue when their app panic when executing a command on a despawned entity ## Solution - Add an error code and a page describing how to debug the issue --- crates/bevy_ecs/src/system/commands/mod.rs | 12 +-- errors/B0003.md | 91 ++++++++++++++++++++++ errors/src/lib.rs | 3 + 3 files changed, 97 insertions(+), 9 deletions(-) create mode 100644 errors/B0003.md diff --git a/crates/bevy_ecs/src/system/commands/mod.rs b/crates/bevy_ecs/src/system/commands/mod.rs index 1c78d473c124b..29abf060243b5 100644 --- a/crates/bevy_ecs/src/system/commands/mod.rs +++ b/crates/bevy_ecs/src/system/commands/mod.rs @@ -712,9 +712,7 @@ pub struct Despawn { impl Command for Despawn { fn write(self, world: &mut World) { if !world.despawn(self.entity) { - warn!("Could not despawn entity {:?} because it doesn't exist in this World.\n\ - If this command was added to a newly spawned entity, ensure that you have not despawned that entity within the same stage.\n\ - This may have occurred due to system order ambiguity, or if the spawning system has multiple command buffers", self.entity); + warn!("error[B0003]: Could not despawn entity {:?} because it doesn't exist in this World.", self.entity); } } } @@ -732,9 +730,7 @@ where if let Some(mut entity) = world.get_entity_mut(self.entity) { entity.insert_bundle(self.bundle); } else { - panic!("Could not insert a bundle (of type `{}`) for entity {:?} because it doesn't exist in this World.\n\ - If this command was added to a newly spawned entity, ensure that you have not despawned that entity within the same stage.\n\ - This may have occurred due to system order ambiguity, or if the spawning system has multiple command buffers", std::any::type_name::(), self.entity); + panic!("error[B0003]: Could not insert a bundle (of type `{}`) for entity {:?} because it doesn't exist in this World.", std::any::type_name::(), self.entity); } } } @@ -753,9 +749,7 @@ where if let Some(mut entity) = world.get_entity_mut(self.entity) { entity.insert(self.component); } else { - panic!("Could not add a component (of type `{}`) to entity {:?} because it doesn't exist in this World.\n\ - If this command was added to a newly spawned entity, ensure that you have not despawned that entity within the same stage.\n\ - This may have occurred due to system order ambiguity, or if the spawning system has multiple command buffers", std::any::type_name::(), self.entity); + panic!("error[B0003]: Could not add a component (of type `{}`) to entity {:?} because it doesn't exist in this World.", std::any::type_name::(), self.entity); } } } diff --git a/errors/B0003.md b/errors/B0003.md new file mode 100644 index 0000000000000..829c4b76f7f8e --- /dev/null +++ b/errors/B0003.md @@ -0,0 +1,91 @@ +# B0003 + +As commands are executed asynchronously, it is possible to issue a command on an entity that will no longer exist at the time of the command execution. + +Erroneous code example: + +```rust,should_panic +use bevy::prelude::*; + +fn main() { + App::new() + .add_plugins(DefaultPlugins) + .add_startup_system(setup) + .add_system(despawning) + .add_system(use_entity.after(despawning)) + .run(); +} + +struct MyEntity(Entity); + +#[derive(Component)] +struct Hello; + +fn setup(mut commands: Commands) { + let entity = commands.spawn().id(); + commands.insert_resource(MyEntity(entity)); +} + +fn despawning(mut commands: Commands, entity: Option>) { + if let Some(my_entity) = entity { + commands.entity(my_entity.0).despawn(); + commands.remove_resource::(); + } +} + +fn use_entity(mut commands: Commands, entity: Option>) { + if let Some(my_entity) = entity { + commands.entity(my_entity.0).insert(Hello); + } +} +``` + +This will panic, as system `use_entity` is executed after system `despawning`. Without the system ordering specified here, the ordering would be random and this code would panic half the time. + +The default panic message is telling you the entity id (`0v0`) and the command that failed (adding a component `Hello`): + +```text +thread 'main' panicked at 'error[B0003]: Could not add a component (of type `use_entity_after_despawn::Hello`) to entity 0v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:752:13 +``` + +But you don't know which system tried to add a component, and which system despawned the entity. + +To get the system that created the command that panics, you can enable the `trace` feature of Bevy. This will add a panic handler that will print more informations: + +```text + 0: bevy_ecs::schedule::stage::system_commands + with name="use_entity_after_despawn::use_entity" + at crates/bevy_ecs/src/schedule/stage.rs:880 + 1: bevy_ecs::schedule::stage + with name=Update + at crates/bevy_ecs/src/schedule/mod.rs:337 + 2: bevy_app::app::frame + at crates/bevy_app/src/app.rs:113 + 3: bevy_app::app::bevy_app + at crates/bevy_app/src/app.rs:126 +thread 'main' panicked at 'error[B0003]: Could not add a component (of type `use_entity_after_despawn::Hello`) to entity 0v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:752:13 +``` + +From the first two lines, you now know that it panics while executing a command from the system `use_entity`. + +To get the system that created the despawn command, you can enable DEBUG logs for crate `bevy_ecs`, for example by setting the environment variable `RUST_LOG=bevy_ecs=debug`. This will log: + +```text +DEBUG stage{name=Update}:system_commands{name="use_entity_after_despawn::despawning"}: bevy_ecs::world: Despawning entity 0v0 +thread 'main' panicked at 'error[B0003]: Could not add a component (of type `use_entity_after_despawn::Hello`) to entity 0v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:752:13 +``` + +From the first line, you know the entity `0v0` was despawned when executing a command from system `despawning`. In a real case, you could have many log lines, you will need to search for the exact entity from the panic message. + +Combining those two, you should get enough informations to understand why this panic is happening and how to fix it: + +```text +DEBUG stage{name=Update}:system_commands{name="use_entity_after_despawn::despawning"}: bevy_ecs::world: Despawning entity 0v0 + 0: bevy_ecs::schedule::stage::system_commands + with name="use_entity_after_despawn::use_entity" + at crates/bevy_ecs/src/schedule/stage.rs:880 + 1: bevy_ecs::schedule::stage + with name=Update + at crates/bevy_ecs/src/schedule/mod.rs:337 +thread 'main' panicked at 'error[B0003]: Could not add a component (of type `use_entity_after_despawn::Hello`) to entity 0v0 because it doesn't exist in this World.', /bevy/crates/bevy_ecs/src/system/commands/mod.rs:752:13 +``` diff --git a/errors/src/lib.rs b/errors/src/lib.rs index efa56cf95b6b1..c396f5d494cf0 100644 --- a/errors/src/lib.rs +++ b/errors/src/lib.rs @@ -3,3 +3,6 @@ pub struct B0001; #[doc = include_str!("../B0002.md")] pub struct B0002; + +#[doc = include_str!("../B0003.md")] +pub struct B0003;