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Reduce internal system order ambiguities, and add an example explaini…
…ng them (bevyengine#7383) # Objective - Bevy should not have any "internal" execution order ambiguities. These clutter the output of user-facing error reporting, and can result in nasty, nondetermistic, very difficult to solve bugs. - Verifying this currently involves repeated non-trivial manual work. ## Solution - [x] add an example to quickly check this - ~~[ ] ensure that this example panics if there are any unresolved ambiguities~~ - ~~[ ] run the example in CI 😈~~ There's one tricky ambiguity left, between UI and animation. I don't have the tools to fix this without system set configuration, so the remaining work is going to be left to bevyengine#7267 or another PR after that. ``` 2023-01-27T18:38:42.989405Z INFO bevy_ecs::schedule::ambiguity_detection: Execution order ambiguities detected, you might want to add an explicit dependency relation between some of these systems: * Parallel systems: -- "bevy_animation::animation_player" and "bevy_ui::flex::flex_node_system" conflicts: ["bevy_transform::components::transform::Transform"] ``` ## Changelog Resolved internal execution order ambiguities for: 1. Transform propagation (ignored, we need smarter filter checking). 2. Gamepad processing (fixed). 3. bevy_winit's window handling (fixed). 4. Cascaded shadow maps and perspectives (fixed). Also fixed a desynchronized state bug that could occur when the `Window` component is removed and then added to the same entity in a single frame.
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| Original file line number | Diff line number | Diff line change |
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| //! By default, Bevy systems run in parallel with each other. | ||
| //! Unless the order is explicitly specified, their relative order is nondeterministic. | ||
| //! | ||
| //! In many cases, this doesn't matter and is in fact desirable! | ||
| //! Consider two systems, one which writes to resource A, and the other which writes to resource B. | ||
| //! By allowing their order to be arbitrary, we can evaluate them greedily, based on the data that is free. | ||
| //! Because their data accesses are **compatible**, there is no **observable** difference created based on the order they are run. | ||
| //! | ||
| //! But if instead we have two systems mutating the same data, or one reading it and the other mutating, | ||
| //! then the actual observed value will vary based on the nondeterministic order of evaluation. | ||
| //! These observable conflicts are called **system execution order ambiguities**. | ||
| //! | ||
| //! This example demonstrates how you might detect and resolve (or silence) these ambiguities. | ||
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| use bevy::{ecs::schedule::ReportExecutionOrderAmbiguities, prelude::*}; | ||
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| fn main() { | ||
| App::new() | ||
| // This resource controls the reporting strategy for system execution order ambiguities | ||
| .insert_resource(ReportExecutionOrderAmbiguities) | ||
| .init_resource::<A>() | ||
| .init_resource::<B>() | ||
| // This pair of systems has an ambiguous order, | ||
| // as their data access conflicts, and there's no order between them. | ||
| .add_system(reads_a) | ||
| .add_system(writes_a) | ||
| // This pair of systems has conflicting data access, | ||
| // but it's resolved with an explicit ordering: | ||
| // the .after relationship here means that we will always double after adding. | ||
| .add_system(adds_one_to_b) | ||
| .add_system(doubles_b.after(adds_one_to_b)) | ||
| // This system isn't ambiguous with adds_one_to_b, | ||
| // due to the transitive ordering created by our constraints: | ||
| // if A is before B is before C, then A must be before C as well. | ||
| .add_system(reads_b.after(doubles_b)) | ||
| // This system will conflict with all of our writing systems | ||
| // but we've silenced its ambiguity with adds_one_to_b. | ||
| // This should only be done in the case of clear false positives: | ||
| // leave a comment in your code justifying the decision! | ||
| .add_system(reads_a_and_b.ambiguous_with(adds_one_to_b)) | ||
| // Be mindful, internal ambiguities are reported too! | ||
| // If there are any ambiguities due solely to DefaultPlugins, | ||
| // or between DefaultPlugins and any of your third party plugins, | ||
| // please file a bug with the repo responsible! | ||
| // Only *you* can prevent nondeterministic bugs due to greedy parallelism. | ||
| .add_plugins(DefaultPlugins) | ||
| .run(); | ||
| } | ||
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| #[derive(Resource, Debug, Default)] | ||
| struct A(usize); | ||
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| #[derive(Resource, Debug, Default)] | ||
| struct B(usize); | ||
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| // Data access is determined solely on the basis of the types of the system's parameters | ||
| // Every implementation of the `SystemParam` and `WorldQuery` traits must declare which data is used | ||
| // and whether or not it is mutably accessed. | ||
| fn reads_a(_a: Res<A>) {} | ||
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| fn writes_a(mut a: ResMut<A>) { | ||
| a.0 += 1; | ||
| } | ||
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| fn adds_one_to_b(mut b: ResMut<B>) { | ||
| b.0 = b.0.saturating_add(1); | ||
| } | ||
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| fn doubles_b(mut b: ResMut<B>) { | ||
| // This will overflow pretty rapidly otherwise | ||
| b.0 = b.0.saturating_mul(2); | ||
| } | ||
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| fn reads_b(b: Res<B>) { | ||
| // This invariant is always true, | ||
| // because we've fixed the system order so doubling always occurs after adding. | ||
| assert!((b.0 % 2 == 0) || (b.0 == usize::MAX)); | ||
| } | ||
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| fn reads_a_and_b(a: Res<A>, b: Res<B>) { | ||
| // Only display the first few steps to avoid burying the ambiguities in the console | ||
| if b.0 < 10 { | ||
| info!("{}, {}", a.0, b.0); | ||
| } | ||
| } |