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https://github.com/bevyengine/bevy
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# Objective The `nondeterministic_system_order` example doesn't actually detect and log its deliberate order ambiguities! It should, tho. ## Solution Update the schedule label, and explain in a comment that you can't turn it on for the whole `Main` schedule in one go (alas, that would be nice, but it makes sense that it doesn't work that way).
100 lines
4.1 KiB
Rust
100 lines
4.1 KiB
Rust
//! By default, Bevy systems run in parallel with each other.
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//! Unless the order is explicitly specified, their relative order is nondeterministic.
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//!
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//! In many cases, this doesn't matter and is in fact desirable!
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//! Consider two systems, one which writes to resource A, and the other which writes to resource B.
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//! By allowing their order to be arbitrary, we can evaluate them greedily, based on the data that is free.
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//! Because their data accesses are **compatible**, there is no **observable** difference created based on the order they are run.
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//!
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//! But if instead we have two systems mutating the same data, or one reading it and the other mutating,
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//! then the actual observed value will vary based on the nondeterministic order of evaluation.
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//! These observable conflicts are called **system execution order ambiguities**.
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//!
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//! This example demonstrates how you might detect and resolve (or silence) these ambiguities.
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use bevy::{
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ecs::schedule::{LogLevel, ScheduleBuildSettings},
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prelude::*,
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};
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fn main() {
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App::new()
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// We can modify the reporting strategy for system execution order ambiguities on a per-schedule basis.
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// You must do this for each schedule you want to inspect; child schedules executed within an inspected
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// schedule do not inherit this modification.
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.edit_schedule(Update, |schedule| {
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schedule.set_build_settings(ScheduleBuildSettings {
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ambiguity_detection: LogLevel::Warn,
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..default()
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});
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})
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.init_resource::<A>()
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.init_resource::<B>()
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.add_systems(
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Update,
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(
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// This pair of systems has an ambiguous order,
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// as their data access conflicts, and there's no order between them.
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reads_a,
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writes_a,
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// This pair of systems has conflicting data access,
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// but it's resolved with an explicit ordering:
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// the .after relationship here means that we will always double after adding.
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adds_one_to_b,
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doubles_b.after(adds_one_to_b),
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// This system isn't ambiguous with adds_one_to_b,
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// due to the transitive ordering created by our constraints:
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// if A is before B is before C, then A must be before C as well.
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reads_b.after(doubles_b),
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// This system will conflict with all of our writing systems
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// but we've silenced its ambiguity with adds_one_to_b.
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// This should only be done in the case of clear false positives:
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// leave a comment in your code justifying the decision!
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reads_a_and_b.ambiguous_with(adds_one_to_b),
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),
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)
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// Be mindful, internal ambiguities are reported too!
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// If there are any ambiguities due solely to DefaultPlugins,
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// or between DefaultPlugins and any of your third party plugins,
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// please file a bug with the repo responsible!
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// Only *you* can prevent nondeterministic bugs due to greedy parallelism.
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.add_plugins(DefaultPlugins)
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.run();
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}
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#[derive(Resource, Debug, Default)]
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struct A(usize);
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#[derive(Resource, Debug, Default)]
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struct B(usize);
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// Data access is determined solely on the basis of the types of the system's parameters
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// Every implementation of the `SystemParam` and `WorldQuery` traits must declare which data is used
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// and whether or not it is mutably accessed.
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fn reads_a(_a: Res<A>) {}
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fn writes_a(mut a: ResMut<A>) {
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a.0 += 1;
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}
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fn adds_one_to_b(mut b: ResMut<B>) {
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b.0 = b.0.saturating_add(1);
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}
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fn doubles_b(mut b: ResMut<B>) {
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// This will overflow pretty rapidly otherwise
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b.0 = b.0.saturating_mul(2);
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}
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fn reads_b(b: Res<B>) {
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// This invariant is always true,
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// because we've fixed the system order so doubling always occurs after adding.
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assert!((b.0 % 2 == 0) || (b.0 == usize::MAX));
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}
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fn reads_a_and_b(a: Res<A>, b: Res<B>) {
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// Only display the first few steps to avoid burying the ambiguities in the console
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if b.0 < 10 {
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info!("{}, {}", a.0, b.0);
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}
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}
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