Split ambiguity detection logic off into its own file

crates/bevy_ecs/src/schedule/ambiguity_detection.rs

@@ -0,0 +1,284 @@
+use crate::component::ComponentId;
+use crate::schedule::{AmbiguityDetection, SystemContainer, SystemStage};
+use crate::world::World;
+
+use bevy_utils::HashMap;
+use fixedbitset::FixedBitSet;
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
+pub enum AmbiguityReportLevel {
+    Off,
+    Minimal,
+    Verbose,
+}
+
+/// Systems that access the same Component or Resource within the same stage
+/// risk an ambiguous order that could result in logic bugs, unless they have an
+/// explicit execution ordering constraint between them.
+///
+/// This occurs because, in the absence of explicit constraints, systems are executed in
+/// an unstable, arbitrary order within each stage that may vary between runs and frames.
+///
+/// Some ambiguities reported by the ambiguity checker may be warranted (to allow two systems to run
+/// without blocking each other) or spurious, as the exact combination of archetypes used may
+/// prevent them from ever conflicting during actual gameplay. You can resolve the warnings produced
+/// by the ambiguity checker by adding `.before` or `.after` to one of the conflicting systems
+/// referencing the other system to force a specific ordering.
+///
+/// The checker may report a system more times than the amount of constraints it would actually need
+/// to have unambiguous order with regards to a group of already-constrained systems.
+///
+/// By default only a warning with the number of unresolved ambiguities detected will be reported per [`SystemStage`].
+/// This behavior can be changed by explicitly adding this resource using the following constructors:
+/// * [`ReportExecutionOrderAmbiguities::off()`] - Disables all messages reported by the ambiguity checker.
+/// * [`ReportExecutionOrderAmbiguities::minimal()`] - Displays only the number of unresolved ambiguities detected by the ambiguity checker.
+/// * [`ReportExecutionOrderAmbiguities::verbose()`] - Displays a full report of ambiguities detected by the ambiguity checker.
+///
+/// The ambiguity checker will ignore ambiguities within official Bevy crates.
+/// To ignore a custom crate, use [`ReportExecutionOrderAmbiguities::ignore`]
+/// with an list of crate names as an argument.
+/// This resource should be added before any bevy internal plugin.
+///
+/// ## Example
+/// ```ignore
+/// # use bevy_app::App;
+/// # use bevy_ecs::schedule::ReportExecutionOrderAmbiguities;
+/// App::new()
+///    .insert_resource(ReportExecutionOrderAmbiguities::verbose().ignore(&["my_external_crate"]));
+/// ```
+pub struct ReportExecutionOrderAmbiguities {
+    pub level: AmbiguityReportLevel,
+    pub ignore_crates: Vec<String>,
+}
+
+/// Returns vector containing all pairs of indices of systems with ambiguous execution order,
+/// along with specific components that have triggered the warning.
+/// Systems must be topologically sorted beforehand.
+pub(super) fn find_ambiguities(
+    systems: &[impl SystemContainer],
+    crates_filter: &[String],
+) -> Vec<(usize, usize, Vec<ComponentId>)> {
+    fn should_ignore_ambiguity(
+        systems: &[impl SystemContainer],
+        index_a: usize,
+        index_b: usize,
+        crates_filter: &[String],
+    ) -> bool {
+        let system_a = &systems[index_a];
+        let system_b = &systems[index_b];
+
+        (match system_a.ambiguity_detection() {
+            AmbiguityDetection::Ignore => true,
+            AmbiguityDetection::Check => false,
+            AmbiguityDetection::IgnoreWithLabel(labels) => {
+                labels.iter().any(|l| system_b.labels().contains(l))
+            }
+        }) || (match system_b.ambiguity_detection() {
+            AmbiguityDetection::Ignore => true,
+            AmbiguityDetection::Check => false,
+            AmbiguityDetection::IgnoreWithLabel(labels) => {
+                labels.iter().any(|l| system_a.labels().contains(l))
+            }
+        }) || (crates_filter.iter().any(|s| system_a.name().starts_with(s))
+            && crates_filter.iter().any(|s| system_b.name().starts_with(s)))
+    }
+
+    let mut ambiguity_set_labels = HashMap::default();
+    for set in systems.iter().flat_map(|c| c.ambiguity_sets()) {
+        let len = ambiguity_set_labels.len();
+        ambiguity_set_labels.entry(set).or_insert(len);
+    }
+    let mut all_ambiguity_sets = Vec::<FixedBitSet>::with_capacity(systems.len());
+    let mut all_dependencies = Vec::<FixedBitSet>::with_capacity(systems.len());
+    let mut all_dependants = Vec::<FixedBitSet>::with_capacity(systems.len());
+    for (index, container) in systems.iter().enumerate() {
+        let mut ambiguity_sets = FixedBitSet::with_capacity(ambiguity_set_labels.len());
+        for set in container.ambiguity_sets() {
+            ambiguity_sets.insert(ambiguity_set_labels[set]);
+        }
+        all_ambiguity_sets.push(ambiguity_sets);
+        let mut dependencies = FixedBitSet::with_capacity(systems.len());
+        for &dependency in container.dependencies() {
+            dependencies.union_with(&all_dependencies[dependency]);
+            dependencies.insert(dependency);
+            all_dependants[dependency].insert(index);
+        }
+
+        all_dependants.push(FixedBitSet::with_capacity(systems.len()));
+        all_dependencies.push(dependencies);
+    }
+    for index in (0..systems.len()).rev() {
+        let mut dependants = FixedBitSet::with_capacity(systems.len());
+        for dependant in all_dependants[index].ones() {
+            dependants.union_with(&all_dependants[dependant]);
+            dependants.insert(dependant);
+        }
+        all_dependants[index] = dependants;
+    }
+    let mut all_relations = all_dependencies
+        .drain(..)
+        .zip(all_dependants.drain(..))
+        .enumerate()
+        .map(|(index, (dependencies, dependants))| {
+            let mut relations = FixedBitSet::with_capacity(systems.len());
+            relations.union_with(&dependencies);
+            relations.union_with(&dependants);
+            relations.insert(index);
+            relations
+        })
+        .collect::<Vec<FixedBitSet>>();
+    let mut ambiguities = Vec::new();
+    let full_bitset: FixedBitSet = (0..systems.len()).collect();
+    let mut processed = FixedBitSet::with_capacity(systems.len());
+    for (index_a, relations) in all_relations.drain(..).enumerate() {
+        // TODO: prove that `.take(index_a)` would be correct here, and uncomment it if so.
+        for index_b in full_bitset.difference(&relations)
+        // .take(index_a)
+        {
+            if !processed.contains(index_b)
+                && all_ambiguity_sets[index_a].is_disjoint(&all_ambiguity_sets[index_b])
+                && !should_ignore_ambiguity(systems, index_a, index_b, crates_filter)
+            {
+                let a_access = systems[index_a].component_access();
+                let b_access = systems[index_b].component_access();
+                if let (Some(a), Some(b)) = (a_access, b_access) {
+                    let conflicts = a.get_conflicts(b);
+                    if !conflicts.is_empty() {
+                        ambiguities.push((index_a, index_b, conflicts));
+                    }
+                } else {
+                    ambiguities.push((index_a, index_b, Vec::new()));
+                }
+            }
+        }
+        processed.insert(index_a);
+    }
+    ambiguities
+}
+
+impl ReportExecutionOrderAmbiguities {
+    /// Disables all messages reported by the ambiguity checker.
+    pub fn off() -> Self {
+        Self {
+            level: AmbiguityReportLevel::Off,
+            ..Default::default()
+        }
+    }
+
+    /// Displays only the number of unresolved ambiguities detected by the ambiguity checker. This is the default behavior.
+    pub fn minimal() -> Self {
+        Self {
+            level: AmbiguityReportLevel::Minimal,
+            ..Default::default()
+        }
+    }
+
+    /// Displays a full report of ambiguities detected by the ambiguity checker.
+    pub fn verbose() -> Self {
+        Self {
+            level: AmbiguityReportLevel::Verbose,
+            ..Default::default()
+        }
+    }
+
+    /// Adds the given crate to be ignored by ambiguity checker. Check [`ReportExecutionOrderAmbiguities`] for more details.
+    pub fn ignore(mut self, crate_prefix: &str) -> Self {
+        self.ignore_crates.push(crate_prefix.to_string());
+        self
+    }
+
+    /// Adds all the given crates to be ignored by ambiguity checker. Check [`ReportExecutionOrderAmbiguities`] for more details.
+    pub fn ignore_all(mut self, crate_prefixes: &[&str]) -> Self {
+        for s in crate_prefixes {
+            self.ignore_crates.push(s.to_string());
+        }
+        self
+    }
+}
+
+impl Default for ReportExecutionOrderAmbiguities {
+    fn default() -> Self {
+        Self {
+            level: AmbiguityReportLevel::Minimal,
+            ignore_crates: vec![],
+        }
+    }
+}
+
+impl SystemStage {
+    /// Logs execution order ambiguities between systems. System orders must be fresh.
+    pub fn report_ambiguities(&self, world: &mut World) {
+        let ambiguity_report =
+            world.get_resource_or_insert_with(ReportExecutionOrderAmbiguities::default);
+
+        if ambiguity_report.level == AmbiguityReportLevel::Off {
+            return;
+        }
+
+        debug_assert!(!self.systems_modified);
+
+        fn write_display_names_of_pairs(
+            offset: usize,
+            systems: &[impl SystemContainer],
+            ambiguities: Vec<(usize, usize, Vec<ComponentId>)>,
+            world: &World,
+        ) -> usize {
+            for (i, (system_a_index, system_b_index, conflicting_indexes)) in
+                ambiguities.iter().enumerate()
+            {
+                let _system_a_name = systems[*system_a_index].name();
+                let _system_b_name = systems[*system_b_index].name();
+
+                let _conflicting_components = conflicting_indexes
+                    .iter()
+                    .map(|id| world.components().get_info(*id).unwrap().name())
+                    .collect::<Vec<_>>();
+
+                let _ambiguity_number = i + offset;
+
+                println!(
+						"{_ambiguity_number}. {_system_a_name} conflicts with {_system_b_name} on {_conflicting_components:?}"
+					);
+            }
+
+            // We can't merge the SystemContainer arrays, so instead we manually keep track of how high we've counted :upside_down:
+            ambiguities.len()
+        }
+
+        let parallel = find_ambiguities(&self.parallel, &ambiguity_report.ignore_crates);
+        let at_start = find_ambiguities(&self.exclusive_at_start, &ambiguity_report.ignore_crates);
+        let before_commands = find_ambiguities(
+            &self.exclusive_before_commands,
+            &ambiguity_report.ignore_crates,
+        );
+        let at_end = find_ambiguities(&self.exclusive_at_end, &ambiguity_report.ignore_crates);
+
+        let mut unresolved_count = parallel.len();
+        unresolved_count += at_start.len();
+        unresolved_count += before_commands.len();
+        unresolved_count += at_end.len();
+
+        if unresolved_count > 0 {
+            println!("One of your stages contains {unresolved_count} pairs of systems with unknown order and conflicting data access. \
+				You may want to add `.before()` or `.after()` constraints between some of these systems to prevent bugs.\n");
+
+            if ambiguity_report.level != AmbiguityReportLevel::Verbose {
+                println!("Set the level of the `ReportExecutionOrderAmbiguities` resource to `AmbiguityReportLevel::Verbose` for more details.");
+            } else {
+                // TODO: clean up this logic once exclusive systems are more compatible with parallel systems
+                // allowing us to merge these collections
+                let mut offset = 1;
+                offset = write_display_names_of_pairs(offset, &self.parallel, parallel, world);
+                offset =
+                    write_display_names_of_pairs(offset, &self.exclusive_at_start, at_start, world);
+                offset = write_display_names_of_pairs(
+                    offset,
+                    &self.exclusive_before_commands,
+                    before_commands,
+                    world,
+                );
+                write_display_names_of_pairs(offset, &self.exclusive_at_end, at_end, world);
+            }
+        }
+    }
+}

crates/bevy_ecs/src/schedule/mod.rs

@@ -3,6 +3,7 @@
 //! When using Bevy ECS, systems are usually not run directly, but are inserted into a
 //!  [`Stage`], which then lives within a [`Schedule`].
 
+mod ambiguity_detection;
 mod executor;
 mod executor_parallel;
 pub mod graph_utils;
@@ -14,6 +15,7 @@ mod system_container;
 mod system_descriptor;
 mod system_set;
 
+pub use ambiguity_detection::*;
 pub use executor::*;
 pub use executor_parallel::*;
 pub use graph_utils::GraphNode;