Test for ambiguous system ordering in CI (bevyengine#13950)

Progress towards bevyengine#7386.

Following discussion
https://discord.com/channels/691052431525675048/1253260494538539048/1253387942311886960

This Pull Request adds an example to detect system order ambiguities,
and also asserts none exist.

A lot of schedules are ignored in ordered to have the test passing, we
should thrive to make them pass, but in other pull requests.

<details><summary>example output <b>summary</b>, without ignored
schedules</summary>
<p>

```txt
$ cargo run --example ambiguity_detection 2>&1 | grep -C 1 "pairs of syst"
2024-06-21T13:17:55.776585Z  WARN bevy_ecs::schedule::schedule: Schedule First has ambiguities.
1 pairs of systems with conflicting data access have indeterminate execution order. Consider adding `before`, `after`, or `ambiguous_with` relationships between these:
 -- bevy_time::time_system (in set TimeSystem) and bevy_ecs::event::event_update_system (in set EventUpdates)
--
2024-06-21T13:17:55.782265Z  WARN bevy_ecs::schedule::schedule: Schedule PreUpdate has ambiguities.
11 pairs of systems with conflicting data access have indeterminate execution order. Consider adding `before`, `after`, or `ambiguous_with` relationships between these:
 -- bevy_pbr::prepass::update_mesh_previous_global_transforms and bevy_asset::server::handle_internal_asset_events
--
2024-06-21T13:17:55.809516Z  WARN bevy_ecs::schedule::schedule: Schedule PostUpdate has ambiguities.
63 pairs of systems with conflicting data access have indeterminate execution order. Consider adding `before`, `after`, or `ambiguous_with` relationships between these:
 -- bevy_ui::accessibility::image_changed and bevy_ecs::schedule::executor::apply_deferred
--
2024-06-21T13:17:55.816287Z  WARN bevy_ecs::schedule::schedule: Schedule Last has ambiguities.
3 pairs of systems with conflicting data access have indeterminate execution order. Consider adding `before`, `after`, or `ambiguous_with` relationships between these:
 -- bevy_gizmos::update_gizmo_meshes<bevy_gizmos::aabb::AabbGizmoConfigGroup> (in set UpdateGizmoMeshes) and bevy_gizmos::update_gizmo_meshes<bevy_gizmos::light::LightGizmoConfigGroup> (in set UpdateGizmoMeshes)
--
2024-06-21T13:17:55.831074Z  WARN bevy_ecs::schedule::schedule: Schedule ExtractSchedule has ambiguities.
296 pairs of systems with conflicting data access have indeterminate execution order. Consider adding `before`, `after`, or `ambiguous_with` relationships between these:
 -- bevy_render::extract_component::extract_components<bevy_sprite::SpriteSource> and bevy_render::render_asset::extract_render_asset<bevy_sprite::mesh2d::material::PreparedMaterial2d<bevy_sprite::mesh2d::color_material::ColorMaterial>>
```

</p>
</details> 

To try locally: 
```sh
CI_TESTING_CONFIG="./.github/example-run/ambiguity_detection.ron" cargo run --example ambiguity_detection --features "bevy_ci_testing,trace,trace_chrome"
```

---------

Co-authored-by: Jan Hohenheim <[email protected]>

.github/example-run/ambiguity_detection.ron

@@ -0,0 +1,2 @@
+(
+)

Cargo.toml

@@ -365,6 +365,7 @@ ron = "0.8.0"
 flate2 = "1.0"
 serde = { version = "1", features = ["derive"] }
 bytemuck = "1.7"
+bevy_render = { path = "crates/bevy_render", version = "0.15.0-dev", default-features = false }
 # Needed to poll Task examples
 futures-lite = "2.0.1"
 async-std = "1.12"
@@ -2980,6 +2981,14 @@ description = "Demonstrates customizing default window settings"
 category = "Window"
 wasm = true
 
+[[example]]
+name = "ambiguity_detection"
+path = "tests/ecs/ambiguity_detection.rs"
+doc-scrape-examples = true
+
+[package.metadata.example.ambiguity_detection]
+hidden = true
+
 [[example]]
 name = "resizing"
 path = "tests/window/resizing.rs"

crates/bevy_dev_tools/src/ci_testing/mod.rs

@@ -6,6 +6,7 @@ mod systems;
 pub use self::config::*;
 
 use bevy_app::prelude::*;
+use bevy_ecs::schedule::IntoSystemConfigs;
 use bevy_time::TimeUpdateStrategy;
 use std::time::Duration;
 
@@ -46,9 +47,11 @@ impl Plugin for CiTestingPlugin {
                 fixed_frame_time,
             )));
         }
-
         app.add_event::<CiTestingCustomEvent>()
             .insert_resource(config)
-            .add_systems(Update, systems::send_events);
+            .add_systems(
+                Update,
+                systems::send_events.before(bevy_window::close_when_requested),
+            );
     }
 }

tests/ecs/ambiguity_detection.rs

@@ -0,0 +1,128 @@
+//! A test to confirm that `bevy` doesn't regress its system ambiguities count when using [`DefaultPlugins`].
+//! This is run in CI.
+
+use bevy::{
+    ecs::schedule::{InternedScheduleLabel, LogLevel, ScheduleBuildSettings, ScheduleLabel},
+    prelude::*,
+    utils::HashMap,
+};
+use bevy_render::{pipelined_rendering::RenderExtractApp, Render, RenderApp};
+
+/// FIXME: bevy should not have any ambiguities, but it takes time to clean these up,
+/// so we're just ignoring those for now.
+///
+/// See [#7386](https://github.com/bevyengine/bevy/issues/7386) for relevant issue.
+pub fn get_ignored_ambiguous_system_schedules() -> Vec<Box<dyn ScheduleLabel>> {
+    vec![
+        Box::new(First),
+        Box::new(PreUpdate),
+        Box::new(Update),
+        Box::new(PostUpdate),
+        Box::new(Last),
+        Box::new(ExtractSchedule),
+        Box::new(Render),
+    ]
+}
+
+/// A test to confirm that `bevy` doesn't regress its system ambiguities count when using [`DefaultPlugins`].
+/// This is run in CI.
+pub fn main() {
+    let mut app = App::new();
+    app.add_plugins(DefaultPlugins);
+
+    let sub_app = app.main_mut();
+    configure_ambiguity_detection(sub_app);
+    let sub_app = app.sub_app_mut(RenderApp);
+    configure_ambiguity_detection(sub_app);
+    let sub_app = app.sub_app_mut(RenderExtractApp);
+    configure_ambiguity_detection(sub_app);
+
+    app.finish();
+    app.cleanup();
+    app.update();
+
+    let sub_app = app.main();
+
+    let ignored_schedules = get_ignored_ambiguous_system_schedules();
+
+    let ambiguities = count_ambiguities(sub_app);
+    let mut unexpected_ambiguities = vec![];
+    for (&label, &count) in ambiguities.0.iter() {
+        if ignored_schedules
+            .iter()
+            .any(|label_to_ignore| **label_to_ignore == *label)
+        {
+            continue;
+        }
+        if count != 0 {
+            unexpected_ambiguities.push(label);
+        }
+    }
+    assert_eq!(
+        unexpected_ambiguities.len(),
+        0,
+        "Main app has unexpected ambiguities among these schedules: {:?}.\n\
+    More Details:\n{:#?}",
+        unexpected_ambiguities,
+        ambiguities
+    );
+
+    let total_ambiguities = ambiguities.total();
+    assert_eq!(
+        total_ambiguities, 82,
+        "Main app does not have an expected conflicting systems count, \
+        you might consider verifying if it's normal, or change the expected number.\n\
+        Details:\n{:#?}",
+        ambiguities
+    );
+
+    // RenderApp is not checked here, because it is not within the App at this point.
+    let sub_app = app.sub_app(RenderExtractApp);
+
+    let ambiguities = count_ambiguities(sub_app);
+    let total_ambiguities = ambiguities.total();
+    assert_eq!(
+        total_ambiguities, 0,
+        "RenderExtractApp contains conflicting systems.",
+    );
+}
+
+/// Contains the number of conflicting systems per schedule.
+#[derive(Debug, Deref, DerefMut)]
+struct AmbiguitiesCount(pub HashMap<InternedScheduleLabel, usize>);
+
+impl AmbiguitiesCount {
+    fn total(&self) -> usize {
+        self.values().sum()
+    }
+}
+
+fn configure_ambiguity_detection(sub_app: &mut SubApp) {
+    let ignored_ambiguous_systems = get_ignored_ambiguous_system_schedules();
+    let mut schedules = sub_app.world_mut().resource_mut::<Schedules>();
+    for (_, schedule) in schedules.iter_mut() {
+        if ignored_ambiguous_systems
+            .iter()
+            .any(|label| **label == *schedule.label())
+        {
+            // Note: you can remove this bypass to get full details about ambiguities.
+            continue;
+        }
+        schedule.set_build_settings(ScheduleBuildSettings {
+            ambiguity_detection: LogLevel::Warn,
+            use_shortnames: false,
+            ..default()
+        });
+    }
+}
+
+/// Returns the number of conflicting systems per schedule.
+fn count_ambiguities(sub_app: &SubApp) -> AmbiguitiesCount {
+    let schedules = sub_app.world().resource::<Schedules>();
+    let mut ambiguities = HashMap::new();
+    for (_, schedule) in schedules.iter() {
+        let ambiguities_in_schedule = schedule.graph().conflicting_systems().len();
+        ambiguities.insert(schedule.label(), ambiguities_in_schedule);
+    }
+    AmbiguitiesCount(ambiguities)
+}