update archetypes if needed before running system in SingleThreadedExecutor (#1586)

fixes #1585 

I copied most of the logic from the `ParallelSystemExecutor` impl, simplifying it a little as systems can't run in parallel

crates/bevy_ecs/src/schedule/executor.rs

@@ -1,4 +1,4 @@
-use crate::{schedule::ParallelSystemContainer, world::World};
+use crate::{archetype::ArchetypeGeneration, schedule::ParallelSystemContainer, world::World};
 use downcast_rs::{impl_downcast, Downcast};
 
 pub trait ParallelSystemExecutor: Downcast + Send + Sync {
@@ -10,13 +10,25 @@ pub trait ParallelSystemExecutor: Downcast + Send + Sync {
 
 impl_downcast!(ParallelSystemExecutor);
 
-#[derive(Default)]
-pub struct SingleThreadedExecutor;
+pub struct SingleThreadedExecutor {
+    /// Last archetypes generation observed by parallel systems.
+    archetype_generation: ArchetypeGeneration,
+}
 
+impl Default for SingleThreadedExecutor {
+    fn default() -> Self {
+        Self {
+            // MAX ensures access information will be initialized on first run.
+            archetype_generation: ArchetypeGeneration::new(usize::MAX),
+        }
+    }
+}
 impl ParallelSystemExecutor for SingleThreadedExecutor {
     fn rebuild_cached_data(&mut self, _: &[ParallelSystemContainer]) {}
 
     fn run_systems(&mut self, systems: &mut [ParallelSystemContainer], world: &mut World) {
+        self.update_archetypes(systems, world);
+
         for system in systems {
             if system.should_run() {
                 system.system_mut().run((), world);
@@ -24,3 +36,30 @@ impl ParallelSystemExecutor for SingleThreadedExecutor {
         }
     }
 }
+
+impl SingleThreadedExecutor {
+    /// Calls system.new_archetype() for each archetype added since the last call to [update_archetypes] and
+    /// updates cached archetype_component_access.
+    fn update_archetypes(&mut self, systems: &mut [ParallelSystemContainer], world: &World) {
+        let archetypes = world.archetypes();
+        let old_generation = self.archetype_generation;
+        let new_generation = archetypes.generation();
+        if old_generation == new_generation {
+            return;
+        }
+
+        let archetype_index_range = if old_generation.value() == usize::MAX {
+            0..archetypes.len()
+        } else {
+            old_generation.value()..archetypes.len()
+        };
+        for archetype in archetypes.archetypes[archetype_index_range].iter() {
+            for container in systems.iter_mut() {
+                let system = container.system_mut();
+                system.new_archetype(archetype);
+            }
+        }
+
+        self.archetype_generation = new_generation;
+    }
+}

crates/bevy_ecs/src/schedule/stage.rs

@@ -1528,4 +1528,49 @@ mod tests {
         stage.run(&mut world_a);
         stage.run(&mut world_b);
     }
+
+    #[test]
+    fn archetype_update_single_executor() {
+        fn query_count_system(
+            mut entity_count: ResMut<usize>,
+            query: Query<crate::entity::Entity>,
+        ) {
+            *entity_count = query.iter().count();
+        }
+
+        let mut world = World::new();
+        world.insert_resource(0_usize);
+        let mut stage = SystemStage::single(query_count_system.system());
+
+        let entity = world.spawn().insert_bundle(()).id();
+        stage.run(&mut world);
+        assert_eq!(*world.get_resource::<usize>().unwrap(), 1);
+
+        world.get_entity_mut(entity).unwrap().insert(1);
+        stage.run(&mut world);
+        assert_eq!(*world.get_resource::<usize>().unwrap(), 1);
+    }
+
+    #[test]
+    fn archetype_update_parallel_executor() {
+        fn query_count_system(
+            mut entity_count: ResMut<usize>,
+            query: Query<crate::entity::Entity>,
+        ) {
+            *entity_count = query.iter().count();
+        }
+
+        let mut world = World::new();
+        world.insert_resource(0_usize);
+        let mut stage = SystemStage::parallel();
+        stage.add_system(query_count_system.system());
+
+        let entity = world.spawn().insert_bundle(()).id();
+        stage.run(&mut world);
+        assert_eq!(*world.get_resource::<usize>().unwrap(), 1);
+
+        world.get_entity_mut(entity).unwrap().insert(1);
+        stage.run(&mut world);
+        assert_eq!(*world.get_resource::<usize>().unwrap(), 1);
+    }
 }