Mark events as read during EventReader::par_read (#13836)

# Objective

- Fix issue #13821  

## Solution

- Rewrote the test to ensure that it actually tests the functionality
correctly. Then made the par_read function correctly change the values
of self.reader.last_event_count.

## Testing

- Rewrote the test for par_read to run the system schedule twice,
checking the output each time

---------

Co-authored-by: Martín Maita <47983254+mnmaita@users.noreply.github.com>

crates/bevy_ecs/src/event/iterators.rs

@@ -1,8 +1,7 @@
 use crate as bevy_ecs;
-use bevy_ecs::{
-    batching::BatchingStrategy,
-    event::{Event, EventId, EventInstance, Events, ManualEventReader},
-};
+#[cfg(feature = "multi_threaded")]
+use bevy_ecs::batching::BatchingStrategy;
+use bevy_ecs::event::{Event, EventId, EventInstance, Events, ManualEventReader};
 use bevy_utils::detailed_trace;
 use std::{iter::Chain, slice::Iter};
 
@@ -141,12 +140,15 @@ impl<'a, E: Event> ExactSizeIterator for EventIteratorWithId<'a, E> {
 
 /// A parallel iterator over `Event`s.
 #[derive(Debug)]
+#[cfg(feature = "multi_threaded")]
 pub struct EventParIter<'a, E: Event> {
     reader: &'a mut ManualEventReader<E>,
     slices: [&'a [EventInstance<E>]; 2],
     batching_strategy: BatchingStrategy,
+    unread: usize,
 }
 
+#[cfg(feature = "multi_threaded")]
 impl<'a, E: Event> EventParIter<'a, E> {
     /// Creates a new parallel iterator over `events` that have not yet been seen by `reader`.
     pub fn new(reader: &'a mut ManualEventReader<E>, events: &'a Events<E>) -> Self {
@@ -168,6 +170,7 @@ impl<'a, E: Event> EventParIter<'a, E> {
             reader,
             slices: [a, b],
             batching_strategy: BatchingStrategy::default(),
+            unread: unread_count,
         }
     }
 
@@ -203,13 +206,13 @@ impl<'a, E: Event> EventParIter<'a, E> {
     /// initialized and run from the ECS scheduler, this should never panic.
     ///
     /// [`ComputeTaskPool`]: bevy_tasks::ComputeTaskPool
-    pub fn for_each_with_id<FN: Fn(&'a E, EventId<E>) + Send + Sync + Clone>(self, func: FN) {
-        #[cfg(any(target_arch = "wasm32", not(feature = "multi_threaded")))]
+    pub fn for_each_with_id<FN: Fn(&'a E, EventId<E>) + Send + Sync + Clone>(mut self, func: FN) {
+        #[cfg(target_arch = "wasm32")]
         {
             self.into_iter().for_each(|(e, i)| func(e, i));
         }
 
-        #[cfg(all(not(target_arch = "wasm32"), feature = "multi_threaded"))]
+        #[cfg(not(target_arch = "wasm32"))]
         {
             let pool = bevy_tasks::ComputeTaskPool::get();
             let thread_count = pool.thread_num();
@@ -233,6 +236,10 @@ impl<'a, E: Event> EventParIter<'a, E> {
                     });
                 }
             });
+
+            // Events are guaranteed to be read at this point.
+            self.reader.last_event_count += self.unread;
+            self.unread = 0;
         }
     }
 
@@ -247,6 +254,7 @@ impl<'a, E: Event> EventParIter<'a, E> {
     }
 }
 
+#[cfg(feature = "multi_threaded")]
 impl<'a, E: Event> IntoIterator for EventParIter<'a, E> {
     type IntoIter = EventIteratorWithId<'a, E>;
     type Item = <Self::IntoIter as Iterator>::Item;

crates/bevy_ecs/src/event/mod.rs

@@ -11,7 +11,9 @@ pub(crate) use base::EventInstance;
 pub use base::{Event, EventId};
 pub use bevy_ecs_macros::Event;
 pub use collections::{Events, SendBatchIds};
-pub use iterators::{EventIterator, EventIteratorWithId, EventParIter};
+#[cfg(feature = "multi_threaded")]
+pub use iterators::EventParIter;
+pub use iterators::{EventIterator, EventIteratorWithId};
 pub use reader::{EventReader, ManualEventReader};
 pub use registry::{EventRegistry, ShouldUpdateEvents};
 pub use update::{
@@ -421,30 +423,33 @@ mod tests {
     #[cfg(feature = "multi_threaded")]
     #[test]
     fn test_events_par_iter() {
-        use std::{collections::HashSet, sync::mpsc};
-
         use crate::prelude::*;
+        use std::sync::atomic::{AtomicUsize, Ordering};
+
+        #[derive(Resource)]
+        struct Counter(AtomicUsize);
 
         let mut world = World::new();
         world.init_resource::<Events<TestEvent>>();
-        for i in 0..100 {
-            world.send_event(TestEvent { i });
+        for _ in 0..100 {
+            world.send_event(TestEvent { i: 1 });
         }
-
         let mut schedule = Schedule::default();
+        schedule.add_systems(
+            |mut events: EventReader<TestEvent>, counter: ResMut<Counter>| {
+                events.par_read().for_each(|event| {
+                    counter.0.fetch_add(event.i, Ordering::Relaxed);
+                });
+            },
+        );
+        world.insert_resource(Counter(AtomicUsize::new(0)));
+        schedule.run(&mut world);
+        let counter = world.remove_resource::<Counter>().unwrap();
+        assert_eq!(counter.0.into_inner(), 100);
 
-        schedule.add_systems(|mut events: EventReader<TestEvent>| {
-            let (tx, rx) = mpsc::channel();
-            events.par_read().for_each(|event| {
-                tx.send(event.i).unwrap();
-            });
-            drop(tx);
-
-            let observed: HashSet<_> = rx.into_iter().collect();
-            assert_eq!(observed, HashSet::from_iter(0..100));
-        });
+        world.insert_resource(Counter(AtomicUsize::new(0)));
         schedule.run(&mut world);
+        let counter = world.remove_resource::<Counter>().unwrap();
+        assert_eq!(counter.0.into_inner(), 0);
     }
-
-    // Peak tests
 }

crates/bevy_ecs/src/event/reader.rs

@@ -1,6 +1,8 @@
 use crate as bevy_ecs;
+#[cfg(feature = "multi_threaded")]
+use bevy_ecs::event::EventParIter;
 use bevy_ecs::{
-    event::{Event, EventIterator, EventIteratorWithId, EventParIter, Events},
+    event::{Event, EventIterator, EventIteratorWithId, Events},
     system::{Local, Res, SystemParam},
 };
 use std::marker::PhantomData;
@@ -66,6 +68,7 @@ impl<'w, 's, E: Event> EventReader<'w, 's, E> {
     /// assert_eq!(counter.into_inner(), 4950);
     /// ```
     ///
+    #[cfg(feature = "multi_threaded")]
     pub fn par_read(&mut self) -> EventParIter<'_, E> {
         self.reader.par_read(&self.events)
     }
@@ -189,6 +192,7 @@ impl<E: Event> ManualEventReader<E> {
     }
 
     /// See [`EventReader::par_read`]
+    #[cfg(feature = "multi_threaded")]
     pub fn par_read<'a>(&'a mut self, events: &'a Events<E>) -> EventParIter<'a, E> {
         EventParIter::new(self, events)
     }