Stageless: move MainThreadExecutor to schedule_v3 (#7444)

# Objective

- Trying to move some of the fixes from https://github.com/bevyengine/bevy/pull/7267 to make that one easier to review
- The MainThreadExecutor is how the render world runs nonsend systems on the main thread for pipelined rendering.
- The multithread executor for stageless wasn't using the MainThreadExecutor.
- MainThreadExecutor was declared in the old executor_parallel module that is getting deleted.
- The way the MainThreadExecutor was getting passed to the scope was actually unsound as the resource could be dropped from the World while the schedule was running

## Solution

- Move MainThreadExecutor to the new multithreaded_executor's file.
- Make the multithreaded executor use the MainThreadExecutor
- Clone the MainThreadExecutor onto the stack and pass that ref in

## Changelog

- Move MainThreadExecutor for stageless migration.

crates/bevy_ecs/src/schedule/executor_parallel.rs

@@ -1,15 +1,12 @@
-use std::sync::Arc;
-
-use crate as bevy_ecs;
 use crate::{
     archetype::ArchetypeComponentId,
     query::Access,
     schedule::{ParallelSystemExecutor, SystemContainer},
-    system::Resource,
+    schedule_v3::MainThreadExecutor,
     world::World,
 };
 use async_channel::{Receiver, Sender};
-use bevy_tasks::{ComputeTaskPool, Scope, TaskPool, ThreadExecutor};
+use bevy_tasks::{ComputeTaskPool, Scope, TaskPool};
 #[cfg(feature = "trace")]
 use bevy_utils::tracing::Instrument;
 use event_listener::Event;
@@ -18,16 +15,6 @@ use fixedbitset::FixedBitSet;
 #[cfg(test)]
 use scheduling_event::*;
 
-/// New-typed [`ThreadExecutor`] [`Resource`] that is used to run systems on the main thread
-#[derive(Resource, Default, Clone)]
-pub struct MainThreadExecutor(pub Arc<ThreadExecutor<'static>>);
-
-impl MainThreadExecutor {
-    pub fn new() -> Self {
-        MainThreadExecutor(Arc::new(ThreadExecutor::new()))
-    }
-}
-
 struct SystemSchedulingMetadata {
     /// Used to signal the system's task to start the system.
     start: Event,
@@ -138,7 +125,10 @@ impl ParallelSystemExecutor for ParallelExecutor {
             }
         }
 
-        let thread_executor = world.get_resource::<MainThreadExecutor>().map(|e| &*e.0);
+        let thread_executor = world
+            .get_resource::<MainThreadExecutor>()
+            .map(|e| e.0.clone());
+        let thread_executor = thread_executor.as_deref();
 
         ComputeTaskPool::init(TaskPool::default).scope_with_executor(
             false,

crates/bevy_ecs/src/schedule_v3/executor/mod.rs

@@ -2,7 +2,7 @@ mod multi_threaded;
 mod simple;
 mod single_threaded;
 
-pub use self::multi_threaded::MultiThreadedExecutor;
+pub use self::multi_threaded::{MainThreadExecutor, MultiThreadedExecutor};
 pub use self::simple::SimpleExecutor;
 pub use self::single_threaded::SingleThreadedExecutor;
 

crates/bevy_ecs/src/schedule_v3/executor/multi_threaded.rs

@@ -1,14 +1,16 @@
-use bevy_tasks::{ComputeTaskPool, Scope, TaskPool};
+use bevy_tasks::{ComputeTaskPool, Scope, TaskPool, ThreadExecutor};
 use bevy_utils::default;
 use bevy_utils::syncunsafecell::SyncUnsafeCell;
 #[cfg(feature = "trace")]
 use bevy_utils::tracing::{info_span, Instrument};
+use std::sync::Arc;
 
 use async_channel::{Receiver, Sender};
 use fixedbitset::FixedBitSet;
 
 use crate::{
     archetype::ArchetypeComponentId,
+    prelude::Resource,
     query::Access,
     schedule_v3::{
         is_apply_system_buffers, BoxedCondition, ExecutorKind, SystemExecutor, SystemSchedule,
@@ -17,6 +19,8 @@ use crate::{
     world::World,
 };
 
+use crate as bevy_ecs;
+
 /// A funky borrow split of [`SystemSchedule`] required by the [`MultiThreadedExecutor`].
 struct SyncUnsafeSchedule<'a> {
     systems: &'a [SyncUnsafeCell<BoxedSystem>],
@@ -145,13 +149,21 @@ impl SystemExecutor for MultiThreadedExecutor {
             }
         }
 
+        let thread_executor = world
+            .get_resource::<MainThreadExecutor>()
+            .map(|e| e.0.clone());
+        let thread_executor = thread_executor.as_deref();
+
         let world = SyncUnsafeCell::from_mut(world);
         let SyncUnsafeSchedule {
             systems,
             mut conditions,
         } = SyncUnsafeSchedule::new(schedule);
 
-        ComputeTaskPool::init(TaskPool::default).scope(|scope| {
+        ComputeTaskPool::init(TaskPool::default).scope_with_executor(
+            false,
+            thread_executor,
+            |scope| {
                 // the executor itself is a `Send` future so that it can run
                 // alongside systems that claim the local thread
                 let executor = async {
@@ -185,7 +197,8 @@ impl SystemExecutor for MultiThreadedExecutor {
                 #[cfg(feature = "trace")]
                 let executor = executor.instrument(executor_span);
                 scope.spawn(executor);
-        });
+            },
+        );
 
         // Do one final apply buffers after all systems have completed
         // SAFETY: all systems have completed, and so no outstanding accesses remain
@@ -574,3 +587,13 @@ fn evaluate_and_fold_conditions(conditions: &mut [BoxedCondition], world: &World
         })
         .fold(true, |acc, res| acc && res)
 }
+
+/// New-typed [`ThreadExecutor`] [`Resource`] that is used to run systems on the main thread
+#[derive(Resource, Default, Clone)]
+pub struct MainThreadExecutor(pub Arc<ThreadExecutor<'static>>);
+
+impl MainThreadExecutor {
+    pub fn new() -> Self {
+        MainThreadExecutor(Arc::new(ThreadExecutor::new()))
+    }
+}

crates/bevy_render/src/pipelined_rendering.rs

@@ -2,7 +2,8 @@ use async_channel::{Receiver, Sender};
 
 use bevy_app::{App, AppLabel, Plugin, SubApp};
 use bevy_ecs::{
-    schedule::{MainThreadExecutor, StageLabel, SystemStage},
+    schedule::{StageLabel, SystemStage},
+    schedule_v3::MainThreadExecutor,
     system::Resource,
     world::{Mut, World},
 };