tick local executor (#6121)

# Objective

- #4466 broke local tasks running.
- Fixes #6120

## Solution

- Add system for ticking local executors on main thread into bevy_core where the tasks pools are initialized.
- Add ticking local executors into thread executors

## Changelog

- tick all thread local executors in task pool.

## Notes

- ~~Not 100% sure about this PR. Ticking the local executor for the main thread in scope feels a little kludgy as it requires users of bevy_tasks to be calling scope periodically for those tasks to make progress.~~ took this out in favor of a system that ticks the local executors.

crates/bevy_core/Cargo.toml

@@ -20,3 +20,6 @@ bevy_utils = { path = "../bevy_utils", version = "0.9.0-dev" }
 
 # other
 bytemuck = "1.5"
+
+[dev-dependencies]
+crossbeam-channel = "0.5.0"

crates/bevy_core/src/lib.rs

@@ -22,6 +22,11 @@ use bevy_utils::{Duration, HashSet, Instant};
 use std::borrow::Cow;
 use std::ops::Range;
 
+#[cfg(not(target_arch = "wasm32"))]
+use bevy_ecs::schedule::IntoSystemDescriptor;
+#[cfg(not(target_arch = "wasm32"))]
+use bevy_tasks::tick_global_task_pools_on_main_thread;
+
 /// Adds core functionality to Apps.
 #[derive(Default)]
 pub struct CorePlugin;
@@ -35,6 +40,13 @@ impl Plugin for CorePlugin {
             .unwrap_or_default()
             .create_default_pools();
 
+        #[cfg(not(target_arch = "wasm32"))]
+        app.add_system_to_stage(
+            bevy_app::CoreStage::Last,
+            tick_global_task_pools_on_main_thread.at_end(),
+        );
+
+        app.register_type::<Entity>().register_type::<Name>();
         app.register_type::<Entity>()
             .register_type::<Name>()
             .register_type::<Range<f32>>()
@@ -97,3 +109,42 @@ fn register_math_types(app: &mut App) {
 /// Wraps to 0 when it reaches the maximum u32 value
 #[derive(Default, Resource, Clone, Copy)]
 pub struct FrameCount(pub u32);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use bevy_tasks::prelude::{AsyncComputeTaskPool, ComputeTaskPool, IoTaskPool};
+
+    #[test]
+    fn runs_spawn_local_tasks() {
+        let mut app = App::new();
+        app.add_plugin(CorePlugin);
+
+        let (async_tx, async_rx) = crossbeam_channel::unbounded();
+        AsyncComputeTaskPool::get()
+            .spawn_local(async move {
+                async_tx.send(()).unwrap();
+            })
+            .detach();
+
+        let (compute_tx, compute_rx) = crossbeam_channel::unbounded();
+        ComputeTaskPool::get()
+            .spawn_local(async move {
+                compute_tx.send(()).unwrap();
+            })
+            .detach();
+
+        let (io_tx, io_rx) = crossbeam_channel::unbounded();
+        IoTaskPool::get()
+            .spawn_local(async move {
+                io_tx.send(()).unwrap();
+            })
+            .detach();
+
+        app.run();
+
+        async_rx.try_recv().unwrap();
+        compute_rx.try_recv().unwrap();
+        io_rx.try_recv().unwrap();
+    }
+}

crates/bevy_tasks/src/lib.rs

@@ -18,6 +18,8 @@ mod single_threaded_task_pool;
 pub use single_threaded_task_pool::{Scope, TaskPool, TaskPoolBuilder};
 
 mod usages;
+#[cfg(not(target_arch = "wasm32"))]
+pub use usages::tick_global_task_pools_on_main_thread;
 pub use usages::{AsyncComputeTaskPool, ComputeTaskPool, IoTaskPool};
 
 mod iter;

crates/bevy_tasks/src/task_pool.rs

@@ -8,7 +8,7 @@ use std::{
 };
 
 use concurrent_queue::ConcurrentQueue;
-use futures_lite::{future, pin};
+use futures_lite::{future, pin, FutureExt};
 
 use crate::Task;
 
@@ -117,9 +117,16 @@ impl TaskPool {
 
                 thread_builder
                     .spawn(move || {
-                        let shutdown_future = ex.run(shutdown_rx.recv());
-                        // Use unwrap_err because we expect a Closed error
-                        future::block_on(shutdown_future).unwrap_err();
+                        TaskPool::LOCAL_EXECUTOR.with(|local_executor| {
+                            let tick_forever = async move {
+                                loop {
+                                    local_executor.tick().await;
+                                }
+                            };
+                            let shutdown_future = ex.run(tick_forever.or(shutdown_rx.recv()));
+                            // Use unwrap_err because we expect a Closed error
+                            future::block_on(shutdown_future).unwrap_err();
+                        });
                     })
                     .expect("Failed to spawn thread.")
             })
@@ -314,6 +321,24 @@ impl TaskPool {
     {
         Task::new(TaskPool::LOCAL_EXECUTOR.with(|executor| executor.spawn(future)))
     }
+
+    /// Runs a function with the local executor. Typically used to tick
+    /// the local executor on the main thread as it needs to share time with
+    /// other things.
+    ///
+    /// ```rust
+    /// use bevy_tasks::TaskPool;
+    ///
+    /// TaskPool::new().with_local_executor(|local_executor| {
+    ///     local_executor.try_tick();
+    /// });
+    /// ```
+    pub fn with_local_executor<F, R>(&self, f: F) -> R
+    where
+        F: FnOnce(&async_executor::LocalExecutor) -> R,
+    {
+        Self::LOCAL_EXECUTOR.with(f)
+    }
 }
 
 impl Default for TaskPool {

crates/bevy_tasks/src/usages.rs

@@ -109,3 +109,29 @@ impl Deref for IoTaskPool {
         &self.0
     }
 }
+
+/// Used by `bevy_core` to tick the global tasks pools on the main thread.
+/// This will run a maximum of 100 local tasks per executor per call to this function.
+#[cfg(not(target_arch = "wasm32"))]
+pub fn tick_global_task_pools_on_main_thread() {
+    COMPUTE_TASK_POOL
+        .get()
+        .unwrap()
+        .with_local_executor(|compute_local_executor| {
+            ASYNC_COMPUTE_TASK_POOL
+                .get()
+                .unwrap()
+                .with_local_executor(|async_local_executor| {
+                    IO_TASK_POOL
+                        .get()
+                        .unwrap()
+                        .with_local_executor(|io_local_executor| {
+                            for _ in 0..100 {
+                                compute_local_executor.try_tick();
+                                async_local_executor.try_tick();
+                                io_local_executor.try_tick();
+                            }
+                        });
+                });
+        });
+}