Thread executor for running tasks on specific threads. (#7087)

# Objective

- Spawn tasks from other threads onto an async executor, but limit those tasks to run on a specific thread.
- This is a continuation of trying to break up some of the changes in pipelined rendering.
- Eventually this will be used to allow `NonSend` systems to run on the main thread in pipelined rendering #6503 and also to solve #6552.
- For this specific PR this allows for us to store a thread executor in a thread local, rather than recreating a scope executor for every scope which should save on a little work.

## Solution

- We create a Executor that does a runtime check for what thread it's on before creating a !Send ticker. The ticker is the only way for the executor to make progress.

---

## Changelog

- create a ThreadExecutor that can only be ticked on one thread.

crates/bevy_tasks/src/lib.rs

@@ -22,6 +22,11 @@ mod usages;
 pub use usages::tick_global_task_pools_on_main_thread;
 pub use usages::{AsyncComputeTaskPool, ComputeTaskPool, IoTaskPool};
 
+#[cfg(not(target_arch = "wasm32"))]
+mod thread_executor;
+#[cfg(not(target_arch = "wasm32"))]
+pub use thread_executor::{ThreadExecutor, ThreadExecutorTicker};
+
 mod iter;
 pub use iter::ParallelIterator;
 

crates/bevy_tasks/src/task_pool.rs

@@ -10,7 +10,7 @@ use async_task::FallibleTask;
 use concurrent_queue::ConcurrentQueue;
 use futures_lite::{future, pin, FutureExt};
 
-use crate::Task;
+use crate::{thread_executor::ThreadExecutor, Task};
 
 struct CallOnDrop(Option<Arc<dyn Fn() + Send + Sync + 'static>>);
 
@@ -108,6 +108,7 @@ pub struct TaskPool {
 impl TaskPool {
     thread_local! {
         static LOCAL_EXECUTOR: async_executor::LocalExecutor<'static> = async_executor::LocalExecutor::new();
+        static THREAD_EXECUTOR: ThreadExecutor<'static> = ThreadExecutor::new();
     }
 
     /// Create a `TaskPool` with the default configuration.
@@ -271,6 +272,7 @@ impl TaskPool {
         F: for<'scope> FnOnce(&'scope Scope<'scope, 'env, T>),
         T: Send + 'static,
     {
+        Self::THREAD_EXECUTOR.with(|thread_executor| {
             // SAFETY: This safety comment applies to all references transmuted to 'env.
             // Any futures spawned with these references need to return before this function completes.
             // This is guaranteed because we drive all the futures spawned onto the Scope
@@ -278,16 +280,15 @@ impl TaskPool {
             // transmute the lifetimes to 'env here to appease the compiler as it is unable to validate safety.
             let executor: &async_executor::Executor = &self.executor;
             let executor: &'env async_executor::Executor = unsafe { mem::transmute(executor) };
-        let task_scope_executor = &async_executor::Executor::default();
+            let thread_executor: &'env ThreadExecutor<'env> =
-        let task_scope_executor: &'env async_executor::Executor =
+                unsafe { mem::transmute(thread_executor) };
-            unsafe { mem::transmute(task_scope_executor) };
             let spawned: ConcurrentQueue<FallibleTask<T>> = ConcurrentQueue::unbounded();
             let spawned_ref: &'env ConcurrentQueue<FallibleTask<T>> =
                 unsafe { mem::transmute(&spawned) };
 
             let scope = Scope {
                 executor,
-            task_scope_executor,
+                thread_executor,
                 spawned: spawned_ref,
                 scope: PhantomData,
                 env: PhantomData,
@@ -312,6 +313,7 @@ impl TaskPool {
                 // Pin the futures on the stack.
                 pin!(get_results);
 
+                let thread_ticker = thread_executor.ticker().unwrap();
                 loop {
                     if let Some(result) = future::block_on(future::poll_once(&mut get_results)) {
                         break result;
@@ -319,11 +321,12 @@ impl TaskPool {
 
                     std::panic::catch_unwind(|| {
                         executor.try_tick();
-                    task_scope_executor.try_tick();
+                        thread_ticker.try_tick();
                     })
                     .ok();
                 }
             }
+        })
     }
 
     /// Spawns a static future onto the thread pool. The returned Task is a future. It can also be
@@ -395,7 +398,7 @@ impl Drop for TaskPool {
 #[derive(Debug)]
 pub struct Scope<'scope, 'env: 'scope, T> {
     executor: &'scope async_executor::Executor<'scope>,
-    task_scope_executor: &'scope async_executor::Executor<'scope>,
+    thread_executor: &'scope ThreadExecutor<'scope>,
     spawned: &'scope ConcurrentQueue<FallibleTask<T>>,
     // make `Scope` invariant over 'scope and 'env
     scope: PhantomData<&'scope mut &'scope ()>,
@@ -425,7 +428,7 @@ impl<'scope, 'env, T: Send + 'scope> Scope<'scope, 'env, T> {
     ///
     /// For more information, see [`TaskPool::scope`].
     pub fn spawn_on_scope<Fut: Future<Output = T> + 'scope + Send>(&self, f: Fut) {
-        let task = self.task_scope_executor.spawn(f).fallible();
+        let task = self.thread_executor.spawn(f).fallible();
         // ConcurrentQueue only errors when closed or full, but we never
         // close and use an unbounded queue, so it is safe to unwrap
         self.spawned.push(task).unwrap();

crates/bevy_tasks/src/thread_executor.rs

@@ -0,0 +1,128 @@
+use std::{
+    marker::PhantomData,
+    thread::{self, ThreadId},
+};
+
+use async_executor::{Executor, Task};
+use futures_lite::Future;
+
+/// An executor that can only be ticked on the thread it was instantiated on. But
+/// can spawn `Send` tasks from other threads.
+///
+/// # Example
+/// ```rust
+/// # use std::sync::{Arc, atomic::{AtomicI32, Ordering}};
+/// use bevy_tasks::ThreadExecutor;
+///
+/// let thread_executor = ThreadExecutor::new();
+/// let count = Arc::new(AtomicI32::new(0));
+///
+/// // create some owned values that can be moved into another thread
+/// let count_clone = count.clone();
+///
+/// std::thread::scope(|scope| {
+///     scope.spawn(|| {
+///         // we cannot get the ticker from another thread
+///         let not_thread_ticker = thread_executor.ticker();
+///         assert!(not_thread_ticker.is_none());
+///         
+///         // but we can spawn tasks from another thread
+///         thread_executor.spawn(async move {
+///             count_clone.fetch_add(1, Ordering::Relaxed);
+///         }).detach();
+///     });
+/// });
+///
+/// // the tasks do not make progress unless the executor is manually ticked
+/// assert_eq!(count.load(Ordering::Relaxed), 0);
+///
+/// // tick the ticker until task finishes
+/// let thread_ticker = thread_executor.ticker().unwrap();
+/// thread_ticker.try_tick();
+/// assert_eq!(count.load(Ordering::Relaxed), 1);
+/// ```
+#[derive(Debug)]
+pub struct ThreadExecutor<'task> {
+    executor: Executor<'task>,
+    thread_id: ThreadId,
+}
+
+impl<'task> Default for ThreadExecutor<'task> {
+    fn default() -> Self {
+        Self {
+            executor: Executor::new(),
+            thread_id: thread::current().id(),
+        }
+    }
+}
+
+impl<'task> ThreadExecutor<'task> {
+    /// create a new [`ThreadExecutor`]
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Spawn a task on the thread executor
+    pub fn spawn<T: Send + 'task>(
+        &self,
+        future: impl Future<Output = T> + Send + 'task,
+    ) -> Task<T> {
+        self.executor.spawn(future)
+    }
+
+    /// Gets the [`ThreadExecutorTicker`] for this executor.
+    /// Use this to tick the executor.
+    /// It only returns the ticker if it's on the thread the executor was created on
+    /// and returns `None` otherwise.
+    pub fn ticker<'ticker>(&'ticker self) -> Option<ThreadExecutorTicker<'task, 'ticker>> {
+        if thread::current().id() == self.thread_id {
+            return Some(ThreadExecutorTicker {
+                executor: &self.executor,
+                _marker: PhantomData::default(),
+            });
+        }
+        None
+    }
+}
+
+/// Used to tick the [`ThreadExecutor`]. The executor does not
+/// make progress unless it is manually ticked on the thread it was
+/// created on.
+#[derive(Debug)]
+pub struct ThreadExecutorTicker<'task, 'ticker> {
+    executor: &'ticker Executor<'task>,
+    // make type not send or sync
+    _marker: PhantomData<*const ()>,
+}
+impl<'task, 'ticker> ThreadExecutorTicker<'task, 'ticker> {
+    /// Tick the thread executor.
+    pub async fn tick(&self) {
+        self.executor.tick().await;
+    }
+
+    /// Synchronously try to tick a task on the executor.
+    /// Returns false if if does not find a task to tick.
+    pub fn try_tick(&self) -> bool {
+        self.executor.try_tick()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::sync::Arc;
+
+    #[test]
+    fn test_ticker() {
+        let executor = Arc::new(ThreadExecutor::new());
+        let ticker = executor.ticker();
+        assert!(ticker.is_some());
+
+        std::thread::scope(|s| {
+            s.spawn(|| {
+                let ticker = executor.ticker();
+                assert!(ticker.is_none());
+            });
+        });
+    }
+}