await tasks to cancel (#6696)

# Objective

- Fixes https://github.com/bevyengine/bevy/issues/6603

## Solution

- `Task`s will cancel when dropped, but wait until they return Pending before they actually get canceled. That means that if a task panics, it's possible for that error to get propagated to the scope and the scope gets dropped, while scoped tasks in other threads are still running. This is a big problem since scoped task can hold life-timed values that are dropped as the scope is dropped leading to UB.

---

## Changelog

- changed `Scope` to use `FallibleTask` and await the cancellation of all remaining tasks when it's dropped.

crates/bevy_tasks/src/task_pool.rs

@@ -2,11 +2,11 @@ use std::{
     future::Future,
     marker::PhantomData,
     mem,
-    pin::Pin,
     sync::Arc,
     thread::{self, JoinHandle},
 };
 
+use async_task::FallibleTask;
 use concurrent_queue::ConcurrentQueue;
 use futures_lite::{future, pin, FutureExt};
 
@@ -248,8 +248,8 @@ impl TaskPool {
         let task_scope_executor = &async_executor::Executor::default();
         let task_scope_executor: &'env async_executor::Executor =
             unsafe { mem::transmute(task_scope_executor) };
-        let spawned: ConcurrentQueue<async_executor::Task<T>> = ConcurrentQueue::unbounded();
-        let spawned_ref: &'env ConcurrentQueue<async_executor::Task<T>> =
+        let spawned: ConcurrentQueue<FallibleTask<T>> = ConcurrentQueue::unbounded();
+        let spawned_ref: &'env ConcurrentQueue<FallibleTask<T>> =
             unsafe { mem::transmute(&spawned) };
 
         let scope = Scope {
@@ -267,10 +267,10 @@ impl TaskPool {
         if spawned.is_empty() {
             Vec::new()
         } else {
-            let get_results = async move {
-                let mut results = Vec::with_capacity(spawned.len());
-                while let Ok(task) = spawned.pop() {
-                    results.push(task.await);
+            let get_results = async {
+                let mut results = Vec::with_capacity(spawned_ref.len());
+                while let Ok(task) = spawned_ref.pop() {
+                    results.push(task.await.unwrap());
                 }
 
                 results
@@ -279,23 +279,8 @@ impl TaskPool {
             // Pin the futures on the stack.
             pin!(get_results);
 
-            // SAFETY: This function blocks until all futures complete, so we do not read/write
-            // the data from futures outside of the 'scope lifetime. However,
-            // rust has no way of knowing this so we must convert to 'static
-            // here to appease the compiler as it is unable to validate safety.
-            let get_results: Pin<&mut (dyn Future<Output = Vec<T>> + 'static + Send)> = get_results;
-            let get_results: Pin<&'static mut (dyn Future<Output = Vec<T>> + 'static + Send)> =
-                unsafe { mem::transmute(get_results) };
-
-            // The thread that calls scope() will participate in driving tasks in the pool
-            // forward until the tasks that are spawned by this scope() call
-            // complete. (If the caller of scope() happens to be a thread in
-            // this thread pool, and we only have one thread in the pool, then
-            // simply calling future::block_on(spawned) would deadlock.)
-            let mut spawned = task_scope_executor.spawn(get_results);
-
             loop {
-                if let Some(result) = future::block_on(future::poll_once(&mut spawned)) {
+                if let Some(result) = future::block_on(future::poll_once(&mut get_results)) {
                     break result;
                 };
 
@@ -378,7 +363,7 @@ impl Drop for TaskPool {
 pub struct Scope<'scope, 'env: 'scope, T> {
     executor: &'scope async_executor::Executor<'scope>,
     task_scope_executor: &'scope async_executor::Executor<'scope>,
-    spawned: &'scope ConcurrentQueue<async_executor::Task<T>>,
+    spawned: &'scope ConcurrentQueue<FallibleTask<T>>,
     // make `Scope` invariant over 'scope and 'env
     scope: PhantomData<&'scope mut &'scope ()>,
     env: PhantomData<&'env mut &'env ()>,
@@ -394,7 +379,7 @@ impl<'scope, 'env, T: Send + 'scope> Scope<'scope, 'env, T> {
     ///
     /// For more information, see [`TaskPool::scope`].
     pub fn spawn<Fut: Future<Output = T> + 'scope + Send>(&self, f: Fut) {
-        let task = self.executor.spawn(f);
+        let task = self.executor.spawn(f).fallible();
         // ConcurrentQueue only errors when closed or full, but we never
         // close and use an unbouded queue, so it is safe to unwrap
         self.spawned.push(task).unwrap();
@@ -407,13 +392,26 @@ 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);
+        let task = self.task_scope_executor.spawn(f).fallible();
         // ConcurrentQueue only errors when closed or full, but we never
         // close and use an unbouded queue, so it is safe to unwrap
         self.spawned.push(task).unwrap();
     }
 }
 
+impl<'scope, 'env, T> Drop for Scope<'scope, 'env, T>
+where
+    T: 'scope,
+{
+    fn drop(&mut self) {
+        future::block_on(async {
+            while let Ok(task) = self.spawned.pop() {
+                task.cancel().await;
+            }
+        });
+    }
+}
+
 #[cfg(test)]
 #[allow(clippy::disallowed_types)]
 mod tests {