Stageless: close the finish channel so executor doesn't deadlock (#7448)

# Objective

- Fix panic_when_hierachy_cycle test hanging
- The problem is that the scope only awaits one task at a time in get_results. In stageless this task is the multithreaded executor. That tasks hangs when a system panics and cannot make anymore progress. This wasn't a problem before because the executor was spawned after all the system tasks had been spawned. But in stageless the executor is spawned before all the system tasks are spawned.

## Solution

- We can catch unwind on each system and close the finish channel if one panics. This then causes the receiver end of the finish channel to panic too.
- this might have a small perf impact, but when running many_foxes it seems to be within the noise. So less than 40us.

## Other possible solutions

- It might be possible to fairly poll all the tasks in get_results in the scope. If we could do that then the scope could panic whenever one of tasks panics. It would require a data structure that we could both poll the futures through a shared ref and also push to it. I tried FuturesUnordered, but it requires an exclusive ref to poll it.
- The catch unwind could be moved onto when we create the tasks for scope instead. We would then need something like a oneshot async channel to inform get_results if a task panics.

Author: hymm

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

@@ -1,3 +1,5 @@
+use std::panic::AssertUnwindSafe;
+
 use bevy_tasks::{ComputeTaskPool, Scope, TaskPool, ThreadExecutor};
 use bevy_utils::default;
 use bevy_utils::syncunsafecell::SyncUnsafeCell;
@@ -175,11 +177,10 @@ impl SystemExecutor for MultiThreadedExecutor {
 
                         if self.num_running_systems > 0 {
                             // wait for systems to complete
-                            let index = self
-                                .receiver
-                                .recv()
-                                .await
-                                .unwrap_or_else(|error| unreachable!("{}", error));
+                            let index =
+                                self.receiver.recv().await.expect(
+                                    "A system has panicked so the executor cannot continue.",
+                                );
 
                             self.finish_system_and_signal_dependents(index);
 
@@ -429,14 +430,22 @@ impl MultiThreadedExecutor {
         let task = async move {
             #[cfg(feature = "trace")]
             let system_guard = system_span.enter();
-            // SAFETY: access is compatible
-            unsafe { system.run_unsafe((), world) };
+            let res = std::panic::catch_unwind(AssertUnwindSafe(|| {
+                // SAFETY: access is compatible
+                unsafe { system.run_unsafe((), world) };
+            }));
             #[cfg(feature = "trace")]
             drop(system_guard);
-            sender
-                .send(system_index)
-                .await
-                .unwrap_or_else(|error| unreachable!("{}", error));
+            if res.is_err() {
+                // close the channel to propagate the error to the
+                // multithreaded executor
+                sender.close();
+            } else {
+                sender
+                    .send(system_index)
+                    .await
+                    .unwrap_or_else(|error| unreachable!("{}", error));
+            }
         };
 
         #[cfg(feature = "trace")]
@@ -479,13 +488,21 @@ impl MultiThreadedExecutor {
             let task = async move {
                 #[cfg(feature = "trace")]
                 let system_guard = system_span.enter();
-                apply_system_buffers(&unapplied_systems, systems, world);
+                let res = std::panic::catch_unwind(AssertUnwindSafe(|| {
+                    apply_system_buffers(&unapplied_systems, systems, world);
+                }));
                 #[cfg(feature = "trace")]
                 drop(system_guard);
-                sender
-                    .send(system_index)
-                    .await
-                    .unwrap_or_else(|error| unreachable!("{}", error));
+                if res.is_err() {
+                    // close the channel to propagate the error to the
+                    // multithreaded executor
+                    sender.close();
+                } else {
+                    sender
+                        .send(system_index)
+                        .await
+                        .unwrap_or_else(|error| unreachable!("{}", error));
+                }
             };
 
             #[cfg(feature = "trace")]
@@ -495,13 +512,21 @@ impl MultiThreadedExecutor {
             let task = async move {
                 #[cfg(feature = "trace")]
                 let system_guard = system_span.enter();
-                system.run((), world);
+                let res = std::panic::catch_unwind(AssertUnwindSafe(|| {
+                    system.run((), world);
+                }));
                 #[cfg(feature = "trace")]
                 drop(system_guard);
-                sender
-                    .send(system_index)
-                    .await
-                    .unwrap_or_else(|error| unreachable!("{}", error));
+                if res.is_err() {
+                    // close the channel to propagate the error to the
+                    // multithreaded executor
+                    sender.close();
+                } else {
+                    sender
+                        .send(system_index)
+                        .await
+                        .unwrap_or_else(|error| unreachable!("{}", error));
+                }
             };
 
             #[cfg(feature = "trace")]