wait for render app when main world is dropped (#11737)

# Objective

- Try not to drop the render world on the render thread, and drop the
main world after the render world.
- The render world has a drop check that will panic if it is dropped off
the main thread.

## Solution

- Keep track of where the render world is and wait for it to come back
when the channel resource is dropped.

---

## Changelog

- Wait for the render world when the main world is dropped.

## Migration Guide

- If you were using the pipelined rendering channels,
`MainToRenderAppSender` and `RenderToMainAppReceiver`, they have been
combined into the single resource `RenderAppChannels`.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Friz64 <friz64@protonmail.com>

crates/bevy_render/src/pipelined_rendering.rs

@@ -17,13 +17,52 @@ use crate::RenderApp;
 #[derive(Debug, Clone, Copy, Hash, PartialEq, Eq, AppLabel)]
 pub struct RenderExtractApp;
 
-/// Channel to send the render app from the main thread to the rendering thread
+/// Channels used by the main app to send and receive the render app.
 #[derive(Resource)]
-pub struct MainToRenderAppSender(pub Sender<SubApp>);
+pub struct RenderAppChannels {
+    app_to_render_sender: Sender<SubApp>,
+    render_to_app_receiver: Receiver<SubApp>,
+    render_app_in_render_thread: bool,
+}
 
-/// Channel to send the render app from the render thread to the main thread
-#[derive(Resource)]
-pub struct RenderToMainAppReceiver(pub Receiver<SubApp>);
+impl RenderAppChannels {
+    /// Create a `RenderAppChannels` from a [`async_channel::Receiver`] and [`async_channel::Sender`]
+    pub fn new(
+        app_to_render_sender: Sender<SubApp>,
+        render_to_app_receiver: Receiver<SubApp>,
+    ) -> Self {
+        Self {
+            app_to_render_sender,
+            render_to_app_receiver,
+            render_app_in_render_thread: false,
+        }
+    }
+
+    /// Send the `render_app` to the rendering thread.
+    pub fn send_blocking(&mut self, render_app: SubApp) {
+        self.app_to_render_sender.send_blocking(render_app).unwrap();
+        self.render_app_in_render_thread = true;
+    }
+
+    /// Receive the `render_app` from the rendering thread.
+    pub async fn recv(&mut self) -> SubApp {
+        let render_app = self.render_to_app_receiver.recv().await.unwrap();
+        self.render_app_in_render_thread = false;
+        render_app
+    }
+}
+
+impl Drop for RenderAppChannels {
+    fn drop(&mut self) {
+        if self.render_app_in_render_thread {
+            // Any non-send data in the render world was initialized on the main thread.
+            // So on dropping the main world and ending the app, we block and wait for
+            // the render world to return to drop it. Which allows the non-send data
+            // drop methods to run on the correct thread.
+            self.render_to_app_receiver.recv_blocking().ok();
+        }
+    }
+}
 
 /// The [`PipelinedRenderingPlugin`] can be added to your application to enable pipelined rendering.
 /// This moves rendering into a different thread, so that the Nth frame's rendering can
@@ -96,8 +135,10 @@ impl Plugin for PipelinedRenderingPlugin {
 
         render_to_app_sender.send_blocking(render_app).unwrap();
 
-        app.insert_resource(MainToRenderAppSender(app_to_render_sender));
-        app.insert_resource(RenderToMainAppReceiver(render_to_app_receiver));
+        app.insert_resource(RenderAppChannels::new(
+            app_to_render_sender,
+            render_to_app_receiver,
+        ));
 
         std::thread::spawn(move || {
             #[cfg(feature = "trace")]
@@ -136,21 +177,19 @@ impl Plugin for PipelinedRenderingPlugin {
 // runs extract, and then sends the rendering world back to the render thread.
 fn update_rendering(app_world: &mut World, _sub_app: &mut App) {
     app_world.resource_scope(|world, main_thread_executor: Mut<MainThreadExecutor>| {
+        world.resource_scope(|world, mut render_channels: Mut<RenderAppChannels>| {
             // we use a scope here to run any main thread tasks that the render world still needs to run
             // while we wait for the render world to be received.
             let mut render_app = ComputeTaskPool::get()
                 .scope_with_executor(true, Some(&*main_thread_executor.0), |s| {
-                s.spawn(async {
-                    let receiver = world.get_resource::<RenderToMainAppReceiver>().unwrap();
-                    receiver.0.recv().await.unwrap()
-                });
+                    s.spawn(async { render_channels.recv().await });
                 })
                 .pop()
                 .unwrap();
 
             render_app.extract(world);
 
-        let sender = world.resource::<MainToRenderAppSender>();
-        sender.0.send_blocking(render_app).unwrap();
+            render_channels.send_blocking(render_app);
+        });
     });
 }