From 392795989a12d472d6531906017eace55da74417 Mon Sep 17 00:00:00 2001
From: Mike <mike.hsu@gmail.com>
Date: Sun, 11 Dec 2022 18:34:15 +0000
Subject: [PATCH] run clear trackers on render world (#6878)

# Objective

- Fixes https://github.com/bevyengine/bevy/issues/6417

## Solution

- clear_trackers was not being called on the render world. This causes the removed components vecs to continuously grow. This PR adds clear trackers to the end of RenderStage::Cleanup

## Migration Guide

The call to `clear_trackers` in `App` has been moved from the schedule to App::update for the main world and calls to `clear_trackers` have been added for sub_apps in the same function. This was due to needing stronger guarantees. If clear_trackers isn't called on a world it can lead to memory leaks in `RemovedComponents`.
---
 crates/bevy_app/src/app.rs                 | 8 +++++---
 crates/bevy_ecs/src/system/system_param.rs | 7 +++----
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/crates/bevy_app/src/app.rs b/crates/bevy_app/src/app.rs
index c6613825591ec..d88a82059126c 100644
--- a/crates/bevy_app/src/app.rs
+++ b/crates/bevy_app/src/app.rs
@@ -107,9 +107,7 @@ impl Default for App {
         #[cfg(feature = "bevy_reflect")]
         app.init_resource::<AppTypeRegistry>();
 
-        app.add_default_stages()
-            .add_event::<AppExit>()
-            .add_system_to_stage(CoreStage::Last, World::clear_trackers);
+        app.add_default_stages().add_event::<AppExit>();
 
         #[cfg(feature = "bevy_ci_testing")]
         {
@@ -150,9 +148,13 @@ impl App {
         #[cfg(feature = "trace")]
         let _bevy_frame_update_span = info_span!("frame").entered();
         self.schedule.run(&mut self.world);
+
         for sub_app in self.sub_apps.values_mut() {
             (sub_app.runner)(&mut self.world, &mut sub_app.app);
+            sub_app.app.world.clear_trackers();
         }
+
+        self.world.clear_trackers();
     }
 
     /// Starts the application by calling the app's [runner function](Self::set_runner).
diff --git a/crates/bevy_ecs/src/system/system_param.rs b/crates/bevy_ecs/src/system/system_param.rs
index 8ec8953d486f7..c9ef8ab840eb3 100644
--- a/crates/bevy_ecs/src/system/system_param.rs
+++ b/crates/bevy_ecs/src/system/system_param.rs
@@ -816,10 +816,9 @@ unsafe impl<T: FromWorld + Send + 'static> SystemParamState for LocalState<T> {
 /// note that the `RemovedComponents` list will not be automatically cleared for you,
 /// and will need to be manually flushed using [`World::clear_trackers`]
 ///
-/// For users of `bevy` itself, this is automatically done in a system added by `MinimalPlugins`
-/// or `DefaultPlugins` at the end of each pass of the game loop during the `CoreStage::Last`
-/// stage. As such `RemovedComponents` systems should be scheduled after the stage where
-/// removal occurs but before `CoreStage::Last`.
+/// For users of `bevy` and `bevy_app`, this is automatically done in `bevy_app::App::update`.
+/// For the main world, [`World::clear_trackers`] is run after the main schedule is run and after
+/// `SubApp`'s have run.
 ///
 /// # Examples
 ///