Fix App::get_added_plugins not working inside finish and cleanup (bevyengine#20506)

# Objective

This assertion fails when called inside finish or cleanup:
```rs
assert_eq!(
    app.is_plugin_added::<ImagePlugin>(),
    app.get_added_plugins::<ImagePlugin>().len() > 0
);
```

## Solution

- Do the hokey pokey one plugin at a time
- swap_remove to stay O(1), then push and swap back to preserve plugin
order

## Testing

- Someone should write unit tests for this and probably document that it
hokey pokeys, although i dont think anyone will
`get_added_plugins<Self>` because you have a `self` param in finish and
cleanup anyways.

Author: atlv24

crates/bevy_app/src/app.rs

@@ -256,27 +256,29 @@ impl App {
     /// plugins are ready, but can be useful for situations where you want to use [`App::update`].
     pub fn finish(&mut self) {
         // plugins installed to main should see all sub-apps
-        let plugins = core::mem::take(&mut self.main_mut().plugin_registry);
-        for plugin in &plugins {
-            plugin.finish(self);
+        // do hokey pokey with a boxed zst plugin (doesn't allocate)
+        let mut hokeypokey: Box<dyn Plugin> = Box::new(HokeyPokey);
+        for i in 0..self.main().plugin_registry.len() {
+            core::mem::swap(&mut self.main_mut().plugin_registry[i], &mut hokeypokey);
+            hokeypokey.finish(self);
+            core::mem::swap(&mut self.main_mut().plugin_registry[i], &mut hokeypokey);
         }
-        let main = self.main_mut();
-        main.plugin_registry = plugins;
-        main.plugins_state = PluginsState::Finished;
+        self.main_mut().plugins_state = PluginsState::Finished;
         self.sub_apps.iter_mut().skip(1).for_each(SubApp::finish);
     }
 
     /// Runs [`Plugin::cleanup`] for each plugin. This is usually called by the event loop after
     /// [`App::finish`], but can be useful for situations where you want to use [`App::update`].
     pub fn cleanup(&mut self) {
         // plugins installed to main should see all sub-apps
-        let plugins = core::mem::take(&mut self.main_mut().plugin_registry);
-        for plugin in &plugins {
-            plugin.cleanup(self);
+        // do hokey pokey with a boxed zst plugin (doesn't allocate)
+        let mut hokeypokey: Box<dyn Plugin> = Box::new(HokeyPokey);
+        for i in 0..self.main().plugin_registry.len() {
+            core::mem::swap(&mut self.main_mut().plugin_registry[i], &mut hokeypokey);
+            hokeypokey.cleanup(self);
+            core::mem::swap(&mut self.main_mut().plugin_registry[i], &mut hokeypokey);
         }
-        let main = self.main_mut();
-        main.plugin_registry = plugins;
-        main.plugins_state = PluginsState::Cleaned;
+        self.main_mut().plugins_state = PluginsState::Cleaned;
         self.sub_apps.iter_mut().skip(1).for_each(SubApp::cleanup);
     }
 
@@ -1390,6 +1392,12 @@ impl App {
     }
 }
 
+// Used for doing hokey pokey in finish and cleanup
+pub(crate) struct HokeyPokey;
+impl Plugin for HokeyPokey {
+    fn build(&self, _: &mut App) {}
+}
+
 type RunnerFn = Box<dyn FnOnce(App) -> AppExit>;
 
 fn run_once(mut app: App) -> AppExit {
@@ -1526,6 +1534,38 @@ mod tests {
         }
     }
 
+    struct PluginF;
+
+    impl Plugin for PluginF {
+        fn build(&self, _app: &mut App) {}
+
+        fn finish(&self, app: &mut App) {
+            // Ensure other plugins are available during finish
+            assert_eq!(
+                app.is_plugin_added::<PluginA>(),
+                !app.get_added_plugins::<PluginA>().is_empty(),
+            );
+        }
+
+        fn cleanup(&self, app: &mut App) {
+            // Ensure other plugins are available during finish
+            assert_eq!(
+                app.is_plugin_added::<PluginA>(),
+                !app.get_added_plugins::<PluginA>().is_empty(),
+            );
+        }
+    }
+
+    struct PluginG;
+
+    impl Plugin for PluginG {
+        fn build(&self, _app: &mut App) {}
+
+        fn finish(&self, app: &mut App) {
+            app.add_plugins(PluginB);
+        }
+    }
+
     #[test]
     fn can_add_two_plugins() {
         App::new().add_plugins((PluginA, PluginB));
@@ -1595,6 +1635,39 @@ mod tests {
         app.finish();
     }
 
+    #[test]
+    fn test_get_added_plugins_works_during_finish_and_cleanup() {
+        let mut app = App::new();
+        app.add_plugins(PluginA);
+        app.add_plugins(PluginF);
+        app.finish();
+    }
+
+    #[test]
+    fn test_adding_plugin_works_during_finish() {
+        let mut app = App::new();
+        app.add_plugins(PluginA);
+        app.add_plugins(PluginG);
+        app.finish();
+        assert_eq!(
+            app.main().plugin_registry[0].name(),
+            "bevy_app::main_schedule::MainSchedulePlugin"
+        );
+        assert_eq!(
+            app.main().plugin_registry[1].name(),
+            "bevy_app::app::tests::PluginA"
+        );
+        assert_eq!(
+            app.main().plugin_registry[2].name(),
+            "bevy_app::app::tests::PluginG"
+        );
+        // PluginG adds PluginB during finish
+        assert_eq!(
+            app.main().plugin_registry[3].name(),
+            "bevy_app::app::tests::PluginB"
+        );
+    }
+
     #[test]
     fn test_derive_app_label() {
         use super::AppLabel;

crates/bevy_app/src/sub_app.rs

@@ -401,25 +401,29 @@ impl SubApp {
 
     /// Runs [`Plugin::finish`] for each plugin.
     pub fn finish(&mut self) {
-        let plugins = core::mem::take(&mut self.plugin_registry);
-        self.run_as_app(|app| {
-            for plugin in &plugins {
-                plugin.finish(app);
-            }
-        });
-        self.plugin_registry = plugins;
+        // do hokey pokey with a boxed zst plugin (doesn't allocate)
+        let mut hokeypokey: Box<dyn Plugin> = Box::new(crate::HokeyPokey);
+        for i in 0..self.plugin_registry.len() {
+            core::mem::swap(&mut self.plugin_registry[i], &mut hokeypokey);
+            self.run_as_app(|app| {
+                hokeypokey.finish(app);
+            });
+            core::mem::swap(&mut self.plugin_registry[i], &mut hokeypokey);
+        }
         self.plugins_state = PluginsState::Finished;
     }
 
     /// Runs [`Plugin::cleanup`] for each plugin.
     pub fn cleanup(&mut self) {
-        let plugins = core::mem::take(&mut self.plugin_registry);
-        self.run_as_app(|app| {
-            for plugin in &plugins {
-                plugin.cleanup(app);
-            }
-        });
-        self.plugin_registry = plugins;
+        // do hokey pokey with a boxed zst plugin (doesn't allocate)
+        let mut hokeypokey: Box<dyn Plugin> = Box::new(crate::HokeyPokey);
+        for i in 0..self.plugin_registry.len() {
+            core::mem::swap(&mut self.plugin_registry[i], &mut hokeypokey);
+            self.run_as_app(|app| {
+                hokeypokey.cleanup(app);
+            });
+            core::mem::swap(&mut self.plugin_registry[i], &mut hokeypokey);
+        }
         self.plugins_state = PluginsState::Cleaned;
     }