fix run-once runners (#10195)

# Objective

- After #9826, there are issues on "run once runners"
- example `without_winit` crashes:
```
2023-10-19T22:06:01.810019Z  INFO bevy_render::renderer: AdapterInfo { name: "llvmpipe (LLVM 15.0.7, 256 bits)", vendor: 65541, device: 0, device_type: Cpu, driver: "llvmpipe", driver_info: "Mesa 23.2.1 - kisak-mesa PPA (LLVM 15.0.7)", backend: Vulkan }
2023-10-19T22:06:02.860331Z  WARN bevy_audio::audio_output: No audio device found.
2023-10-19T22:06:03.215154Z  INFO bevy_diagnostic::system_information_diagnostics_plugin::internal: SystemInfo { os: "Linux 22.04 Ubuntu", kernel: "6.2.0-1014-azure", cpu: "Intel(R) Xeon(R) CPU E5-2673 v3 @ 2.40GHz", core_count: "2", memory: "6.8 GiB" }
thread 'main' panicked at crates/bevy_render/src/pipelined_rendering.rs:91:14:
Unable to get RenderApp. Another plugin may have removed the RenderApp before PipelinedRenderingPlugin
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```
- example `headless` runs the app twice with the `run_once` schedule

## Solution

- Expose a more complex state of an app than just "ready"
- Also block adding plugins to an app after it has finished or cleaned
up its plugins as that wouldn't work anyway

## Migration Guide

* `app.ready()` has been replaced by `app.plugins_state()` which will
return more details on the current state of plugins in the app

crates/bevy_app/src/app.rs

@@ -76,6 +76,7 @@ pub struct App {
     plugin_name_added: HashSet<String>,
     /// A private counter to prevent incorrect calls to `App::run()` from `Plugin::build()`
     building_plugin_depth: usize,
+    plugins_state: PluginsState,
 }
 
 impl Debug for App {
@@ -194,6 +195,19 @@ impl Default for App {
     }
 }
 
+/// Plugins state in the application
+#[derive(PartialEq, Eq, Debug, Clone, Copy)]
+pub enum PluginsState {
+    /// Plugins are being added.
+    Adding,
+    /// All plugins already added are ready.
+    Ready,
+    /// Finish has been executed for all plugins added.
+    Finished,
+    /// Cleanup has been executed for all plugins added.
+    Cleaned,
+}
+
 // Dummy plugin used to temporary hold the place in the plugin registry
 struct PlaceholderPlugin;
 impl Plugin for PlaceholderPlugin {
@@ -221,6 +235,7 @@ impl App {
             plugin_name_added: Default::default(),
             main_schedule_label: Box::new(Main),
             building_plugin_depth: 0,
+            plugins_state: PluginsState::Adding,
         }
     }
 
@@ -288,7 +303,7 @@ impl App {
             panic!("App::run() was called from within Plugin::build(), which is not allowed.");
         }
 
-        if app.ready() {
+        if app.plugins_state() == PluginsState::Ready {
             // If we're already ready, we finish up now and advance one frame.
             // This prevents black frames during the launch transition on iOS.
             app.finish();
@@ -300,20 +315,25 @@ impl App {
         (runner)(app);
     }
 
-    /// Check that [`Plugin::ready`] of all plugins returns true. This is usually called by the
+    /// Check the state of all plugins already added to this app. This is usually called by the
     /// event loop, but can be useful for situations where you want to use [`App::update`]
-    pub fn ready(&self) -> bool {
+    #[inline]
-        for plugin in &self.plugin_registry {
+    pub fn plugins_state(&self) -> PluginsState {
-            if !plugin.ready(self) {
+        match self.plugins_state {
-                return false;
+            PluginsState::Adding => {
+                for plugin in &self.plugin_registry {
+                    if !plugin.ready(self) {
+                        return PluginsState::Adding;
+                    }
+                }
+                PluginsState::Ready
             }
+            state => state,
         }
-        true
     }
 
     /// Run [`Plugin::finish`] for each plugin. This is usually called by the event loop once all
-    /// plugins are [`App::ready`], but can be useful for situations where you want to use
+    /// plugins are ready, but can be useful for situations where you want to use [`App::update`].
-    /// [`App::update`].
     pub fn finish(&mut self) {
         // temporarily remove the plugin registry to run each plugin's setup function on app.
         let plugin_registry = std::mem::take(&mut self.plugin_registry);
@@ -321,6 +341,7 @@ impl App {
             plugin.finish(self);
         }
         self.plugin_registry = plugin_registry;
+        self.plugins_state = PluginsState::Finished;
     }
 
     /// Run [`Plugin::cleanup`] for each plugin. This is usually called by the event loop after
@@ -332,6 +353,7 @@ impl App {
             plugin.cleanup(self);
         }
         self.plugin_registry = plugin_registry;
+        self.plugins_state = PluginsState::Cleaned;
     }
 
     /// Adds [`State<S>`] and [`NextState<S>`] resources, [`OnEnter`] and [`OnExit`] schedules
@@ -696,6 +718,14 @@ impl App {
     /// [`PluginGroup`]:super::PluginGroup
     #[track_caller]
     pub fn add_plugins<M>(&mut self, plugins: impl Plugins<M>) -> &mut Self {
+        if matches!(
+            self.plugins_state(),
+            PluginsState::Cleaned | PluginsState::Finished
+        ) {
+            panic!(
+                "Plugins cannot be added after App::cleanup() or App::finish() has been called."
+            );
+        }
         plugins.add_to_app(self);
         self
     }
@@ -947,14 +977,20 @@ impl App {
 }
 
 fn run_once(mut app: App) {
-    while !app.ready() {
+    let plugins_state = app.plugins_state();
-        #[cfg(not(target_arch = "wasm32"))]
+    if plugins_state != PluginsState::Cleaned {
-        bevy_tasks::tick_global_task_pools_on_main_thread();
+        while app.plugins_state() == PluginsState::Adding {
+            #[cfg(not(target_arch = "wasm32"))]
+            bevy_tasks::tick_global_task_pools_on_main_thread();
+        }
+        app.finish();
+        app.cleanup();
     }
-    app.finish();
-    app.cleanup();
 
-    app.update();
+    // if plugins where cleaned before the runner start, an update already ran
+    if plugins_state != PluginsState::Cleaned {
+        app.update();
+    }
 }
 
 /// An event that indicates the [`App`] should exit. This will fully exit the app process at the

crates/bevy_app/src/schedule_runner.rs

@@ -1,6 +1,7 @@
 use crate::{
     app::{App, AppExit},
     plugin::Plugin,
+    PluginsState,
 };
 use bevy_ecs::event::{Events, ManualEventReader};
 use bevy_utils::{Duration, Instant};
@@ -71,8 +72,9 @@ impl Plugin for ScheduleRunnerPlugin {
     fn build(&self, app: &mut App) {
         let run_mode = self.run_mode;
         app.set_runner(move |mut app: App| {
-            if !app.ready() {
+            let plugins_state = app.plugins_state();
-                while !app.ready() {
+            if plugins_state != PluginsState::Cleaned {
+                while app.plugins_state() == PluginsState::Adding {
                     #[cfg(not(target_arch = "wasm32"))]
                     bevy_tasks::tick_global_task_pools_on_main_thread();
                 }
@@ -83,7 +85,10 @@ impl Plugin for ScheduleRunnerPlugin {
             let mut app_exit_event_reader = ManualEventReader::<AppExit>::default();
             match run_mode {
                 RunMode::Once => {
-                    app.update();
+                    // if plugins where cleaned before the runner start, an update already ran
+                    if plugins_state != PluginsState::Cleaned {
+                        app.update();
+                    }
                 }
                 RunMode::Loop { wait } => {
                     let mut tick = move |app: &mut App,

crates/bevy_winit/src/lib.rs

@@ -20,7 +20,7 @@ use system::{changed_windows, create_windows, despawn_windows, CachedWindow};
 pub use winit_config::*;
 pub use winit_windows::*;
 
-use bevy_app::{App, AppExit, Last, Plugin};
+use bevy_app::{App, AppExit, Last, Plugin, PluginsState};
 use bevy_ecs::event::{Events, ManualEventReader};
 use bevy_ecs::prelude::*;
 use bevy_ecs::system::{SystemParam, SystemState};
@@ -378,8 +378,6 @@ pub fn winit_runner(mut app: App) {
         ResMut<CanvasParentResizeEventChannel>,
     )> = SystemState::from_world(&mut app.world);
 
-    let mut finished_and_setup_done = app.ready();
-
     // setup up the event loop
     let event_handler = move |event: Event<()>,
                               event_loop: &EventLoopWindowTarget<()>,
@@ -387,14 +385,13 @@ pub fn winit_runner(mut app: App) {
         #[cfg(feature = "trace")]
         let _span = bevy_utils::tracing::info_span!("winit event_handler").entered();
 
-        if !finished_and_setup_done {
+        if app.plugins_state() != PluginsState::Cleaned {
-            if !app.ready() {
+            if app.plugins_state() != PluginsState::Ready {
                 #[cfg(not(target_arch = "wasm32"))]
                 tick_global_task_pools_on_main_thread();
             } else {
                 app.finish();
                 app.cleanup();
-                finished_and_setup_done = true;
             }
 
             if let Some(app_exit_events) = app.world.get_resource::<Events<AppExit>>() {
@@ -775,7 +772,7 @@ pub fn winit_runner(mut app: App) {
                         }
                     };
 
-                    if finished_and_setup_done && should_update {
+                    if app.plugins_state() == PluginsState::Cleaned && should_update {
                         // reset these on each update
                         runner_state.wait_elapsed = false;
                         runner_state.window_event_received = false;