Handle Ctrl+C in the terminal properly (#14001)

# Objective

Fixes #13995.

## Solution

Override the default `Ctrl+C` handler with one that sends `AppExit`
event to every app with `TerminalCtrlCHandlerPlugin`.

## Testing

Tested by running the `3d_scene` example and hitting `Ctrl+C` in the
terminal.

---

## Changelog

Handles `Ctrl+C` in the terminal gracefully.

## Migration Guide

If you are overriding the `Ctrl+C` handler then you should call
`TerminalCtrlCHandlerPlugin::gracefully_exit` from your handler. It will
tell the app to exit.

crates/bevy_app/Cargo.toml

@@ -26,6 +26,9 @@ bevy_tasks = { path = "../bevy_tasks", version = "0.14.0-dev" }
 downcast-rs = "1.2.0"
 thiserror = "1.0"
 
+[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
+ctrlc = "3.4.4"
+
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 wasm-bindgen = { version = "0.2" }
 web-sys = { version = "0.3", features = ["Window"] }

crates/bevy_app/src/lib.rs

@@ -14,6 +14,8 @@ mod plugin;
 mod plugin_group;
 mod schedule_runner;
 mod sub_app;
+#[cfg(not(target_arch = "wasm32"))]
+mod terminal_ctrl_c_handler;
 
 pub use app::*;
 pub use bevy_derive::DynamicPlugin;
@@ -23,6 +25,8 @@ pub use plugin::*;
 pub use plugin_group::*;
 pub use schedule_runner::*;
 pub use sub_app::*;
+#[cfg(not(target_arch = "wasm32"))]
+pub use terminal_ctrl_c_handler::*;
 
 #[allow(missing_docs)]
 pub mod prelude {

crates/bevy_app/src/terminal_ctrl_c_handler.rs

@@ -0,0 +1,73 @@
+use std::sync::atomic::{AtomicBool, Ordering};
+
+use bevy_ecs::event::EventWriter;
+
+use crate::{App, AppExit, Plugin, Update};
+
+pub use ctrlc;
+
+/// Indicates that all [`App`]'s should exit.
+static SHOULD_EXIT: AtomicBool = AtomicBool::new(false);
+
+/// Gracefully handles `Ctrl+C` by emitting a [`AppExit`] event. This plugin is part of the `DefaultPlugins`.
+///
+/// ```no_run
+/// # use bevy_app::{App, NoopPluginGroup as MinimalPlugins, PluginGroup, TerminalCtrlCHandlerPlugin};
+/// fn main() {
+///     App::new()
+///         .add_plugins(MinimalPlugins)
+///         .add_plugins(TerminalCtrlCHandlerPlugin)
+///         .run();
+/// }
+/// ```
+///
+/// If you want to setup your own `Ctrl+C` handler, you should call the
+/// [`TerminalCtrlCHandlerPlugin::gracefully_exit`] function in your handler if you want bevy to gracefully exit.
+/// ```no_run
+/// # use bevy_app::{App, NoopPluginGroup as DefaultPlugins, PluginGroup, TerminalCtrlCHandlerPlugin, ctrlc};
+/// fn main() {
+///     // Your own `Ctrl+C` handler
+///     ctrlc::set_handler(move || {
+///         // Other clean up code ...
+///
+///         TerminalCtrlCHandlerPlugin::gracefully_exit();
+///     });
+///
+///     App::new()
+///         .add_plugins(DefaultPlugins)
+///         .run();
+/// }
+/// ```
+#[derive(Default)]
+pub struct TerminalCtrlCHandlerPlugin;
+
+impl TerminalCtrlCHandlerPlugin {
+    /// Sends the [`AppExit`] event to all apps using this plugin to make them gracefully exit.
+    pub fn gracefully_exit() {
+        SHOULD_EXIT.store(true, Ordering::Relaxed);
+    }
+
+    /// Sends a [`AppExit`] event when the user presses `Ctrl+C` on the terminal.
+    fn exit_on_flag(mut events: EventWriter<AppExit>) {
+        if SHOULD_EXIT.load(Ordering::Relaxed) {
+            events.send(AppExit::from_code(130));
+        }
+    }
+}
+
+impl Plugin for TerminalCtrlCHandlerPlugin {
+    fn build(&self, app: &mut App) {
+        let result = ctrlc::try_set_handler(move || {
+            Self::gracefully_exit();
+        });
+        match result {
+            Ok(()) => {}
+            Err(ctrlc::Error::MultipleHandlers) => {
+                bevy_utils::tracing::info!("Skipping installing `Ctrl+C` handler as one was already installed. Please call `TerminalCtrlCHandlerPlugin::gracefully_exit` in your own `Ctrl+C` handler if you want Bevy to gracefully exit on `Ctrl+C`.");
+            }
+            Err(err) => bevy_utils::tracing::warn!("Failed to set `Ctrl+C` handler: {err}"),
+        }
+
+        app.add_systems(Update, TerminalCtrlCHandlerPlugin::exit_on_flag);
+    }
+}

crates/bevy_internal/src/default_plugins.rs

@@ -59,6 +59,11 @@ impl PluginGroup for DefaultPlugins {
             .add(bevy_window::WindowPlugin::default())
             .add(bevy_a11y::AccessibilityPlugin);
 
+        #[cfg(not(target_arch = "wasm32"))]
+        {
+            group = group.add(bevy_app::TerminalCtrlCHandlerPlugin);
+        }
+
         #[cfg(feature = "bevy_asset")]
         {
             group = group.add(bevy_asset::AssetPlugin::default());