Added HeadlessPlugins (#15203) (#15260)

Added a `HeadlessPlugins` plugin group, that adds more default
functionality (like logging) than the `MinimumPlugins`. Fixes #15203
Changed the headless example to use the new plugin group.

I am not entirely sure if the list of plugins is correct. Are there ones
that should be added / removed?

----
The `TerminalCtrlCHandlerPlugin` has interesting effects in the headless
example: Installing it a second time it will give a log message about
skipping installation, because it is already installed. Ctrl+C will
terminate the application in that case. However, _not_ installing it the
second time (so only on the app that runs once) has the effect that the
app that runs continuously cannot be stopped using Ctrl+C.
This implies that, even though the second app did not install the Ctrl+C
handler, it did _something_ because it was keeping the one from the
first app alive.
Not sure if this is a problem or issue, or can be labeled a wierd quirk
of having multiple Apps in one executable.

crates/bevy_dev_tools/src/ci_testing/mod.rs

@@ -17,8 +17,9 @@ use std::time::Duration;
 /// (`ci_testing_config.ron` by default) and executes its specified actions. For a reference of the
 /// allowed configuration, see [`CiTestingConfig`].
 ///
-/// This plugin is included within `DefaultPlugins` and `MinimalPlugins` when the `bevy_ci_testing`
-/// feature is enabled. It is recommended to only used this plugin during testing (manual or
+/// This plugin is included within `DefaultPlugins`, `HeadlessPlugins` and `MinimalPlugins`
+/// when the `bevy_ci_testing` feature is enabled.
+/// It is recommended to only used this plugin during testing (manual or
 /// automatic), and disable it during regular development and for production builds.
 #[derive(Default)]
 pub struct CiTestingPlugin;

crates/bevy_internal/src/default_plugins.rs

@@ -71,7 +71,56 @@ plugin_group! {
     ///
     /// [`DefaultPlugins`] contains all the plugins typically required to build
     /// a *Bevy* application which includes a *window* and presentation components.
-    /// For *headless* cases – without a *window* or presentation, see [`MinimalPlugins`].
+    /// For *headless* cases – without a *window* or presentation, see [`HeadlessPlugins`].
+    /// For the absolute minimum number of plugins needed to run a Bevy application, see [`MinimalPlugins`].
+}
+
+plugin_group! {
+    /// This plugin group will add all the default plugins for a headless (no *window* or rendering) *Bevy* application:
+    pub struct HeadlessPlugins {
+        bevy_app:::PanicHandlerPlugin,
+        bevy_log:::LogPlugin,
+        bevy_core:::TaskPoolPlugin,
+        bevy_core:::TypeRegistrationPlugin,
+        bevy_core:::FrameCountPlugin,
+        bevy_time:::TimePlugin,
+        bevy_transform:::TransformPlugin,
+        bevy_hierarchy:::HierarchyPlugin,
+        bevy_diagnostic:::DiagnosticsPlugin,
+        bevy_app:::ScheduleRunnerPlugin,
+        #[custom(cfg(not(target_arch = "wasm32")))]
+        bevy_app:::TerminalCtrlCHandlerPlugin,
+        #[cfg(feature = "bevy_asset")]
+        bevy_asset:::AssetPlugin,
+        #[cfg(feature = "bevy_scene")]
+        bevy_scene:::ScenePlugin,
+        #[cfg(feature = "bevy_animation")]
+        bevy_animation:::AnimationPlugin,
+        #[cfg(feature = "bevy_state")]
+        bevy_state::app:::StatesPlugin,
+        #[cfg(feature = "bevy_ci_testing")]
+        bevy_dev_tools::ci_testing:::CiTestingPlugin,
+        #[doc(hidden)]
+        :IgnoreAmbiguitiesPlugin,
+    }
+    /// This group of plugins is intended for use for *headless* programs, for example: dedicated game servers.
+    /// See the [*Bevy* *headless* example](https://github.com/bevyengine/bevy/blob/main/examples/app/headless.rs)
+    ///
+    /// [`HeadlessPlugins`] obeys *Cargo* *feature* flags. Users may exert control over this plugin group
+    /// by disabling `default-features` in their `Cargo.toml` and enabling only those features
+    /// that they wish to use.
+    ///
+    /// [`HeadlessPlugins`] contains all the plugins typically required to build
+    /// a *Bevy* application. In contrast with [`DefaultPlugins`], it leaves out *window* and presentation components.
+    /// This allows applications built using this plugin group to run on devices that do not have a screen or rendering
+    /// capabilities.
+    /// It includes a [schedule runner (`ScheduleRunnerPlugin`)](crate::app::ScheduleRunnerPlugin)
+    /// to provide functionality that would otherwise be driven by a windowed application's
+    /// *event loop* or *message loop*.
+    ///
+    /// Windowed applications that wish to use a reduced set of plugins should consider the
+    /// [`DefaultPlugins`] plugin group which can be controlled with *Cargo* *feature* flags.
+    /// For the absolute minimum number of plugins needed to run a Bevy application, see [`MinimalPlugins`].
 }
 
 #[derive(Default)]
@@ -110,12 +159,12 @@ plugin_group! {
         #[cfg(feature = "bevy_ci_testing")]
         bevy_dev_tools::ci_testing:::CiTestingPlugin,
     }
-    /// This group of plugins is intended for use for minimal, *headless* programs –
-    /// see the [*Bevy* *headless* example](https://github.com/bevyengine/bevy/blob/main/examples/app/headless.rs)
-    /// – and includes a [schedule runner (`ScheduleRunnerPlugin`)](crate::app::ScheduleRunnerPlugin)
+    /// This plugin group represents the absolute minimum, bare-bones, bevy application.
+    /// Use this if you want to have absolute control over the plugins used.
+    /// If you are looking to make a *headless* application - without a *window* or rendering,
+    /// it is usually best to use [`HeadlessPlugins`].
+    ///
+    /// It includes a [schedule runner (`ScheduleRunnerPlugin`)](crate::app::ScheduleRunnerPlugin)
     /// to provide functionality that would otherwise be driven by a windowed application's
     /// *event loop* or *message loop*.
-    ///
-    /// Windowed applications that wish to use a reduced set of plugins should consider the
-    /// [`DefaultPlugins`] plugin group which can be controlled with *Cargo* *feature* flags.
 }

crates/bevy_internal/src/prelude.rs

@@ -2,7 +2,8 @@
 pub use crate::{
     app::prelude::*, core::prelude::*, ecs::prelude::*, hierarchy::prelude::*, input::prelude::*,
     log::prelude::*, math::prelude::*, reflect::prelude::*, time::prelude::*,
-    transform::prelude::*, utils::prelude::*, window::prelude::*, DefaultPlugins, MinimalPlugins,
+    transform::prelude::*, utils::prelude::*, window::prelude::*, DefaultPlugins, HeadlessPlugins,
+    MinimalPlugins,
 };
 
 pub use bevy_derive::{bevy_main, Deref, DerefMut};

examples/app/headless.rs

@@ -8,21 +8,26 @@
 //! # replace "*" with the most recent version of bevy
 //! ```
 
-use bevy::{app::ScheduleRunnerPlugin, prelude::*, utils::Duration};
+use bevy::{app::ScheduleRunnerPlugin, log::LogPlugin, prelude::*, utils::Duration};
 
 fn main() {
     // This app runs once
     App::new()
-        .add_plugins(MinimalPlugins.set(ScheduleRunnerPlugin::run_once()))
+        .add_plugins(HeadlessPlugins.set(ScheduleRunnerPlugin::run_once()))
         .add_systems(Update, hello_world_system)
         .run();
 
     // This app loops forever at 60 fps
     App::new()
         .add_plugins(
-            MinimalPlugins.set(ScheduleRunnerPlugin::run_loop(Duration::from_secs_f64(
-                1.0 / 60.0,
-            ))),
+            HeadlessPlugins
+                .set(ScheduleRunnerPlugin::run_loop(Duration::from_secs_f64(
+                    1.0 / 60.0,
+                )))
+                // The log and ctrl+c plugin can only be registered once globally,
+                // which means we need to disable it here, because it was already registered with the
+                // app that runs once.
+                .disable::<LogPlugin>(),
         )
         .add_systems(Update, counter)
         .run();