Disabling default features causes app with DefaultPlugins to exit

[beeryt]

Bevy version

0.10.0

What you did

I wrote a simple little reproducible program called eager-hello:

use bevy::prelude::*;
fn main() {
  App::new()
    .add_plugins(DefaultPlugins)
    .add_system(|| println!("hello"))
    .run();

With the following Cargo.toml, the program is not as eager as I had hoped:

[dependencies.bevy]
version = "0.10.0"
default-features = false

What went wrong

The expected output would be:

hello
hello
hello
...

Unfortunately I only get:

hello

Additional information

If I remove the default-features = false from Cargo.toml the program works as expected.
However, I want a simple program with as few dependencies as possible.

I've tried disabling the window exit condition:

use bevy::prelude::*;
fn main() {
    App::new()
        .add_plugins(DefaultPlugins.set(WindowPlugin {
            primary_window: None,
            exit_condition: bevy::window::ExitCondition::DontExit,
            close_when_requested: false,
        }))
        .add_system(|| println!("hello"))
        .run();
}

I've tried disabling the window plugin:

use bevy::prelude::*;
fn main() {
  App::new()
    .add_plugins(DefaultPlugins.build().disable::<WindowPlugin>())
    .add_system(|| println!("hello"))
    .run();

I tried to get further insight by looking at the logging, but nothing seems obvious for why the app exits after one update cycle.
Here is the log from where the WindowPlugin was disabled:

2023-03-16T22:34:41.104978Z DEBUG bevy_app::plugin_group: added plugin: bevy_core::TaskPoolPlugin
2023-03-16T22:34:41.105039Z DEBUG bevy_app::app: added plugin: bevy_core::TaskPoolPlugin
2023-03-16T22:34:41.105230Z TRACE bevy_core::task_pool_options: Assigning 8 cores to default task pools
2023-03-16T22:34:41.105256Z TRACE bevy_core::task_pool_options: IO Threads: 2
2023-03-16T22:34:41.105403Z TRACE bevy_core::task_pool_options: Async Compute Threads: 2
2023-03-16T22:34:41.105526Z TRACE bevy_core::task_pool_options: Compute Threads: 4
2023-03-16T22:34:41.105817Z DEBUG bevy_app::plugin_group: added plugin: bevy_core::TypeRegistrationPlugin
2023-03-16T22:34:41.105847Z DEBUG bevy_app::app: added plugin: bevy_core::TypeRegistrationPlugin
2023-03-16T22:34:41.107118Z DEBUG bevy_app::plugin_group: added plugin: bevy_core::FrameCountPlugin
2023-03-16T22:34:41.107157Z DEBUG bevy_app::app: added plugin: bevy_core::FrameCountPlugin
2023-03-16T22:34:41.107285Z DEBUG bevy_app::plugin_group: added plugin: bevy_time::TimePlugin
2023-03-16T22:34:41.107304Z DEBUG bevy_app::app: added plugin: bevy_time::TimePlugin
2023-03-16T22:34:41.107570Z DEBUG bevy_app::plugin_group: added plugin: bevy_transform::TransformPlugin
2023-03-16T22:34:41.107593Z DEBUG bevy_app::app: added plugin: bevy_transform::TransformPlugin
2023-03-16T22:34:41.107762Z DEBUG bevy_app::app: added plugin: bevy_hierarchy::valid_parent_check_plugin::ValidParentCheckPlugin<bevy_transform::components::global_transform::GlobalTransform>
2023-03-16T22:34:41.108071Z DEBUG bevy_app::plugin_group: added plugin: bevy_hierarchy::HierarchyPlugin
2023-03-16T22:34:41.108096Z DEBUG bevy_app::app: added plugin: bevy_hierarchy::HierarchyPlugin
2023-03-16T22:34:41.108325Z DEBUG bevy_app::plugin_group: added plugin: bevy_diagnostic::DiagnosticsPlugin
2023-03-16T22:34:41.108348Z DEBUG bevy_app::app: added plugin: bevy_diagnostic::DiagnosticsPlugin
2023-03-16T22:34:41.108437Z DEBUG bevy_app::plugin_group: added plugin: bevy_input::InputPlugin
2023-03-16T22:34:41.108464Z DEBUG bevy_app::app: added plugin: bevy_input::InputPlugin
2023-03-16T22:34:41.109808Z DEBUG bevy_app::plugin_group: added plugin: bevy_a11y::AccessibilityPlugin
2023-03-16T22:34:41.109839Z DEBUG bevy_app::app: added plugin: bevy_a11y::AccessibilityPlugin
2023-03-16T22:34:41.131299Z  INFO bevy_diagnostic::system_information_diagnostics_plugin::internal: SystemInfo { os: "Linux 20.04 Ubuntu", kernel: "5.4.0-139-generic", cpu: "Intel(R) Core(TM) i5-8250U CPU @ 1.60GHz", core_count: "4", memory: "15.5 GiB" }
hello

I even tried disabling plugins one-by-one, but no matter what I tried, I still couldn't get the app to continue:

use bevy::{
    a11y::AccessibilityPlugin, diagnostic::DiagnosticsPlugin, input::InputPlugin, prelude::*,
    time::TimePlugin,
};
fn main() {
    App::new()
        .add_plugins(
            DefaultPlugins
                .build()
                .disable::<WindowPlugin>()
                .disable::<InputPlugin>()
                .disable::<DiagnosticsPlugin>()
                .disable::<FrameCountPlugin>()
                .disable::<AccessibilityPlugin>()
                .disable::<TransformPlugin>()
                .disable::<HierarchyPlugin>()
                .disable::<TypeRegistrationPlugin>()
                .disable::<TimePlugin>()
                .disable::<TaskPoolPlugin>(),
        )
        .add_system(|| println!("hello"))
        .run();
}

[tim-blackbird]

The default App runner runs the app once and exits.

If you want to have a window you'll need to enable the bevy_winit feature, which sets a runner for you.
Alternatively, in case you don't want a window you can add the ScheduleRunnerPlugin (which comes included in MinimalPlugins)

[beeryt]

There it is, thanks!

use bevy::{app::ScheduleRunnerPlugin, prelude::*};
fn main() {
    App::new()
        .add_plugins(DefaultPlugins.build().disable::<WindowPlugin>())
        .add_plugin(ScheduleRunnerPlugin)
        .add_system(|| println!("hello"))
        .run();
}

[beeryt]

Is there still something which could be updated here? From (briefly) reading the docs it isn't clear that there is an implicit dependency on bevy_winit for the runner.

I assume this is part of WinitPlugin so maybe the doc page for DefaultPlugins could say:

WinitPlugin -- with feature bevy_winit (which also calls app::set_runner)

[beeryt]

Also, would it make sense for bevy to log that no app runner was found maybe at debug or trace level?

DEBUG bevy_app::app: no app runner detected, defaulting to single-shot

It looks like the docs for App::set_runner do specify that it is usually set by built-in plugins, but here was a scenario where the default built-in plugins did not do so.