Error logs are silently lost if LogPlugin is added late.

[shanecelis]

Problem

Perfectly good error logs are silently lost if DefaultPlugins or LogPlugin has not been added early enough.

I found this behavior on the main branch but I expect it goes pretty far back.

Example

I was mucking around with mesh2d_manual for reasons and I made this mistake: I put ExtractResourcePlugin ahead of DefaultPlugins.

fn main() {
    App::new()
        .init_resource::<DistBuffer>()
        .add_plugins(ExtractResourcePlugin::<DistBuffer>::default())
        // Error message that "Render app did not exist..." is never shown.
        .add_plugins((DefaultPlugins, ColoredMesh2dPlugin))
        .add_systems(Startup, star)
        .run();
}

I was befuddled by this because I could see the error message in the code and assumed I must not be hitting that execution path. I added some eprintln! and figured out it was the LogPlugin that needed to come first before the error was reported. To provoke the error again and log it, I added LogPlugin manually.

fn main() {
    App::new()
        .add_plugins(LogPlugin::default())
        .add_plugins(ExtractResourcePlugin::<DistBuffer>::default())
        .add_plugins((DefaultPlugins.build().disable::<LogPlugin>(), ColoredMesh2dPlugin))
        .init_resource::<DistBuffer>()
        .add_systems(Startup, star)
        .run();
}

This then showed me the right error message:

2024-05-17T05:12:34.125757Z ERROR bevy_render::extract_resource: Render app did not exist when trying to add `extract_resource` for <mesh2d_manual::DistBuffer>.

This is a great error message and would have set me straight if I had seen it.

Suggested Solutions

Fix in the small

One could have ExtractResourcePlugin come before DefaultPlugins if it did its work in finish() rather than build(). I've seen a few plugins opt for this when dealing with the RenderApp. Maybe that should be done but I think that's avoiding a larger issue.

Accumulate Log Counts

Perhaps errors emitted before LogPlugin ought to be accumulated somehow so they aren't silently lost until LogPlugin is added. Keeping an accounting of the missing log types would be helpful. Then when LogPlugin is added, it could emit this warning if there are missing log types that we care about:

2024-05-17T05:12:34.125757Z WARNING bevy_log::LogPlugin: Missed logs: 2 warn, 1 error; consider adding LogPlugin or DefaultPlugins to App earlier.

Maybe a LogCountPlugin could be devised that only collects those counts.

And if the App never adds LogPlugin, maybe those counts could be made available somewhere.

Conclusion

I'm happy to submit a PR for either of these solutions if desired.

[alice-i-cecile]

Ultimately caused by #1255. We need real plugin ordering in some form to avoid this sort of footgun.