Issue ordering exclusive systems accross SystemSet

[NiklasEi]

Bevy version

main 58474d7

Operating system & version

Ubuntu 18.04

What you did

I tried ordering exclusive systems based on labels. Some of them are in a SystemSet, but all are in the same Stage and have the default location.

What you expected to happen

The systems should run in the correct order.

What actually happened

The first frame has the wrong order. Later frames are correct.

Additional information

Minimal example

Here the ordering should be 1., A, 2., but the first frame prints A, 1., 2.. Removing the SystemSet fixes this and prints the correct order from the first frame on.

[hymm]

This is a states issue. On the first frame the state needs to run on_enter, so in the loop in runs the "a system" first, then loops to run the on_update systems. Basically it has to ignore the ordering. After the first frame it no longer needs to do the on_enter, so starts with the on_update systems and is able to respect the ordering.

Ordering is only respected when system run criteria return true on the same loop.

[alice-i-cecile]

Unfortunately this gets the needs-rfc tag, because of the mess that is states powered by run criteria. See #2801 for bread crumbs.

[alice-i-cecile]

Fixed by #7267.