Ensure Rollback Order is Stable After Rollback
#82
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If the de/spawning of a `Rollback` entity is rolled back, we must ensure the `RollbackOrdered` resource reflects that. I've also added an `EntityChecksumPlugin` which will flag if clients desync on the number of active `Rollback` entities, or the total number ever spawned.
Because Entity derives it's ordering, the generation value will change the order drastically. Unfortunately, Bevy will pick a different generation during rollback, destroying ordering. `RollbackOrdered` is now the only way to get a stable ordering for `Rollback`.
johanhelsing
approved these changes
Oct 31, 2023
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Awesome! I'm not seeing any desyncs in #78 anymore either. Would be really nice to have a unit test for this, but also really nice to get it in quickly so main isn't broken :P
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| #[derive(Component, Hash, PartialEq, Eq, Clone, Copy, Debug)] | ||
| pub struct Rollback(Entity); |
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Aaaah. I totally forgot about #58, and thought we we're still using "rollback ids"
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Objective
If a
Rollbackentity is spawned, then rollback causes that entity to no longer be spawned (or vice versa), the generation of thatEntitywill be different across clients that did/not rollback. BecauseEntityderives itsPartialOrd, the generation value changes the order of the entities, and likewise forRollback. Any code relying on a stable ordering ofRollbackcomponents will then likely desync.This should not happen!
Solution
PartialOrdandOrdimplementations forRollback, since they are not stable across clients, and therefore using them is a footgun waiting to happen.RollbackOrdered::pushto simply stack the values in the orderAddRollbackCommandis called. I believe this ordering is stable across clients so it can be used.CloneforRollbackOrderedand added it to the rollback schedule to account forRollbackentities de/spawned during rollback.Rollbackentities, and the total number spawnedRollbackentities is a part of the checksum. These are cheap since both are singlelen()calls.Notes
I've tested rebasing #78 onto this PR and from my testing, all desync issues are resolved. At a minimum, the frequency of desync issues is so drastically reduced that I was not able to replicate them no matter how randomly and quickly I switched peers/inputs.