Enable desync detection in p2p example

[johanhelsing]

This is a very useful feature, we should either enable it by default, or at least inform about it in example code.

Unfortunately, I think it's currently broken? I get desyncs as soon as the game starts without giving any input.

@HeatXD

[HeatXD]

Mm just tested it in the normal examples and it worked fine. i will look at this.

[johanhelsing]

looks like the actual checksums are different. I added a log statement in GameStateCell::save:

Peer 1

GGRS Event: Synchronizing { addr: 127.0.0.1:7000, total: 5, count: 1 }
GGRS Event: Synchronizing { addr: 127.0.0.1:7000, total: 5, count: 2 }
GGRS Event: Synchronizing { addr: 127.0.0.1:7000, total: 5, count: 3 }
GGRS Event: Synchronizing { addr: 127.0.0.1:7000, total: 5, count: 4 }
frame: 0, checksum: Some(377796134171315194)
frame: 0, checksum: Some(377796134171315194)
GGRS Event: Synchronized { addr: 127.0.0.1:7000 }
frame: 1, checksum: Some(7837177710828900633)
frame: 2, checksum: Some(15495205509060815232)
frame: 3, checksum: Some(11699597999696539875)
frame: 4, checksum: Some(11020218334025782011)

Peer 2:

GGRS Event: Synchronizing { addr: 127.0.0.1:7001, total: 5, count: 1 }
GGRS Event: Synchronizing { addr: 127.0.0.1:7001, total: 5, count: 2 }
GGRS Event: Synchronizing { addr: 127.0.0.1:7001, total: 5, count: 3 }
GGRS Event: Synchronizing { addr: 127.0.0.1:7001, total: 5, count: 4 }
frame: 0, checksum: Some(11950335332055537129)
frame: 0, checksum: Some(11950335332055537129)
GGRS Event: Synchronized { addr: 127.0.0.1:7001 }
frame: 1, checksum: Some(10010806704571646955)
frame: 2, checksum: Some(16190560606418438605)
frame: 3, checksum: Some(3571966077237507259)
frame: 4, checksum: Some(9411370770399689895)

[HeatXD]

Does bevy's worlds have some non gameplay relevant state which is included in the snapshot?
making the desync function fire off?

[johanhelsing]

At least resource.reflect_hash() seems to return a different value each time I start the app, even if I disable the increase frame system. Maybe the hash is seeded or something?

[johanhelsing]

seems they're using AHasher::default in bevy

[johanhelsing]

bevy_utils depends on ahash with default features, which includes its std feature, which means the hasher keys will be generated on the first invocation: https://docs.rs/ahash/latest/ahash/struct.AHasher.html#method.default

In other words, as far as I can see the desync detection mode can't be used without patching bevy.

[johanhelsing]

Was able to make it work with this branch: https://github.com/johanhelsing/bevy/tree/fixed-reflect-hash-0.9.1

[gschup]

very interesting! let's see how this goes :)

[johanhelsing]

Tested it in Cargo Space as well, works great!

And I had no desyncs 🎉 (or at least not any for my components that implement reflect_hash)

[gschup]

is this PR still a draft?

[johanhelsing]

We need bevy 0.10 first (because we needed the deterministic, bevyengine/bevy#7583), but otherwise I think it's ready.

[johanhelsing]

Did a Ggrsevent use get lost in the merge?

[gschup]

yes, I made a mistake but pushed a fix already :) Sorry, my bad!

[johanhelsing]

Yeah, no problem! Just trying to help. Thanks for the super quick release!