Predicted spawn methods quite literally do not work.

[moonheart08]

The most accurate description of these methods, at this time, is "repeatedly retry spawning the entity, deleting it every tick until either we stop trying or the real one comes in."

This doesn't work, and breaks basically everything that has longer-term state, including:

• Audio.
• Sprite animations.
• UI.
• Anything predicted from the newly spawned entity itself.

I'd hazard to say it's an outright antipattern and we must do something smarter than that.

One of the best ways to handle predicting discrete events is to give those events unique identifiers, saying that "okay, here's the identifier for this event, if the server says it happens then we're all good otherwise we mispredicted and gotta undo it"

I've also noticed content has tried to come up with various solutions to unique identifiers (for rng, mostly). The following do not work in the general case:

• Coordinates (The game's coordinate math is not deterministic.)
• EntityUid (nondeterministic)
• NetEntity (breaks prediction of events caused by predicted entities, as it doesn't exist w/o the server)
• Sequence numbers (Order dependent)

The following solutions work in the general case:

Unfortunately, that's pretty few general solutions.
So here's the things we can use as unique identifier, sometimes:

• NetEntity (for entities not spawned as part of prediction)
• Sequence numbers (It's just incrementing an integer, as long as the order stays the same there's no problem)

I'm going to try to figure out a proper predicted spawn (likely involving sequence numbers) because the current methods cannot stay.

[moonheart08]

i have a mental model for this figured out but im about to go to bed so have schizonotes:

• this problem is literally just merge conflicts. We need to merge entity A (client predicted ent, if any) and entity B (server ent).
• Our current solution is essentially just to always take entity B but constantly create an entity A while we wait for B to exist.
• If we instead assign unique sequence numbers and use them to uniquely identify entity A and entity B in a way that is consistent, we can merge the entities in a way that reconciles to "entity B, but with all the long-term information from entity A."
• We can also use this same model to properly reconcile entity A if entity B never exists (like making sure we don't cause audio choppiness.)
• the consequences for mispredicts here are more severe, if your identifier isnt unique across the sim then you may merge the wrong entities.

[slarticodefast]

See also #5845