Steps to Reproduce the Crash

Join a multiplayer world
Exit the world
Try to a singleplayer world
You will get an error that prevents you from joining

Mod Pack URL (Optional)

No response

Mod Pack Version (Optional)

No response

Extra Notes (Optional)

This was tested with only JEI on the instance.
I did a little debugging and the recipe serializer jei:jei_shaped is somewhat related to it

Crash Report

https://mclo.gs/1fxfCiH#L368

It is the interaction between RegistryManager/RegistrySnapshot.

Set a breakpoint at:

net.minecraft.core.MappedRegistry#clear
with condition this.toString().contains("recipe")

net.minecraft.core.MappedRegistry#registerIdMapping
with condition key.toString().contains("jei_shaped")

net.neoforged.neoforge.registries.RegistrySnapshot#RegistrySnapshot(net.minecraft.core.Registry, boolean) (this is the constructor)
with condition registry.toString().contains("recipe_serializer")

--

Loading game, taking a vanilla snapshot: Ids and registry all fine

Still loading game, now a frozen snapshot: Still ids and registry fine

Going to load a Multiplayer world

Apply snapshot is called and will clear partially the registry (only byId and toId)

Logged in, now I will disconnect

Now I will join my Singleplayer world

Some snapshot will be taken and notice the size of the maps does not match

when registry.getId(key) runs and get a key that is not mapped, it will give -1
Now snapshot is poisoned with a -1 Id

Some more clearing

It will now try add to the mapping that cursed -1 that it got from the snapshot

Setting -1 here causes the ArrayIndexOutOfBoundsException

Hello,

Sorry JEI does not sync anything at startup in 1.21.1, so I am not sure how it can be the cause of this particular issue.
The JEI shaped recipes only exist on the client and aren't added to the vanilla registry.
Can you share your findings about jei:jei_shaped?

Fun fact: when you lose connection/is disconnected on that multiplayer, looks like a revertToFrozen is called and makes joining singleplayer normally.

When I click disconnect, this is where it goes:

When I'm disconnected (like losing connection), this is where it goes:

So we can see that when I click disconnect, that revertToFrozen is never called.

Did this as a test and it fixes this issue
(calling this on client side only)

public class FixSnapshot {
	private static final VarHandle DISCONNECTION_HANDLED;

	static {
		DISCONNECTION_HANDLED = ReflectionHelper.getFieldFromClass(Connection.class, "disconnectionHandled", boolean.class, false);
	}

	public FixSnapshot() {
		var gamebus = NeoForge.EVENT_BUS;
		gamebus.addListener(this::callRevertToFrozenOnDisconnect);
	}

	private void callRevertToFrozenOnDisconnect(ClientPlayerNetworkEvent.LoggingOut event) {
		if (Minecraft.getInstance().getConnection() instanceof ClientPacketListener listener) {
			var handled = (boolean)DISCONNECTION_HANDLED.get(listener.getConnection());
			if (!listener.getConnection().isMemoryConnection() && !handled) {
				RegistryManager.revertToFrozen();
			}
		}
	}
}

Tested and it does not call twice or in on wrong moment like login
Works for single player (will skip), multiplayer clicking quit (will trigger), or multiplayer being disconnected (will skip)

Thanks for the analysis @pietro-lopes , it's rare that someone is so motivated to help dig in!

I wasn't aware of this functionality/optimization in the NeoForge registry manager when I registered my serializers to it. After looking at it for a bit, I think there is a potential that this is a NeoForge bug.

The NeoForge devs have a much deeper understanding of this, so for now the most efficient route to fixing it is to follow @ChampionAsh5357 's new NeoForge issue:

• neoforged/NeoForge#1932

If the issue is in JEI after all, I'll be happy to accept their guidance to fix it!

Will be fixed by neoforged/NeoForge#1944