random error on slime removing

[cb31455]

Stack trace

java.lang.IndexOutOfBoundsException: Index 0 out of bounds for length 0
	at java.base/jdk.internal.util.Preconditions.outOfBounds(Preconditions.java:100) ~[?:?]
	at java.base/jdk.internal.util.Preconditions.outOfBoundsCheckIndex(Preconditions.java:106) ~[?:?]
	at java.base/jdk.internal.util.Preconditions.checkIndex(Preconditions.java:302) ~[?:?]
	at java.base/java.util.Objects.checkIndex(Objects.java:365) ~[?:?]
	at java.base/java.util.ArrayList.get(ArrayList.java:428) ~[?:?]
	at org.bukkit.event.entity.EntityTransformEvent.<init>(EntityTransformEvent.java:24) ~[paper-api-1.21.4-R0.1-SNAPSHOT.jar:?]
	at org.bukkit.craftbukkit.event.CraftEventFactory.callEntityTransformEvent(CraftEventFactory.java:624) ~[paper-1.21.4.jar:1.21.4-226-a838a88]
	at net.minecraft.world.entity.monster.Slime.remove(Slime.java:249) ~[paper-1.21.4.jar:1.21.4-226-a838a88]
	at net.minecraft.world.entity.Entity.discard(Entity.java:631) ~[paper-1.21.4.jar:1.21.4-226-a838a88]
	at org.bukkit.craftbukkit.entity.CraftEntity.remove(CraftEntity.java:407) ~[paper-1.21.4.jar:1.21.4-226-a838a88]
	at org.bukkit.craftbukkit.scheduler.CraftTask.run(CraftTask.java:78) ~[paper-1.21.4.jar:1.21.4-226-a838a88]
	at org.bukkit.craftbukkit.scheduler.CraftScheduler.mainThreadHeartbeat(CraftScheduler.java:474) ~[paper-1.21.4.jar:1.21.4-226-a838a88]
	at net.minecraft.server.MinecraftServer.tickChildren(MinecraftServer.java:1659) ~[paper-1.21.4.jar:1.21.4-226-a838a88]
	at net.minecraft.server.MinecraftServer.tickServer(MinecraftServer.java:1529) ~[paper-1.21.4.jar:1.21.4-226-a838a88]
	at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1251) ~[paper-1.21.4.jar:1.21.4-226-a838a88]
	at net.minecraft.server.MinecraftServer.lambda$spin$2(MinecraftServer.java:310) ~[paper-1.21.4.jar:1.21.4-226-a838a88]
	at java.base/java.lang.Thread.run(Thread.java:1575) ~[?:?]

Plugin and Datapack List

ChipsMc, Protocollib.

Actions to reproduce (if known)

dunno, this just happens. i do not know if its mine or something else problem, but no plugin releated text in crash report

Paper version

This server is running Paper version 1.21.4-226-ver/1.21.4@a838a88 (2025-04-17T19:20:57Z) (Implementing API version 1.21.4-R0.1-SNAPSHOT)
You are 1 version(s) behind
Download the new version at: https://papermc.io/downloads/paper
Previous version: 1.21.4-224-0cf7315 (MC: 1.21.4)

Other

No response

[lynxplay]

Eh, from a quick glance this would happen if you call .remove() on an already removed slime in that bukkit task.
Given PL certainly won't be doing that, this feels like a plugin issue.
Can you confirm that by reading through the presumably proprietary ChipsMc plugin?

[cb31455]

hi. but i do not call remove slime. i mean, i call remove on Mob interface, but thats the full list. i never touch just slime, but this error comes only slime. anyway, i can fix this in my codes thx to your advice, but still strange.

Thats the full code of remove:
@eventhandler(priority = EventPriority.LOWEST)
public void unloadChunk(ChunkUnloadEvent event) {
GarbageCollector.clearMobs(event.getChunk().getEntities(),true);
}

public static int clearMobs(Entity [] inEnts)
{
int numDel = 0;
for (Entity ents : inEnts) {
boolean isremove = false;
if (ents instanceof Enemy && !(ents instanceof EnderDragon))
if (!isMobOnDefField((Enemy) ents))
isremove = true;
if (ents instanceof Ambient)
isremove = true;
if (ents instanceof Item)
isremove = true;
if (ents instanceof TextDisplay)
isremove = true;
if (ents instanceof AreaEffectCloud)
isremove = true;
if (ents instanceof ExperienceOrb)
isremove = true;
if (ents instanceof Projectile && !(ents instanceof Trident))
isremove = true;
if (isremove && !dataPack.getLocOfFarmArea(ents.getLocation())) {
ents.remove();
numDel++;
}
if (ents instanceof Wolf tame && tame.getOwner() != null && tame.getCustomName() != null) {
tame.teleport(Rules.dogBase.clone().setDirection(Rules.dogBase.toVector().subtract(tame.getLocation().toVector()).normalize()));
}
}
return numDel;
}

[cb31455]

if(ents != null && ents.isValid())
i hope this line will fixes. i added it inside loop

[lynxplay]

Well a Slime is a Mob so yea. Calling .remove() on an already removed entity is generally invalid.
We could create a protection against this, tho I am not too sure we want to as that is a pretty sensitive bit of code.

The above lines look good yes 👍
I'll close the issue for now, if the above does not fix it, feel free to write here and I'll reopen it for further investigations.