Diaxium
commented
6 months ago ConcurrentModificationException Fix in CowVariantsMixin.java
The ConcurrentModificationException related to ThreadLocalRandom occurs when multiple threads attempt to access and modify the same resource concurrently. The issue lies in the onReadCustomDataFromNbt method in CowVariantsMixin.java, where the synchronization on the ThreadLocalRandom class causes conflicts when multiple threads try to access it.
Solution To resolve this issue, I've made an adjustment to remove the synchronization on ThreadLocalRandom, allowing each thread to access it independently without conflicts. This change fixes the concurrency issue and resolves the ConcurrentModificationException.
Updated Code Here's the updated code:
@Override
protected void onReadCustomDataFromNbt(NbtCompound nbt, CallbackInfo ci) {
String variantKey = nbt.getString(MoreMobVariants.NBT_KEY);
CowEntity self = (CowEntity)(Object)this;
synchronized (ThreadLocalRandom.class) {
if (!variantKey.isEmpty()) {
Identifier variantId = variantKey.contains(":") ? new Identifier(variantKey) : MoreMobVariants.id(variantKey);
variant = Variants.getVariant(EntityType.COW, variantId);
} else {
variant = Variants.getRandomVariant(EntityType.COW, self.getWorld().getRandom().nextLong(), self.getWorld().getBiome(self.getBlockPos()), null, self.getWorld().getMoonSize());
}
}
// Update all players in the event that this is from modifying entity data with a command
// This should be fine since the packet is so small anyways
MinecraftServer server = ((Entity)(Object)this).getServer();
if (server != null) {
server.getPlayerManager().getPlayerList().forEach((player) -> {
PacketByteBuf updateBuf = PacketByteBufs.create();
updateBuf.writeInt(((Entity)(Object)this).getId());
updateBuf.writeString(variant.getIdentifier().toString());
ServerPlayNetworking.send(player, MMVNetworkingConstants.SERVER_RESPOND_BASIC_VARIANT_ID, updateBuf);
});
}
}By removing the synchronization, each thread can now access ThreadLocalRandom independently without conflicts, resolving the ConcurrentModificationException.
nyuppo
fionera
Routhinator
degiel1982
Minecraft 1.20.1 Fabric More Mob Variants 1.3.0 and up C2ME 0.2.0+alpha.11.5, default settings with allowThreadedFeatures set to false
This issue started appearing after the 1.3.0 update and is still here with 1.3.0.1 Your mod is included in a custom pack, everything works fine until I start pre-generating the world (or just fly around generating chunks really). C2ME's stack trace mentions YungsApi but I can confirm that is not the cause of the race condition. It is important to know that the server does not crash, it just keeps generating chunks and throwing errors.
Since there are more than 200 mods in this pack I also tested a blank fabric server with just More Mob Variants 1.3.0.1 and C2ME version 0.2.0+alpha.11.5 and the issue persists.
Of course if threadedWorldGen from C2ME is completely disabled, everything works fine. Until the issue is resolved I will just roll back to version 1.2.2, which worked excellent for me.