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Fix ImposterProtoChunk leaking live block entities to worldgen

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embeddedt 2026-05-24 23:08:07 -04:00
parent 494203ef5a
commit 33851c1cb6
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src/main/java/org/embeddedt/modernfix/common/mixin/perf/release_protochunks/ImposterProtoChunkMixin.java Normal file
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package org.embeddedt.modernfix.common.mixin.perf.release_protochunks;
import com.llamalad7.mixinextras.injector.ModifyExpressionValue;
import com.llamalad7.mixinextras.sugar.Local;
import net.minecraft.core.BlockPos;
import net.minecraft.world.level.block.entity.BlockEntity;
import net.minecraft.world.level.chunk.ImposterProtoChunk;
import org.embeddedt.modernfix.ModernFix;
import org.spongepowered.asm.mixin.Final;
import org.spongepowered.asm.mixin.Mixin;
import org.spongepowered.asm.mixin.Shadow;
import org.spongepowered.asm.mixin.injection.At;
@Mixin(ImposterProtoChunk.class)
public class ImposterProtoChunkMixin {
@Shadow
@Final
private boolean allowWrites;
/**
* @author embeddedt
* @reason This is a workaround for a very complicated and subtle vanilla issue. Vanilla uses ImposterProtoChunk as
* a way of exposing fully generated chunks to other chunks that are still working on earlier generation stages.
* The problem is that these fully generated chunks may be in two different states: promoted to FULL and already
* visible to the level (with real BlockEntity objects), or in a loaded but not yet promoted state, where the postload
* hook has not yet run to convert the NBT-serialized block entities from the disk into real BlockEntity objects.
* The former state is the problematic one. If such a chunk is exposed to worldgen, features/structures may try
* to interact with the block entity (e.g. by calling setChanged on it). This has the potential to deadlock.
* <p></p>
* The solution we use here is to simply hide the existence of any "real" BE that has a level attached from worldgen.
* This is consistent with what other code would observe if the fully generated chunk were to be saved to disk
* and then reloaded (ending up in the latter state), so it should not break well-behaved mods.
* <p></p>
* This problem occurs rather often with `mixin.perf.release_protochunks` enabled, because it significantly increases
* the chance of a promoted LevelChunk wrapped in ImposterProtoChunk being used for world generation.
*/
@ModifyExpressionValue(method = "getBlockEntity", at = @At(value = "INVOKE", target = "Lnet/minecraft/world/level/chunk/LevelChunk;getBlockEntity(Lnet/minecraft/core/BlockPos;)Lnet/minecraft/world/level/block/entity/BlockEntity;"))
private BlockEntity avoidLeakingLiveBE(BlockEntity original, @Local(ordinal = 0, argsOnly = true) BlockPos pos) {
if (!this.allowWrites && original != null && original.getLevel() != null) {
ModernFix.LOGGER.debug("Blocked accessing the main level BlockEntity at {} from the ImposterProtoChunk wrapper, as this is unsafe during worldgen.", pos, new Exception("Stacktrace"));
return null;
} else {
return original;
}
}
}