From 33851c1cb640ed17ae94ad25d38635606f55a5c2 Mon Sep 17 00:00:00 2001
From: embeddedt <42941056+embeddedt@users.noreply.github.com>
Date: Sun, 24 May 2026 23:08:07 -0400
Subject: [PATCH] Fix ImposterProtoChunk leaking live block entities to
 worldgen

---
 .../ImposterProtoChunkMixin.java              | 46 +++++++++++++++++++
 1 file changed, 46 insertions(+)
 create mode 100644 src/main/java/org/embeddedt/modernfix/common/mixin/perf/release_protochunks/ImposterProtoChunkMixin.java

diff --git a/src/main/java/org/embeddedt/modernfix/common/mixin/perf/release_protochunks/ImposterProtoChunkMixin.java b/src/main/java/org/embeddedt/modernfix/common/mixin/perf/release_protochunks/ImposterProtoChunkMixin.java
new file mode 100644
index 00000000..e97d27d3
--- /dev/null
+++ b/src/main/java/org/embeddedt/modernfix/common/mixin/perf/release_protochunks/ImposterProtoChunkMixin.java
@@ -0,0 +1,46 @@
+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;
+        }
+    }
+}