Paper/patches/server/0681-Fix-swamp-hut-cat-generation-deadlock.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Spottedleaf <Spottedleaf@users.noreply.github.com>
Date: Sat, 12 Mar 2022 06:31:13 -0800
Subject: [PATCH] Fix swamp hut cat generation deadlock

The worldgen thread will attempt to get structure references
via the world's getChunkAt method, which is fine if the gen is
not cancelled - but if the chunk was unloaded, the call will block
indefinitely. Instead of using the world state, we use the already
supplied ServerLevelAccessor which will always have the chunk available.

diff --git a/src/main/java/net/minecraft/world/entity/animal/Cat.java b/src/main/java/net/minecraft/world/entity/animal/Cat.java
index f0e5428394f8de8f7ac05b8cbc34826d153eeaef..d44807c16712afd37efdbf434d1afb12a7c3d343 100644
--- a/src/main/java/net/minecraft/world/entity/animal/Cat.java
+++ b/src/main/java/net/minecraft/world/entity/animal/Cat.java
@@ -357,7 +357,7 @@ public class Cat extends TamableAnimal implements VariantHolder<Holder<CatVarian
         BuiltInRegistries.CAT_VARIANT.getRandomElementOf(tagkey, world.getRandom()).ifPresent(this::setVariant);
         ServerLevel worldserver = world.getLevel();
 
-        if (worldserver.structureManager().getStructureWithPieceAt(this.blockPosition(), StructureTags.CATS_SPAWN_AS_BLACK).isValid()) {
+        if (worldserver.structureManager().getStructureWithPieceAt(this.blockPosition(), StructureTags.CATS_SPAWN_AS_BLACK, world).isValid()) { // Paper - Fix swamp hut cat generation deadlock
             this.setVariant((Holder) BuiltInRegistries.CAT_VARIANT.getHolderOrThrow(CatVariant.ALL_BLACK));
             this.setPersistenceRequired();
         }
diff --git a/src/main/java/net/minecraft/world/level/StructureManager.java b/src/main/java/net/minecraft/world/level/StructureManager.java
index ceec33ced3d0b65a3e800378e250d0166d0c195f..54972cce2314eff774250101df43a9b7074e9604 100644
--- a/src/main/java/net/minecraft/world/level/StructureManager.java
+++ b/src/main/java/net/minecraft/world/level/StructureManager.java
@@ -48,7 +48,12 @@ public class StructureManager {
     }
 
     public List<StructureStart> startsForStructure(ChunkPos pos, Predicate<Structure> predicate) {
-        Map<Structure, LongSet> map = this.level.getChunk(pos.x, pos.z, ChunkStatus.STRUCTURE_REFERENCES).getAllReferences();
+        // Paper start - Fix swamp hut cat generation deadlock
+        return this.startsForStructure(pos, predicate, null);
+    }
+    public List<StructureStart> startsForStructure(ChunkPos pos, Predicate<Structure> predicate, @Nullable ServerLevelAccessor levelAccessor) {
+        Map<Structure, LongSet> map = (levelAccessor == null ? this.level : levelAccessor).getChunk(pos.x, pos.z, ChunkStatus.STRUCTURE_REFERENCES).getAllReferences();
+        // Paper end - Fix swamp hut cat generation deadlock
         Builder<StructureStart> builder = ImmutableList.builder();
 
         for (Entry<Structure, LongSet> entry : map.entrySet()) {
@@ -116,10 +121,20 @@ public class StructureManager {
     }
 
     public StructureStart getStructureWithPieceAt(BlockPos pos, Predicate<Holder<Structure>> predicate) {
+        // Paper start - Fix swamp hut cat generation deadlock
+        return this.getStructureWithPieceAt(pos, predicate, null);
+    }
+
+    public StructureStart getStructureWithPieceAt(BlockPos pos, TagKey<Structure> tag, @Nullable ServerLevelAccessor levelAccessor) {
+        return this.getStructureWithPieceAt(pos, structure -> structure.is(tag), levelAccessor);
+    }
+
+    public StructureStart getStructureWithPieceAt(BlockPos pos, Predicate<Holder<Structure>> predicate, @Nullable ServerLevelAccessor levelAccessor) {
+        // Paper end - Fix swamp hut cat generation deadlock
         Registry<Structure> registry = this.registryAccess().registryOrThrow(Registries.STRUCTURE);
 
         for (StructureStart structureStart : this.startsForStructure(
-            new ChunkPos(pos), structure -> registry.getHolder(registry.getId(structure)).map(predicate::test).orElse(false)
+            new ChunkPos(pos), structure -> registry.getHolder(registry.getId(structure)).map(predicate::test).orElse(false), levelAccessor // Paper - Fix swamp hut cat generation deadlock
         )) {
             if (this.structureHasPieceAt(pos, structureStart)) {
                 return structureStart;