Paper/patches/server/0796-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 1bc8334691147a0f52ad0e97236c9e007b95ba34..770b15f5413eb38890587aa6af2a56f15bf5934f 100644
--- a/src/main/java/net/minecraft/world/entity/animal/Cat.java
+++ b/src/main/java/net/minecraft/world/entity/animal/Cat.java
@@ -363,7 +363,7 @@ public class Cat extends TamableAnimal implements VariantHolder<CatVariant> {
         });
         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 deadlock
             this.setVariant((CatVariant) BuiltInRegistries.CAT_VARIANT.getOrThrow(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 b33a015b834873f279bf33a64974ef440a37df79..09c85ed428b8eaf51f8b3c6e45cce925f05ab354 100644
--- a/src/main/java/net/minecraft/world/level/StructureManager.java
+++ b/src/main/java/net/minecraft/world/level/StructureManager.java
@@ -44,7 +44,12 @@ 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
+        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
         ImmutableList.Builder<StructureStart> builder = ImmutableList.builder();
 
         for(Map.Entry<Structure, LongSet> entry : map.entrySet()) {
@@ -108,13 +113,18 @@ public class StructureManager {
     }
 
     public StructureStart getStructureWithPieceAt(BlockPos pos, TagKey<Structure> structureTag) {
+        // Paper start
+        return this.getStructureWithPieceAt(pos, structureTag, null);
+    }
+    public StructureStart getStructureWithPieceAt(BlockPos pos, TagKey<Structure> structureTag, @Nullable ServerLevelAccessor levelAccessor) {
+        // Paper end
         Registry<Structure> registry = this.registryAccess().registryOrThrow(Registries.STRUCTURE);
 
         for(StructureStart structureStart : this.startsForStructure(new ChunkPos(pos), (structure) -> {
             return registry.getHolder(registry.getId(structure)).map((reference) -> {
                 return reference.is(structureTag);
             }).orElse(false);
-        })) {
+        }, levelAccessor)) { // Paper
             if (this.structureHasPieceAt(pos, structureStart)) {
                 return structureStart;
             }