SPIGOT-4975: NPE on WorldGenStronghold When Using Multiple Worlds

nms-patches/ChunkGenerator.patch

@@ -0,0 +1,22 @@
+--- a/net/minecraft/server/ChunkGenerator.java
++++ b/net/minecraft/server/ChunkGenerator.java
+@@ -148,6 +148,19 @@
+                 ChunkCoordIntPair chunkcoordintpair = ichunkaccess.getPos();
+                 StructureStart structurestart = StructureStart.a;
+ 
++                // CraftBukkit start
++                if (structuregenerator == WorldGenerator.STRONGHOLD) {
++                    synchronized (structuregenerator) {
++                        if (structuregenerator.a(chunkgenerator, seededrandom, chunkcoordintpair.x, chunkcoordintpair.z)) {
++                            BiomeBase biomebase = this.getWorldChunkManager().getBiome(new BlockPosition(chunkcoordintpair.d() + 9, 0, chunkcoordintpair.e() + 9));
++                            StructureStart structurestart1 = structuregenerator.a().create(structuregenerator, chunkcoordintpair.x, chunkcoordintpair.z, biomebase, StructureBoundingBox.a(), 0, chunkgenerator.getSeed());
++
++                            structurestart1.a(this, definedstructuremanager, chunkcoordintpair.x, chunkcoordintpair.z, biomebase);
++                            structurestart = structurestart1.e() ? structurestart1 : StructureStart.a;
++                        }
++                    }
++                } else
++                // CraftBukkit end
+                 if (structuregenerator.a(chunkgenerator, seededrandom, chunkcoordintpair.x, chunkcoordintpair.z)) {
+                     BiomeBase biomebase = this.getWorldChunkManager().getBiome(new BlockPosition(chunkcoordintpair.d() + 9, 0, chunkcoordintpair.e() + 9));
+                     StructureStart structurestart1 = structuregenerator.a().create(structuregenerator, chunkcoordintpair.x, chunkcoordintpair.z, biomebase, StructureBoundingBox.a(), 0, chunkgenerator.getSeed());

nms-patches/WorldGenStronghold.patch

@@ -0,0 +1,11 @@
+--- a/net/minecraft/server/WorldGenStronghold.java
++++ b/net/minecraft/server/WorldGenStronghold.java
+@@ -67,7 +67,7 @@
+ 
+     @Nullable
+     @Override
+-    public BlockPosition getNearestGeneratedFeature(World world, ChunkGenerator<? extends GeneratorSettingsDefault> chunkgenerator, BlockPosition blockposition, int i, boolean flag) {
++    public synchronized BlockPosition getNearestGeneratedFeature(World world, ChunkGenerator<? extends GeneratorSettingsDefault> chunkgenerator, BlockPosition blockposition, int i, boolean flag) { // CraftBukkit - synchronized
+         if (!chunkgenerator.getWorldChunkManager().a(this)) {
+             return null;
+         } else {