Paper/nms-patches/ChunkProviderServer.patch

--- a/net/minecraft/server/ChunkProviderServer.java
+++ b/net/minecraft/server/ChunkProviderServer.java
@@ -15,6 +15,11 @@
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 
+// CraftBukkit start
+import org.bukkit.craftbukkit.chunkio.ChunkIOExecutor;
+import org.bukkit.event.world.ChunkUnloadEvent;
+// CraftBukkit end
+
 public class ChunkProviderServer implements IChunkProvider {
 
     private static final Logger a = LogManager.getLogger();
@@ -70,19 +75,82 @@
         Chunk chunk = this.getLoadedChunkAt(i, j);
 
         if (chunk == null) {
+            // CraftBukkit start
+            ChunkRegionLoader loader = null;
+
+            if (this.chunkLoader instanceof ChunkRegionLoader) {
+                loader = (ChunkRegionLoader) this.chunkLoader;
+            }
+            if (loader != null && loader.chunkExists(world, i, j)) {
+                chunk = ChunkIOExecutor.syncChunkLoad(world, loader, this, i, j);
+            }
+        }
+
+        return chunk;
+    }
+
+    @Nullable
+    public Chunk originalGetOrLoadChunkAt(int i, int j) {
+        // CraftBukkit end
+        Chunk chunk = this.getLoadedChunkAt(i, j);
+
+        if (chunk == null) {
             chunk = this.loadChunk(i, j);
             if (chunk != null) {
                 this.chunks.put(ChunkCoordIntPair.a(i, j), chunk);
                 chunk.addEntities();
-                chunk.loadNearby(this, this.chunkGenerator);
+                chunk.loadNearby(this, this.chunkGenerator, false); // CraftBukkit
             }
         }
 
         return chunk;
     }
 
+    // CraftBukkit start
+    public Chunk getChunkIfLoaded(int x, int z) {
+        return chunks.get(ChunkCoordIntPair.a(x, z));
+    }
+    // CraftBukkit end
+
     public Chunk getChunkAt(int i, int j) {
-        Chunk chunk = this.getOrLoadChunkAt(i, j);
+        return getChunkAt(i, j, null);
+    }
+
+    public Chunk getChunkAt(int i, int j, Runnable runnable) {
+        return getChunkAt(i, j, runnable, true);
+    }
+
+    public Chunk getChunkAt(int i, int j, Runnable runnable, boolean generate) {
+        Chunk chunk = getChunkIfLoaded(i, j);
+        ChunkRegionLoader loader = null;
+
+        if (this.chunkLoader instanceof ChunkRegionLoader) {
+            loader = (ChunkRegionLoader) this.chunkLoader;
+
+        }
+        // We can only use the queue for already generated chunks
+        if (chunk == null && loader != null && loader.chunkExists(world, i, j)) {
+            if (runnable != null) {
+                ChunkIOExecutor.queueChunkLoad(world, loader, this, i, j, runnable);
+                return null;
+            } else {
+                chunk = ChunkIOExecutor.syncChunkLoad(world, loader, this, i, j);
+            }
+        } else if (chunk == null && generate) {
+            chunk = originalGetChunkAt(i, j);
+        }
+
+        // If we didn't load the chunk async and have a callback run it now
+        if (runnable != null) {
+            runnable.run();
+        }
+
+        return chunk;
+    }
+
+    public Chunk originalGetChunkAt(int i, int j) {
+        Chunk chunk = this.originalGetOrLoadChunkAt(i, j);
+        // CraftBukkit end
 
         if (chunk == null) {
             long k = ChunkCoordIntPair.a(i, j);
@@ -101,7 +169,7 @@
 
             this.chunks.put(k, chunk);
             chunk.addEntities();
-            chunk.loadNearby(this, this.chunkGenerator);
+            chunk.loadNearby(this, this.chunkGenerator, true); // CraftBukkit
         }
 
         return chunk;
@@ -147,10 +215,12 @@
 
     public boolean a(boolean flag) {
         int i = 0;
-        ArrayList arraylist = Lists.newArrayList(this.chunks.values());
 
-        for (int j = 0; j < arraylist.size(); ++j) {
-            Chunk chunk = (Chunk) arraylist.get(j);
+        // CraftBukkit start
+        Iterator iterator = this.chunks.values().iterator();
+        while (iterator.hasNext()) {
+            Chunk chunk = (Chunk) iterator.next();
+            // CraftBukkit end
 
             if (flag) {
                 this.saveChunkNOP(chunk);
@@ -183,10 +253,12 @@
                     Chunk chunk = (Chunk) this.chunks.get(olong);
 
                     if (chunk != null && chunk.d) {
-                        chunk.removeEntities();
-                        this.saveChunk(chunk);
-                        this.saveChunkNOP(chunk);
-                        this.chunks.remove(olong);
+                        // CraftBukkit start - move unload logic to own method
+                        if (!unloadChunk(chunk, true)) {
+                            continue;
+                        }
+                        // CraftBukkit end
+
                         ++i;
                     }
                 }
@@ -198,6 +270,40 @@
         return false;
     }
 
+    // CraftBukkit start
+    public boolean unloadChunk(Chunk chunk, boolean save) {
+        ChunkUnloadEvent event = new ChunkUnloadEvent(chunk.bukkitChunk, save);
+        this.world.getServer().getPluginManager().callEvent(event);
+        if (event.isCancelled()) {
+            return false;
+        }
+        save = event.isSaveChunk();
+
+        // Update neighbor counts
+        for (int x = -2; x < 3; x++) {
+            for (int z = -2; z < 3; z++) {
+                if (x == 0 && z == 0) {
+                    continue;
+                }
+
+                Chunk neighbor = this.getChunkIfLoaded(chunk.locX + x, chunk.locZ + z);
+                if (neighbor != null) {
+                    neighbor.setNeighborUnloaded(-x, -z);
+                    chunk.setNeighborUnloaded(x, z);
+                }
+            }
+        }
+        // Moved from unloadChunks above
+        chunk.removeEntities();
+        if (save) {
+            this.saveChunk(chunk);
+            this.saveChunkNOP(chunk);
+        }
+        this.chunks.remove(chunk.chunkKey);
+        return true;
+    }
+    // CraftBukkit end
+
     public boolean e() {
         return !this.world.savingDisabled;
     }