Paper/nms-patches/ChunkProviderServer.patch

--- a/net/minecraft/server/ChunkProviderServer.java
+++ b/net/minecraft/server/ChunkProviderServer.java
@@ -16,6 +16,11 @@
 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();
@@ -34,7 +39,7 @@
         this.chunkLoader = ichunkloader;
         this.chunkGenerator = chunkgenerator;
         this.asyncTaskHandler = iasynctaskhandler;
-        this.chunkScheduler = new ChunkTaskScheduler(2, worldserver, chunkgenerator, ichunkloader, iasynctaskhandler);
+        this.chunkScheduler = new ChunkTaskScheduler(0, worldserver, chunkgenerator, ichunkloader, iasynctaskhandler); // CraftBukkit - very buggy, broken in lots of __subtle__ ways. Same goes for async chunk loading. Also Bukkit API / plugins can't handle async events at all anyway.
         this.batchScheduler = new SchedulerBatch<>(this.chunkScheduler);
     }
 
@@ -112,6 +117,22 @@
         }
     }
 
+    // CraftBukkit start
+    public Chunk generateChunk(int x, int z) {
+        try {
+            this.batchScheduler.b();
+            ChunkCoordIntPair pos = new ChunkCoordIntPair(x, z);
+            this.chunkScheduler.forcePolluteCache(pos);
+            this.batchScheduler.a(pos);
+            CompletableFuture<ProtoChunk> completablefuture = this.batchScheduler.c();
+
+            return (Chunk) completablefuture.thenApply(this::a).join();
+        } catch (RuntimeException runtimeexception) {
+            throw this.a(x, z, (Throwable) runtimeexception);
+        }
+    }
+    // CraftBukkit end
+
     public IChunkAccess a(int i, int j, boolean flag) {
         Chunk chunk = this.getChunkAt(i, j, true, false);
 
@@ -249,10 +270,12 @@
                         Chunk chunk = (Chunk) this.chunks.get(olong);
 
                         if (chunk != null) {
-                            chunk.removeEntities();
-                            this.saveChunk(chunk);
-                            this.chunks.remove(olong);
-                            this.lastChunk = null;
+                            // CraftBukkit start - move unload logic to own method
+                            if (!unloadChunk(chunk, true)) {
+                                continue;
+                            }
+                            // CraftBukkit end
+
                             ++i;
                         }
                     }
@@ -265,6 +288,42 @@
         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.getChunkAt(chunk.locX + x, chunk.locZ + z, false, false);
+                if (neighbor != null) {
+                    neighbor.setNeighborUnloaded(-x, -z);
+                    chunk.setNeighborUnloaded(x, z);
+                }
+            }
+        }
+        // Moved from unloadChunks above
+        synchronized (this.chunkLoader) {
+            chunk.removeEntities();
+            if (save) {
+                this.saveChunk(chunk);
+            }
+            this.chunks.remove(chunk.chunkKey);
+            this.lastChunk = null;
+        }
+        return true;
+    }
+    // CraftBukkit end
+
     public boolean d() {
         return !this.world.savingDisabled;
     }