Paper/patches/server/0421-Allow-multiple-callbacks-to-schedule-for-Callback-Ex.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Tue, 21 Apr 2020 03:51:53 -0400
Subject: [PATCH] Allow multiple callbacks to schedule for Callback Executor

ChunkMapDistance polls multiple entries for pendingChunkUpdates

Each of these have the potential to move a chunk in and out of
"Loaded" state, which will result in multiple callbacks being
needed within a single tick of ChunkMapDistance

Use an ArrayDeque to store this Queue

We make sure to also implement a pattern that is recursion safe too.

diff --git a/src/main/java/net/minecraft/server/level/ChunkMap.java b/src/main/java/net/minecraft/server/level/ChunkMap.java
index 2b4c35d9bd186fd7c3650a7ad791cd67fb64e635..e7c78106aa34d948a77cf72d5716e17d10beae72 100644
--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
+++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
@@ -178,17 +178,29 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
     public final CallbackExecutor callbackExecutor = new CallbackExecutor();
     public static final class CallbackExecutor implements java.util.concurrent.Executor, Runnable {
 
-        private final java.util.Queue<Runnable> queue = new java.util.ArrayDeque<>();
+        // Paper start - replace impl with recursive safe multi entry queue
+        // it's possible to schedule multiple tasks currently, so it's vital we change this impl
+        // If we recurse into the executor again, we will append to another queue, ensuring task order consistency
+        private java.util.Queue<Runnable> queue = new java.util.ArrayDeque<>(); // Paper - remove final
 
         @Override
         public void execute(Runnable runnable) {
+            if (this.queue == null) {
+                this.queue = new java.util.ArrayDeque<>();
+            }
             this.queue.add(runnable);
         }
 
         @Override
         public void run() {
+            if (this.queue == null) {
+                return;
+            }
+            java.util.Queue<Runnable> queue = this.queue;
+            this.queue = null;
+            // Paper end
             Runnable task;
-            while ((task = this.queue.poll()) != null) {
+            while ((task = queue.poll()) != null) { // Paper
                 task.run();
             }
         }