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

Spigot-Server-Patches/0491-Allow-multiple-callbacks-to-schedule-for-Callback-Ex.patch

@@ -0,0 +1,52 @@
+From 429c6c3f9e55451e7f2e0065879884f3e0e13217 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
+
+diff --git a/src/main/java/net/minecraft/server/PlayerChunkMap.java b/src/main/java/net/minecraft/server/PlayerChunkMap.java
+index 8b2eed1051..ee0cabadc8 100644
+--- a/src/main/java/net/minecraft/server/PlayerChunkMap.java
++++ b/src/main/java/net/minecraft/server/PlayerChunkMap.java
+@@ -87,25 +87,22 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
+     public final CallbackExecutor callbackExecutor = new CallbackExecutor();
+     public static final class CallbackExecutor implements java.util.concurrent.Executor, Runnable {
+ 
+-        private Runnable queued;
++        // Paper start - replace impl with Deque - possible multiple is needed in single pass
++        private java.util.Deque<Runnable> queued = new java.util.ArrayDeque<>();
+ 
+         @Override
+         public void execute(Runnable runnable) {
+-            if (queued != null) {
+-                MinecraftServer.LOGGER.fatal("Failed to schedule runnable", new IllegalStateException("Already queued")); // Paper - make sure this is printed
+-                throw new IllegalStateException("Already queued");
+-            }
+-            queued = runnable;
++            queued.add(runnable); // Paper
+         }
+ 
+         @Override
+         public void run() {
+-            Runnable task = queued;
+-            queued = null;
+-            if (task != null) {
++            Runnable task;
++            while ((task = queued.pollFirst()) != null) {
+                 task.run();
+             }
+         }
++        // Paper end
+     };
+     // CraftBukkit end
+ 
+-- 
+2.25.1
+