From 0f4ba3f787e0860be4e312a4eaaa2a9556624bf8 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 8b2eed10..ee0cabad 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.windows.1