From aa1a9266fcdeb4ffc727142df482ebca0d03828d Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Sat, 11 Apr 2020 21:23:42 -0400
Subject: [PATCH] Delay unsafe actions until after entity ticking is done

This will help prevent many cases of unregistering entities during entity ticking

diff --git a/src/main/java/net/minecraft/server/PlayerChunk.java b/src/main/java/net/minecraft/server/PlayerChunk.java
index a9a2ce3d3f..24cb88559b 100644
--- a/src/main/java/net/minecraft/server/PlayerChunk.java
+++ b/src/main/java/net/minecraft/server/PlayerChunk.java
@@ -490,8 +490,11 @@ public class PlayerChunk {
                         // Minecraft will apply the chunks tick lists to the world once the chunk got loaded, and then store the tick
                         // lists again inside the chunk once the chunk becomes inaccessible and set the chunk's needsSaving flag.
                         // These actions may however happen deferred, so we manually set the needsSaving flag already here.
+                        // Paper start
+                        ((WorldServer) chunk.world).doIfNotEntityTicking(() -> { // Paper
                         chunk.setNeedsSaving(true);
                         chunk.unloadCallback();
+                        }); // Paper
                     });
                 }
             }).exceptionally((throwable) -> {
diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
index cd8266f675..84a3367b87 100644
--- a/src/main/java/net/minecraft/server/WorldServer.java
+++ b/src/main/java/net/minecraft/server/WorldServer.java
@@ -60,6 +60,16 @@ public class WorldServer extends World {
     private final Queue<Entity> entitiesToAdd = Queues.newArrayDeque();
     public final List<EntityPlayer> players = Lists.newArrayList(); // Paper - private -> public
     boolean tickingEntities;
+    // Paper start
+    List<java.lang.Runnable> afterEntityTickingTasks = Lists.newArrayList();
+    public void doIfNotEntityTicking(java.lang.Runnable run) {
+        if (tickingEntities) {
+            afterEntityTickingTasks.add(run);
+        } else {
+            run.run();
+        }
+    }
+    // Paper end
     private final MinecraftServer server;
     private final WorldNBTStorage dataManager;
     public boolean savingDisabled;
@@ -519,6 +529,16 @@ public class WorldServer extends World {
             timings.entityTick.stopTiming(); // Spigot
 
             this.tickingEntities = false;
+            // Paper start
+            for (java.lang.Runnable run : this.afterEntityTickingTasks) {
+                try {
+                    run.run();
+                } catch (Exception e) {
+                    LOGGER.error("Error in After Entity Ticking Task", e);
+                }
+            }
+            this.afterEntityTickingTasks.clear();
+            // Paper end
             this.getMinecraftServer().midTickLoadChunks(); // Paper
 
             try (co.aikar.timings.Timing ignored = this.timings.newEntities.startTiming()) { // Paper - timings
diff --git a/src/main/java/org/bukkit/craftbukkit/CraftWorld.java b/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
index 1fbb1344fc..f56131e3a5 100644
--- a/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
+++ b/src/main/java/org/bukkit/craftbukkit/CraftWorld.java
@@ -2469,7 +2469,7 @@ public class CraftWorld implements World {
 
         CompletableFuture<Chunk> ret = new CompletableFuture<>();
         this.world.getChunkProvider().getChunkAtAsynchronously(x, z, gen, (net.minecraft.server.Chunk chunk) -> {
-            ret.complete(chunk == null ? null : chunk.bukkitChunk);
+            this.world.doIfNotEntityTicking(() -> ret.complete(chunk == null ? null : chunk.bukkitChunk));
         });
 
         return ret;
-- 
2.25.1