handle PacketPlayInKeepAlive async

In 1.12.2, Mojang moved the processing of PacketPlayInKeepAlive off the main
thread, while entirely correct for the server, this causes issues with
plugins which are expecting the PlayerQuitEvent on the main thread.

In order to counteract some bad behavior, we will post handling of the
disconnection to the main thread, but leave the actual processing of the packet
on the main thread.

Spigot-Server-Patches/0243-handle-PacketPlayInKeepAlive-async.patch

@@ -0,0 +1,41 @@
+From ba1fd1fc65bec591a26a28ec246bfc10b3c5a438 Mon Sep 17 00:00:00 2001
+From: Shane Freeder <theboyetronic@gmail.com>
+Date: Thu, 5 Oct 2017 01:54:07 +0100
+Subject: [PATCH] handle PacketPlayInKeepAlive async
+
+In 1.12.2, Mojang moved the processing of PacketPlayInKeepAlive off the main
+thread, while entirely correct for the server, this causes issues with
+plugins which are expecting the PlayerQuitEvent on the main thread.
+
+In order to counteract some bad behavior, we will post handling of the
+disconnection to the main thread, but leave the actual processing of the packet
+on the main thread.
+
+diff --git a/src/main/java/net/minecraft/server/PlayerConnection.java b/src/main/java/net/minecraft/server/PlayerConnection.java
+index d8f9a093b..eac4beb56 100644
+--- a/src/main/java/net/minecraft/server/PlayerConnection.java
++++ b/src/main/java/net/minecraft/server/PlayerConnection.java
+@@ -2225,14 +2225,18 @@ public class PlayerConnection implements PacketListenerPlayIn, ITickable {
+     }
+ 
+     public void a(PacketPlayInKeepAlive packetplayinkeepalive) {
+-        PlayerConnectionUtils.ensureMainThread(packetplayinkeepalive, this, this.player.x()); // CraftBukkit
++        //PlayerConnectionUtils.ensureMainThread(packetplayinkeepalive, this, this.player.x()); // CraftBukkit // Paper - Keep this on the main thread
+         if (this.g && packetplayinkeepalive.a() == this.h) {
+             int i = (int) (this.d() - this.f);
+ 
+             this.player.ping = (this.player.ping * 3 + i) / 4;
+             this.g = false;
+         } else if (!this.player.getName().equals(this.minecraftServer.Q())) {
+-            this.disconnect(new ChatMessage("disconnect.timeout", new Object[0]));
++            // Paper start - This needs to be handled on the main thread for plugins
++            minecraftServer.postToMainThread(() -> {
++                    this.disconnect(new ChatMessage("disconnect.timeout", new Object[0]));
++            });
++            // Paper end
+         }
+ 
+     }
+-- 
+2.14.2
+