Fix Longstanding Broken behavior of PlayerJoinEvent

For years, plugin developers have had to delay many things they do
inside of the PlayerJoinEvent by 1 tick to make it actually work.

This all boiled down to 1 reason why: The event fired before the
player was fully ready and joined to the world!

Additionally, if that player logged out on a vehicle, the event
fired before the vehicle was even loaded, so that plugins had no
access to the vehicle during this event either.

This change finally fixes this issue, fully preparing the player
into the world as a fully ready entity, vehicle included.

There should be no plugins that break because of this change, but might
improve consistency with other plugins instead.

For example, if 2 plugins listens to this event, and the first one
teleported the player in the event, then the 2nd plugin actually
would be getting a valid player!

This was very non deterministic. This change will ensure every plugin
receives a deterministic result, and should no longer require 1 tick
delays anymore.

Spigot-Server-Patches/0486-Fix-Longstanding-Broken-behavior-of-PlayerJoinEvent.patch

@@ -0,0 +1,91 @@
+From 71067736c5517211f7e9991cf940b49c220b5e84 Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Sun, 19 Apr 2020 00:05:46 -0400
+Subject: [PATCH] Fix Longstanding Broken behavior of PlayerJoinEvent
+
+For years, plugin developers have had to delay many things they do
+inside of the PlayerJoinEvent by 1 tick to make it actually work.
+
+This all boiled down to 1 reason why: The event fired before the
+player was fully ready and joined to the world!
+
+Additionally, if that player logged out on a vehicle, the event
+fired before the vehicle was even loaded, so that plugins had no
+access to the vehicle during this event either.
+
+This change finally fixes this issue, fully preparing the player
+into the world as a fully ready entity, vehicle included.
+
+There should be no plugins that break because of this change, but might
+improve consistency with other plugins instead.
+
+For example, if 2 plugins listens to this event, and the first one
+teleported the player in the event, then the 2nd plugin actually
+would be getting a valid player!
+
+This was very non deterministic. This change will ensure every plugin
+receives a deterministic result, and should no longer require 1 tick
+delays anymore.
+
+diff --git a/src/main/java/net/minecraft/server/PlayerList.java b/src/main/java/net/minecraft/server/PlayerList.java
+index 72ca7f706..3dae96b66 100644
+--- a/src/main/java/net/minecraft/server/PlayerList.java
++++ b/src/main/java/net/minecraft/server/PlayerList.java
+@@ -188,6 +188,11 @@ public abstract class PlayerList {
+         this.j.put(entityplayer.getUniqueID(), entityplayer);
+         // this.sendAll(new PacketPlayOutPlayerInfo(PacketPlayOutPlayerInfo.EnumPlayerInfoAction.ADD_PLAYER, new EntityPlayer[]{entityplayer})); // CraftBukkit - replaced with loop below
+ 
++        // Paper start - correctly register player BEFORE PlayerJoinEvent, so the entity is valid and doesn't require tick delay hacks
++        worldserver.addPlayerJoin(entityplayer);
++        this.server.getBossBattleCustomData().a(entityplayer); // see commented out section below worldserver.addPlayerJoin(entityplayer);
++        mountSavedVehicle(entityplayer, worldserver, nbttagcompound);
++        // Paper end
+         // CraftBukkit start
+         PlayerJoinEvent playerJoinEvent = new PlayerJoinEvent(cserver.getPlayer(entityplayer), joinMessage);
+         cserver.getPluginManager().callEvent(playerJoinEvent);
+@@ -219,7 +224,7 @@ public abstract class PlayerList {
+                 continue;
+             }
+ 
+-            entityplayer.playerConnection.sendPacket(new PacketPlayOutPlayerInfo(PacketPlayOutPlayerInfo.EnumPlayerInfoAction.ADD_PLAYER, new EntityPlayer[] { entityplayer1}));
++            entityplayer.playerConnection.sendPacket(new PacketPlayOutPlayerInfo(PacketPlayOutPlayerInfo.EnumPlayerInfoAction.ADD_PLAYER, new EntityPlayer[]{entityplayer1}));
+         }
+         entityplayer.sentListPacket = true;
+         // CraftBukkit end
+@@ -228,8 +233,8 @@ public abstract class PlayerList {
+ 
+         // CraftBukkit start - Only add if the player wasn't moved in the event
+         if (entityplayer.world == worldserver && !worldserver.getPlayers().contains(entityplayer)) {
+-            worldserver.addPlayerJoin(entityplayer);
+-            this.server.getBossBattleCustomData().a(entityplayer);
++            //worldserver.addPlayerJoin(entityplayer); // Paper - moved up
++            //this.server.getBossBattleCustomData().a(entityplayer); // Paper - moved up - if mapping changes, update above
+         }
+ 
+         worldserver = server.getWorldServer(entityplayer.dimension);  // CraftBukkit - Update in case join event changed it
+@@ -247,6 +252,11 @@ public abstract class PlayerList {
+             playerconnection.sendPacket(new PacketPlayOutEntityEffect(entityplayer.getId(), mobeffect));
+         }
+ 
++        // Paper start - move vehicle into method so it can be called above - short circuit around that code
++        onPlayerJoinFinish(entityplayer, worldserver, s1);
++    }
++    private void mountSavedVehicle(EntityPlayer entityplayer, WorldServer worldserver, NBTTagCompound nbttagcompound) {
++        // Paper end
+         if (nbttagcompound != null && nbttagcompound.hasKeyOfType("RootVehicle", 10)) {
+             NBTTagCompound nbttagcompound1 = nbttagcompound.getCompound("RootVehicle");
+             // CraftBukkit start
+@@ -288,6 +298,10 @@ public abstract class PlayerList {
+             }
+         }
+ 
++        // Paper start
++    }
++    public void onPlayerJoinFinish(EntityPlayer entityplayer, WorldServer worldserver, String s1) {
++        // Paper end
+         entityplayer.syncInventory();
+         // Paper start - Add to collideRule team if needed
+         final Scoreboard scoreboard = this.getServer().getWorldServer(DimensionManager.OVERWORLD).getScoreboard();
+-- 
+2.25.1
+