PaperMC/paper-server/patches/features/0021-Fix-entity-tracker-desync-when-new-players-are-added.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Spottedleaf <Spottedleaf@users.noreply.github.com>
Date: Tue, 20 Feb 2024 18:24:16 -0800
Subject: [PATCH] Fix entity tracker desync when new players are added to the
 tracker

The delta position packet instructs the client to update
the entity position by a position difference. However, this position
difference is relative to the last position in the entity tracker
state, not the last position which has been sent to the player. As
a result, if the last position the player has recorded is different
than the one stored in the entity tracker (which occurs when a new
player is added to an existing entity tracker state) then the sent
position difference will cause a position desync for the client.

We can resolve this problem by either tracking the last position
sent per-player, or by simply resetting the last sent position
in the entity tracker state every time a new player is added.
Resetting the last sent position every time a new player is
added to the tracker is just easier to do, so that is what
this patch does.

This patch also fixes entities appearing to disappear when
teleporting to players by changing the initial position
in the spawn packet to the entities current tracking position.
When teleporting, the spawn packet will contain the old position
which is most likely in an unloaded chunk - which means that the
client will not tick the entity and thus not lerp the entity
from its old position to its new position.

diff --git a/net/minecraft/network/protocol/game/ClientboundAddEntityPacket.java b/net/minecraft/network/protocol/game/ClientboundAddEntityPacket.java
index db31989ebe3d7021cfd2311439e9a00f819b0841..1373977b339405ef59bb3ea03d195285c96dd3fe 100644
--- a/net/minecraft/network/protocol/game/ClientboundAddEntityPacket.java
+++ b/net/minecraft/network/protocol/game/ClientboundAddEntityPacket.java
@@ -42,9 +42,11 @@ public class ClientboundAddEntityPacket implements Packet<ClientGamePacketListen
         this(
             entity.getId(),
             entity.getUUID(),
-            serverEntity.getPositionBase().x(),
-            serverEntity.getPositionBase().y(),
-            serverEntity.getPositionBase().z(),
+            // Paper start - fix entity tracker desync
+            entity.trackingPosition().x(),
+            entity.trackingPosition().y(),
+            entity.trackingPosition().z(),
+            // Paper end - fix entity tracker desync
             serverEntity.getLastSentXRot(),
             serverEntity.getLastSentYRot(),
             entity.getType(),
diff --git a/net/minecraft/server/level/ChunkMap.java b/net/minecraft/server/level/ChunkMap.java
index f8f145cd9614f7e38d6aa72501fe31837340a8bb..fa48e63fea42f387dcd154c08af54cfb0d3b08af 100644
--- a/net/minecraft/server/level/ChunkMap.java
+++ b/net/minecraft/server/level/ChunkMap.java
@@ -1419,6 +1419,7 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
                         this.serverEntity.addPairing(player);
                         }
                         // Paper end - entity tracking events
+                        this.serverEntity.onPlayerAdd(); // Paper - fix desync when a player is added to the tracker
                     }
                 } else if (this.seenBy.remove(player.connection)) {
                     this.serverEntity.removePairing(player);
diff --git a/net/minecraft/server/level/ServerEntity.java b/net/minecraft/server/level/ServerEntity.java
index 4c891706c2a3efc8e8c44fc1c031e8a1d21efb60..66f7926a2639ade41cb89419e38e12053314c982 100644
--- a/net/minecraft/server/level/ServerEntity.java
+++ b/net/minecraft/server/level/ServerEntity.java
@@ -90,6 +90,13 @@ public class ServerEntity {
         this.trackedDataValues = entity.getEntityData().getNonDefaultValues();
     }
 
+    // Paper start - fix desync when a player is added to the tracker
+    private boolean forceStateResync;
+    public void onPlayerAdd() {
+        this.forceStateResync = true;
+    }
+    // Paper end - fix desync when a player is added to the tracker
+
     public void sendChanges() {
         List<Entity> passengers = this.entity.getPassengers();
         if (!passengers.equals(this.lastPassengers)) {
@@ -125,7 +132,7 @@ public class ServerEntity {
             this.sendDirtyEntityData();
         }
 
-        if (this.tickCount % this.updateInterval == 0 || this.entity.hasImpulse || this.entity.getEntityData().isDirty()) {
+        if (this.forceStateResync || this.tickCount % this.updateInterval == 0 || this.entity.hasImpulse || this.entity.getEntityData().isDirty()) { // Paper - fix desync when a player is added to the tracker
             byte b = Mth.packDegrees(this.entity.getYRot());
             byte b1 = Mth.packDegrees(this.entity.getXRot());
             boolean flag = Math.abs(b - this.lastSentYRot) >= 1 || Math.abs(b1 - this.lastSentXRot) >= 1;
@@ -160,7 +167,7 @@ public class ServerEntity {
                 long l1 = this.positionCodec.encodeY(vec3);
                 long l2 = this.positionCodec.encodeZ(vec3);
                 boolean flag5 = l < -32768L || l > 32767L || l1 < -32768L || l1 > 32767L || l2 < -32768L || l2 > 32767L;
-                if (flag5 || this.teleportDelay > 400 || this.wasRiding || this.wasOnGround != this.entity.onGround()) {
+                if (this.forceStateResync || flag5 || this.teleportDelay > 400 || this.wasRiding || this.wasOnGround != this.entity.onGround()) { // Paper - fix desync when a player is added to the tracker
                     this.wasOnGround = this.entity.onGround();
                     this.teleportDelay = 0;
                     packet = ClientboundEntityPositionSyncPacket.of(this.entity);
@@ -225,6 +232,7 @@ public class ServerEntity {
             }
 
             this.entity.hasImpulse = false;
+            this.forceStateResync = false; // Paper - fix desync when a player is added to the tracker
         }
 
         this.tickCount++;