Paper/patches/server/1037-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/src/main/java/net/minecraft/network/protocol/game/ClientboundAddEntityPacket.java b/src/main/java/net/minecraft/network/protocol/game/ClientboundAddEntityPacket.java
index 1a5e73fd97781f3903e5ef13aa0352c64fbc2cc1..4126d82e83810126eb4a41b4587dc993542f3793 100644
--- a/src/main/java/net/minecraft/network/protocol/game/ClientboundAddEntityPacket.java
+++ b/src/main/java/net/minecraft/network/protocol/game/ClientboundAddEntityPacket.java
@@ -42,9 +42,11 @@ public class ClientboundAddEntityPacket implements Packet<ClientGamePacketListen
         this(
             entity.getId(),
             entity.getUUID(),
-            entityTrackerEntry.getPositionBase().x(),
-            entityTrackerEntry.getPositionBase().y(),
-            entityTrackerEntry.getPositionBase().z(),
+            // Paper start - fix entity tracker desync
+            entity.trackingPosition().x(),
+            entity.trackingPosition().y(),
+            entity.trackingPosition().z(),
+            // Paper end - fix entity tracker desync
             entityTrackerEntry.getLastSentXRot(),
             entityTrackerEntry.getLastSentYRot(),
             entity.getType(),
diff --git a/src/main/java/net/minecraft/server/level/ChunkMap.java b/src/main/java/net/minecraft/server/level/ChunkMap.java
index 344966d3deb640eb99bc9c9e318e6e6780421907..6df79aab2f0a75bbe347dc92e9ed5d62ceec7983 100644
--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
+++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
@@ -1302,6 +1302,7 @@ 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/src/main/java/net/minecraft/server/level/ServerEntity.java b/src/main/java/net/minecraft/server/level/ServerEntity.java
index 1d849ce4e2c85f149af25318b8ffb6dcef6c6788..b4971813d267dffc6507502345e5c8b991541eca 100644
--- a/src/main/java/net/minecraft/server/level/ServerEntity.java
+++ b/src/main/java/net/minecraft/server/level/ServerEntity.java
@@ -96,6 +96,13 @@ 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> list = this.entity.getPassengers();
 
@@ -140,7 +147,7 @@ public class ServerEntity {
             }
         }
 
-        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
             int i;
             int j;
 
@@ -180,7 +187,7 @@ public class ServerEntity {
                 long i1 = this.positionCodec.encodeZ(vec3d);
                 boolean flag6 = k < -32768L || k > 32767L || l < -32768L || l > 32767L || i1 < -32768L || i1 > 32767L;
 
-                if (!flag6 && this.teleportDelay <= 400 && !this.wasRiding && this.wasOnGround == this.entity.onGround()) {
+                if (!this.forceStateResync && !flag6 && this.teleportDelay <= 400 && !this.wasRiding && this.wasOnGround == this.entity.onGround()) { // Paper - fix desync when a player is added to the tracker
                     if ((!flag2 || !flag3) && !(this.entity instanceof AbstractArrow)) {
                         if (flag2) {
                             packet1 = new ClientboundMoveEntityPacket.Pos(this.entity.getId(), (short) ((int) k), (short) ((int) l), (short) ((int) i1), this.entity.onGround());
@@ -244,6 +251,7 @@ public class ServerEntity {
             }
 
             this.entity.hasImpulse = false;
+            this.forceStateResync = false; // Paper - fix desync when a player is added to the tracker
         }
 
         ++this.tickCount;