PaperMC/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.

diff --git a/src/main/java/net/minecraft/server/level/ChunkMap.java b/src/main/java/net/minecraft/server/level/ChunkMap.java
index c96740a82eac9101f74edeb44edf4b64d1d633e0..8b6754525fafd1aaac3292cf69a855d6a42b9523 100644
--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
+++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
@@ -1189,6 +1189,7 @@ 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;