Paper/patches/unapplied/server/1025-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 854baa554da2215a656f976ae2973341c3b2d61c..792b9a72a610cc512a8920d61013b6ba02f71e47 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 21370ed6c7d98d3f3546f0365ac50e5c26ba3bde..af8cb316ac169aa8d98a88765b85bb013b9ba961 100644
--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
+++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
@@ -1269,6 +1269,7 @@ 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 12d86f27d04bffed8c3844e36b42fbc2f84dacff..8ea2f24695f5dad55e21f238b69442513e7a90c6 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() {
         // Paper start - optimise collisions
         if (((ca.spottedleaf.moonrise.patches.chunk_system.entity.ChunkSystemEntity)this.entity).moonrise$isHardColliding()) {
@@ -145,7 +152,7 @@ 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;
 
@@ -185,7 +192,7 @@ 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());
@@ -249,6 +256,7 @@ public class ServerEntity {
             }
 
             this.entity.hasImpulse = false;
+            this.forceStateResync = false; // Paper - fix desync when a player is added to the tracker
         }
 
         ++this.tickCount;