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.

Feature patch

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 f6e1deb2f849d8b01b15cfa69e2f6cd5f2b1512b..f66e40326c510aa3267542b1a24ed75d1ed6d3f1 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 cb3850939955ae068d4776c835522e0b8f228984..f1999729cd1c00071c5e1835ee49ea5fcafa7b05 100644
--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
+++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
@@ -1271,6 +1271,7 @@ 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 5bbc7ceaafc163f12344e5d5d355ad2ff30ddca2..90eb4927fa51ce3df86aa7b6c71f49150a03e337 100644
--- a/src/main/java/net/minecraft/server/level/ServerEntity.java
+++ b/src/main/java/net/minecraft/server/level/ServerEntity.java
@@ -100,6 +100,13 @@ 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()) {
@@ -149,7 +156,7 @@ 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
             byte b0 = Mth.packDegrees(this.entity.getYRot());
             byte b1 = Mth.packDegrees(this.entity.getXRot());
             boolean flag = Math.abs(b0 - this.lastSentYRot) >= 1 || Math.abs(b1 - this.lastSentXRot) >= 1;
@@ -199,7 +206,7 @@ public class ServerEntity {
                     long k = this.positionCodec.encodeZ(vec3d);
                     boolean flag5 = i < -32768L || i > 32767L || j < -32768L || j > 32767L || k < -32768L || k > 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
                         if ((!flag2 || !flag) && !(this.entity instanceof AbstractArrow)) {
                             if (flag2) {
                                 packet1 = new ClientboundMoveEntityPacket.Pos(this.entity.getId(), (short) ((int) i), (short) ((int) j), (short) ((int) k), this.entity.onGround());
@@ -265,6 +272,7 @@ public class ServerEntity {
             }
 
             this.entity.hasImpulse = false;
+            this.forceStateResync = false; // Paper - fix desync when a player is added to the tracker
         }
 
         ++this.tickCount;