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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.

Fixes https://github.com/PaperMC/Folia/issues/197
This commit is contained in:
Spottedleaf 2024-02-20 18:30:53 -08:00
parent 0265fdd4f4
commit 5652259e1a
1 changed files with 85 additions and 0 deletions
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patches/server/0019-Fix-entity-tracker-desync-when-new-players-are-added.patch Normal file
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@ -0,0 +1,85 @@
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 21ec49acb3c1241d9286959c42a7f8363f637e4f..79e507c1463dbb9039b1fed8bdf41c0ee5f4fec3 100644
--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
+++ b/src/main/java/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/src/main/java/net/minecraft/server/level/ServerEntity.java b/src/main/java/net/minecraft/server/level/ServerEntity.java
index 529ab44baaf573b97cf7e89560c548642733188f..16373e0c5ea38199fab3eb289bf2a5fcf0dd7439 100644
--- a/src/main/java/net/minecraft/server/level/ServerEntity.java
+++ b/src/main/java/net/minecraft/server/level/ServerEntity.java
@@ -93,6 +93,13 @@ 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();
@@ -141,7 +148,7 @@ 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;
@@ -177,13 +184,13 @@ public class ServerEntity {
boolean flag4 = false;
boolean flag5 = false;
- if (!(this.entity instanceof net.minecraft.world.entity.decoration.HangingEntity) || this.tickCount > 0 || this.entity instanceof AbstractArrow) { // Paper - Always update position to fix first-tick teleports
+ if (this.forceStateResync || !(this.entity instanceof net.minecraft.world.entity.decoration.HangingEntity) || this.tickCount > 0 || this.entity instanceof AbstractArrow) { // Paper - Always update position to fix first-tick teleports
long k = this.positionCodec.encodeX(vec3d);
long l = this.positionCodec.encodeY(vec3d);
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()&& !(io.papermc.paper.configuration.GlobalConfiguration.get().collisions.sendFullPosForHardCollidingEntities && this.entity.hardCollides())) { // Paper - send full pos for hard colliding entities to prevent collision problems due to desync
+ if (!this.forceStateResync && !flag6 && this.teleportDelay <= 400 && !this.wasRiding && this.wasOnGround == this.entity.onGround()&& !(io.papermc.paper.configuration.GlobalConfiguration.get().collisions.sendFullPosForHardCollidingEntities && this.entity.hardCollides())) { // Paper - send full pos for hard colliding entities to prevent collision problems due to desync
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());
@@ -240,6 +247,7 @@ public class ServerEntity {
}
this.entity.hasImpulse = false;
+ this.forceStateResync = false; // Paper - fix desync when a player is added to the tracker
}
++this.tickCount;