Remove wall-time / unused skip tick protection (#11412)

Spigot still maintains some partial implementation of "tick skipping", a
practice in which the MinecraftServer.currentTick field is updated not
by an increment of one per actual tick, but instead set to
System.currentTimeMillis() / 50. This behaviour means that the tracked
tick may "skip" a tick value in case a previous tick took more than the
expected 50ms.

To compensate for this in important paths, spigot/craftbukkit
implements "wall-time". Instead of incrementing/decrementing ticks on
block entities/entities by one for each call to their tick() method,
they instead increment/decrement important values, like
an ItemEntity's age or pickupDelay, by the difference of
`currentTick - lastTick`, where `lastTick` is the value of
`currentTick` during the last tick() call.

These "fixes" however do not play nicely with minecraft's simulation
distance as entities/block entities implementing the above behaviour
would "catch up" their values when moving from a non-ticking chunk to a
ticking one as their `lastTick` value remains stuck on the last tick in
a ticking chunk and hence lead to a large "catch up" once ticked again.

Paper completely removes the "tick skipping" behaviour (See patch
"Further-improve-server-tick-loop"), making the above precautions
completely unnecessary, which also rids paper of the previous described
incompatibility with non-ticking chunks.

1 files changed, 105 insertions, 0 deletions

diff --git a/patches/server/1026-Fix-entity-tracker-desync-when-new-players-are-added.patch b/patches/server/1026-Fix-entity-tracker-desync-when-new-players-are-added.patch
new file mode 100644
index 0000000000..f767b9ab91
--- /dev/null
+++ b/patches/server/1026-Fix-entity-tracker-desync-when-new-players-are-added.patch
@@ -0,0 +1,105 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Spottedleaf <[email protected]>
+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;