PaperMC/patches/unapplied/server/1056-Remove-wall-time-unused-skip-tick-protection.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Bjarne Koll <lynxplay101@gmail.com>
Date: Mon, 16 Sep 2024 23:07:29 +0200
Subject: [PATCH] Remove wall-time / unused skip tick protection

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.

diff --git a/src/main/java/net/minecraft/world/entity/item/ItemEntity.java b/src/main/java/net/minecraft/world/entity/item/ItemEntity.java
index 03cfa29bdb426a9fb6b1b6be6e897da48d4f2f3e..4423973d4d9a2c3879d98d1d4c8b8c117c677ac5 100644
--- a/src/main/java/net/minecraft/world/entity/item/ItemEntity.java
+++ b/src/main/java/net/minecraft/world/entity/item/ItemEntity.java
@@ -60,7 +60,7 @@ public class ItemEntity extends Entity implements TraceableEntity {
     @Nullable
     public UUID target;
     public final float bobOffs;
-    private int lastTick = MinecraftServer.currentTick - 1; // CraftBukkit
+    // private int lastTick = MinecraftServer.currentTick - 1; // CraftBukkit // Paper - remove anti tick skipping measures / wall time
     public boolean canMobPickup = true; // Paper - Item#canEntityPickup
     private int despawnRate = -1; // Paper - Alternative item-despawn-rate
     public net.kyori.adventure.util.TriState frictionState = net.kyori.adventure.util.TriState.NOT_SET; // Paper - Friction API
@@ -153,12 +153,11 @@ public class ItemEntity extends Entity implements TraceableEntity {
             this.discard(EntityRemoveEvent.Cause.DESPAWN); // CraftBukkit - add Bukkit remove cause
         } else {
             super.tick();
-            // CraftBukkit start - Use wall time for pickup and despawn timers
-            int elapsedTicks = MinecraftServer.currentTick - this.lastTick;
-            if (this.pickupDelay != 32767) this.pickupDelay -= elapsedTicks;
-            if (this.age != -32768) this.age += elapsedTicks;
-            this.lastTick = MinecraftServer.currentTick;
-            // CraftBukkit end
+            // Paper start - remove anti tick skipping measures / wall time - revert to vanilla
+            if (this.pickupDelay > 0 && this.pickupDelay != 32767) {
+                --this.pickupDelay;
+            }
+            // Paper end - remove anti tick skipping measures / wall time - revert to vanilla
 
             this.xo = this.getX();
             this.yo = this.getY();
@@ -211,7 +210,7 @@ public class ItemEntity extends Entity implements TraceableEntity {
                 this.mergeWithNeighbours();
             }
 
-            /* CraftBukkit start - moved up
+            // Paper - remove anti tick skipping measures / wall time - revert to vanilla /* CraftBukkit start - moved up
             if (this.age != -32768) {
                 ++this.age;
             }
@@ -242,12 +241,14 @@ public class ItemEntity extends Entity implements TraceableEntity {
     // Spigot start - copied from above
     @Override
     public void inactiveTick() {
-        // CraftBukkit start - Use wall time for pickup and despawn timers
-        int elapsedTicks = MinecraftServer.currentTick - this.lastTick;
-        if (this.pickupDelay != 32767) this.pickupDelay -= elapsedTicks;
-        if (this.age != -32768) this.age += elapsedTicks;
-        this.lastTick = MinecraftServer.currentTick;
-        // CraftBukkit end
+        // Paper start - remove anti tick skipping measures / wall time - copied from above
+        if (this.pickupDelay > 0 && this.pickupDelay != 32767) {
+            --this.pickupDelay;
+        }
+        if (this.age != -32768) {
+            ++this.age;
+        }
+        // Paper end - remove anti tick skipping measures / wall time - copied from above
 
         if (!this.level().isClientSide && this.age >= this.despawnRate) { // Spigot // Paper - Alternative item-despawn-rate
             // CraftBukkit start - fire ItemDespawnEvent
diff --git a/src/main/java/net/minecraft/world/entity/monster/Zombie.java b/src/main/java/net/minecraft/world/entity/monster/Zombie.java
index 2280004638fd19ed018cb3e77d53a018b34ec516..2b43337ac63d051718a2074fcc46e128a1d65129 100644
--- a/src/main/java/net/minecraft/world/entity/monster/Zombie.java
+++ b/src/main/java/net/minecraft/world/entity/monster/Zombie.java
@@ -96,7 +96,7 @@ public class Zombie extends Monster {
     private boolean canBreakDoors;
     private int inWaterTime;
     public int conversionTime;
-    private int lastTick = MinecraftServer.currentTick; // CraftBukkit - add field
+    // private int lastTick = MinecraftServer.currentTick; // CraftBukkit - add field // Paper - remove anti tick skipping measures / wall time
     private boolean shouldBurnInDay = true; // Paper - Add more Zombie API
 
     public Zombie(EntityType<? extends Zombie> type, Level world) {
@@ -219,10 +219,7 @@ public class Zombie extends Monster {
     public void tick() {
         if (!this.level().isClientSide && this.isAlive() && !this.isNoAi()) {
             if (this.isUnderWaterConverting()) {
-                // CraftBukkit start - Use wall time instead of ticks for conversion
-                int elapsedTicks = MinecraftServer.currentTick - this.lastTick;
-                this.conversionTime -= elapsedTicks;
-                // CraftBukkit end
+                --this.conversionTime; // Paper - remove anti tick skipping measures / wall time
                 if (this.conversionTime < 0) {
                     this.doUnderWaterConversion();
                 }
@@ -239,7 +236,7 @@ public class Zombie extends Monster {
         }
 
         super.tick();
-        this.lastTick = MinecraftServer.currentTick; // CraftBukkit
+        // this.lastTick = MinecraftServer.currentTick; // CraftBukkit // Paper - remove anti tick skipping measures / wall time
     }
 
     @Override
@@ -280,7 +277,7 @@ public class Zombie extends Monster {
     }
     // Paper end - Add more Zombie API
     public void startUnderWaterConversion(int ticksUntilWaterConversion) {
-        this.lastTick = MinecraftServer.currentTick; // CraftBukkit
+        // this.lastTick = MinecraftServer.currentTick; // CraftBukkit // Paper - remove anti tick skipping measures / wall time
         this.conversionTime = ticksUntilWaterConversion;
         this.getEntityData().set(Zombie.DATA_DROWNED_CONVERSION_ID, true);
     }
diff --git a/src/main/java/net/minecraft/world/level/block/entity/BrewingStandBlockEntity.java b/src/main/java/net/minecraft/world/level/block/entity/BrewingStandBlockEntity.java
index a2fafef89d5354e2cb02f5672810909950a57777..bf2c303a314205590a2839e0f729af3a9ff40a86 100644
--- a/src/main/java/net/minecraft/world/level/block/entity/BrewingStandBlockEntity.java
+++ b/src/main/java/net/minecraft/world/level/block/entity/BrewingStandBlockEntity.java
@@ -54,7 +54,7 @@ public class BrewingStandBlockEntity extends BaseContainerBlockEntity implements
     public int fuel;
     protected final ContainerData dataAccess;
     // CraftBukkit start - add fields and methods
-    private int lastTick = MinecraftServer.currentTick;
+    // private int lastTick = MinecraftServer.currentTick; // Paper - remove anti tick skipping measures / wall time
     public List<HumanEntity> transaction = new java.util.ArrayList<HumanEntity>();
     private int maxStack = MAX_STACK;
 
@@ -169,12 +169,10 @@ public class BrewingStandBlockEntity extends BaseContainerBlockEntity implements
         boolean flag1 = blockEntity.brewTime > 0;
         ItemStack itemstack1 = (ItemStack) blockEntity.items.get(3);
 
-        // CraftBukkit start - Use wall time instead of ticks for brewing
-        int elapsedTicks = MinecraftServer.currentTick - blockEntity.lastTick;
-        blockEntity.lastTick = MinecraftServer.currentTick;
+        // Paper - remove anti tick skipping measures / wall time
 
         if (flag1) {
-            blockEntity.brewTime -= elapsedTicks;
+            --blockEntity.brewTime; // Paper - remove anti tick skipping measures / wall time - revert to vanilla
             boolean flag2 = blockEntity.brewTime <= 0; // == -> <=
             // CraftBukkit end