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, 29 insertions, 0 deletions

diff --git a/patches/server/0620-Update-head-rotation-in-missing-places.patch b/patches/server/0620-Update-head-rotation-in-missing-places.patch
new file mode 100644
index 0000000000..a1c7566dc6
--- /dev/null
+++ b/patches/server/0620-Update-head-rotation-in-missing-places.patch
@@ -0,0 +1,29 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Owen1212055 <[email protected]>
+Date: Mon, 21 Jun 2021 21:55:23 -0400
+Subject: [PATCH] Update head rotation in missing places
+
+In certain areas the player's head rotation could be desynced when teleported/moved.
+This is because bukkit uses a separate head rotation field for yaw.
+This issue only applies to players.
+
+diff --git a/src/main/java/net/minecraft/world/entity/Entity.java b/src/main/java/net/minecraft/world/entity/Entity.java
+index f21bf6c8bbf30c59f1588c2105dbd7f80c07a0f1..e34260c80d19869c9bec5ba67f7ce7db9d13a1f1 100644
+--- a/src/main/java/net/minecraft/world/entity/Entity.java
++++ b/src/main/java/net/minecraft/world/entity/Entity.java
+@@ -1844,6 +1844,7 @@ public abstract class Entity implements SyncedDataHolder, Nameable, EntityAccess
+         this.setXRot(Mth.clamp(pitch, -90.0F, 90.0F) % 360.0F);
+         this.yRotO = this.getYRot();
+         this.xRotO = this.getXRot();
++        this.setYHeadRot(yaw); // Paper - Update head rotation
+     }
+ 
+     public void absMoveTo(double x, double y, double z) {
+@@ -1886,6 +1887,7 @@ public abstract class Entity implements SyncedDataHolder, Nameable, EntityAccess
+         this.setXRot(pitch);
+         this.setOldPosAndRot();
+         this.reapplyPosition();
++        this.setYHeadRot(yaw); // Paper - Update head rotation
+     }
+ 
+     public final void setOldPosAndRot() {