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, 20 insertions, 0 deletions

diff --git a/patches/server/0657-Use-a-CHM-for-StructureTemplate.Pallete-cache.patch b/patches/server/0657-Use-a-CHM-for-StructureTemplate.Pallete-cache.patch
new file mode 100644
index 0000000000..82d25835a6
--- /dev/null
+++ b/patches/server/0657-Use-a-CHM-for-StructureTemplate.Pallete-cache.patch
@@ -0,0 +1,20 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Shane Freeder <[email protected]>
+Date: Mon, 12 Jul 2021 12:28:29 +0100
+Subject: [PATCH] Use a CHM for StructureTemplate.Pallete cache
+
+fixes a CME due to this collection being shared across threads
+
+diff --git a/src/main/java/net/minecraft/world/level/levelgen/structure/templatesystem/StructureTemplate.java b/src/main/java/net/minecraft/world/level/levelgen/structure/templatesystem/StructureTemplate.java
+index 02b493435509f7a84a2c26050ca0c732ecf00ca0..33564e62e3181d28b18a957e28b8ec5152d8339f 100644
+--- a/src/main/java/net/minecraft/world/level/levelgen/structure/templatesystem/StructureTemplate.java
++++ b/src/main/java/net/minecraft/world/level/levelgen/structure/templatesystem/StructureTemplate.java
+@@ -859,7 +859,7 @@ public class StructureTemplate {
+     public static final class Palette {
+ 
+         private final List<StructureTemplate.StructureBlockInfo> blocks;
+-        private final Map<Block, List<StructureTemplate.StructureBlockInfo>> cache = Maps.newHashMap();
++        private final Map<Block, List<StructureTemplate.StructureBlockInfo>> cache = Maps.newConcurrentMap(); // Paper - Fix CME due to this collection being shared across threads
+ 
+         Palette(List<StructureTemplate.StructureBlockInfo> infos) {
+             this.blocks = infos;