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, 48 insertions, 0 deletions

diff --git a/patches/server/0334-Don-t-move-existing-players-to-world-spawn.patch b/patches/server/0334-Don-t-move-existing-players-to-world-spawn.patch
new file mode 100644
index 0000000000..4c59557ecb
--- /dev/null
+++ b/patches/server/0334-Don-t-move-existing-players-to-world-spawn.patch
@@ -0,0 +1,48 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Aikar <[email protected]>
+Date: Thu, 9 Apr 2020 21:20:33 -0400
+Subject: [PATCH] Don't move existing players to world spawn
+
+This can cause a nasty server lag the spawn chunks are not kept loaded
+or they aren't finished loading yet, or if the world spawn radius is
+larger than the keep loaded range.
+
+By skipping this, we avoid potential for a large spike on server start.
+
+== AT ==
+public net.minecraft.server.level.ServerPlayer fudgeSpawnLocation(Lnet/minecraft/server/level/ServerLevel;)V
+
+diff --git a/src/main/java/net/minecraft/server/level/ServerPlayer.java b/src/main/java/net/minecraft/server/level/ServerPlayer.java
+index 47982f87ce93658c73ee3d7a5f7dd680e2f9aa36..f8d4bd18f98ed914e116d0cc4a80140e4e8d759f 100644
+--- a/src/main/java/net/minecraft/server/level/ServerPlayer.java
++++ b/src/main/java/net/minecraft/server/level/ServerPlayer.java
+@@ -358,7 +358,7 @@ public class ServerPlayer extends net.minecraft.world.entity.player.Player {
+         this.server = server;
+         this.stats = server.getPlayerList().getPlayerStats(this);
+         this.advancements = server.getPlayerList().getPlayerAdvancements(this);
+-        this.moveTo(this.adjustSpawnLocation(world, world.getSharedSpawnPos()).getBottomCenter(), 0.0F, 0.0F);
++        // this.moveTo(this.adjustSpawnLocation(world, world.getSharedSpawnPos()).getBottomCenter(), 0.0F, 0.0F); // Paper - Don't move existing players to world spawn
+         this.updateOptions(clientOptions);
+         this.object = null;
+ 
+@@ -628,7 +628,7 @@ public class ServerPlayer extends net.minecraft.world.entity.player.Player {
+                 position = Vec3.atCenterOf(world.getSharedSpawnPos());
+             }
+             this.setLevel(world);
+-            this.setPos(position);
++            this.setPosRaw(position.x(), position.y(), position.z()); // Paper - don't register to chunks yet
+         }
+         this.gameMode.setLevel((ServerLevel) world);
+     }
+diff --git a/src/main/java/net/minecraft/server/players/PlayerList.java b/src/main/java/net/minecraft/server/players/PlayerList.java
+index 8cd80ea83ddcfd5052c8d8c19d3edb42538d1e15..2cbdcdf0349e7efa797802d0d339d158153690af 100644
+--- a/src/main/java/net/minecraft/server/players/PlayerList.java
++++ b/src/main/java/net/minecraft/server/players/PlayerList.java
+@@ -226,6 +226,7 @@ public abstract class PlayerList {
+         // Paper start - Entity#getEntitySpawnReason
+         if (optional.isEmpty()) {
+             player.spawnReason = org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason.DEFAULT; // set Player SpawnReason to DEFAULT on first login
++            player.moveTo(player.adjustSpawnLocation(worldserver1, worldserver1.getSharedSpawnPos()).getBottomCenter(), 0.0F, 0.0F);
+         }
+         // Paper end - Entity#getEntitySpawnReason
+         player.setServerLevel(worldserver1);