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, 78 insertions, 0 deletions

diff --git a/patches/server/0707-Throw-exception-on-world-create-while-being-ticked.patch b/patches/server/0707-Throw-exception-on-world-create-while-being-ticked.patch
new file mode 100644
index 0000000000..562543e709
--- /dev/null
+++ b/patches/server/0707-Throw-exception-on-world-create-while-being-ticked.patch
@@ -0,0 +1,78 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Jake Potrebic <[email protected]>
+Date: Tue, 22 Mar 2022 12:44:30 -0700
+Subject: [PATCH] Throw exception on world create while being ticked
+
+There are no plans to support creating worlds while worlds are
+being ticked themselvess.
+
+diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java
+index 94aa901b77b19445a33d5b2b24ba2e947d2a6aef..11b9eb8d7d5f9ad736f2c6784c5d9e745a355093 100644
+--- a/src/main/java/net/minecraft/server/MinecraftServer.java
++++ b/src/main/java/net/minecraft/server/MinecraftServer.java
+@@ -317,6 +317,7 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
+     public final double[] recentTps = new double[ 3 ];
+     // Spigot end
+     public final io.papermc.paper.configuration.PaperConfigurations paperConfigurations; // Paper - add paper configuration files
++    public boolean isIteratingOverLevels = false; // Paper - Throw exception on world create while being ticked
+ 
+     public static <S extends MinecraftServer> S spin(Function<Thread, S> serverFactory) {
+         AtomicReference<S> atomicreference = new AtomicReference();
+@@ -1580,7 +1581,7 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
+         this.getFunctions().tick();
+         MinecraftTimings.commandFunctionsTimer.stopTiming(); // Spigot // Paper
+         this.profiler.popPush("levels");
+-        Iterator iterator = this.getAllLevels().iterator();
++        //Iterator iterator = this.getAllLevels().iterator(); // Paper - Throw exception on world create while being ticked; moved down
+ 
+         // CraftBukkit start
+         // Run tasks that are waiting on processing
+@@ -1612,6 +1613,8 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
+         // Paper end - Perf: Optimize time updates
+         MinecraftTimings.timeUpdateTimer.stopTiming(); // Spigot // Paper
+ 
++        this.isIteratingOverLevels = true; // Paper - Throw exception on world create while being ticked
++        Iterator iterator = this.getAllLevels().iterator(); // Paper - Throw exception on world create while being ticked; move down
+         while (iterator.hasNext()) {
+             ServerLevel worldserver = (ServerLevel) iterator.next();
+             worldserver.hasPhysicsEvent = org.bukkit.event.block.BlockPhysicsEvent.getHandlerList().getRegisteredListeners().length > 0; // Paper - BlockPhysicsEvent
+@@ -1647,6 +1650,7 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
+             this.profiler.pop();
+             worldserver.explosionDensityCache.clear(); // Paper - Optimize explosions
+         }
++        this.isIteratingOverLevels = false; // Paper - Throw exception on world create while being ticked
+ 
+         this.profiler.popPush("connection");
+         MinecraftTimings.connectionTimer.startTiming(); // Spigot // Paper
+diff --git a/src/main/java/org/bukkit/craftbukkit/CraftServer.java b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+index b09c1359e4bd5f287b9ce3682fd7d9defa8f7e61..50a4b51ff9b05786ac763f8277bb67c247c04d31 100644
+--- a/src/main/java/org/bukkit/craftbukkit/CraftServer.java
++++ b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+@@ -916,6 +916,11 @@ public final class CraftServer implements Server {
+         return new ArrayList<World>(this.worlds.values());
+     }
+ 
++    @Override
++    public boolean isTickingWorlds() {
++        return console.isIteratingOverLevels;
++    }
++
+     public DedicatedPlayerList getHandle() {
+         return this.playerList;
+     }
+@@ -1178,6 +1183,7 @@ public final class CraftServer implements Server {
+     @Override
+     public World createWorld(WorldCreator creator) {
+         Preconditions.checkState(this.console.getAllLevels().iterator().hasNext(), "Cannot create additional worlds on STARTUP");
++        //Preconditions.checkState(!this.console.isIteratingOverLevels, "Cannot create a world while worlds are being ticked"); // Paper - Cat - Temp disable. We'll see how this goes.
+         Preconditions.checkArgument(creator != null, "WorldCreator cannot be null");
+ 
+         String name = creator.name();
+@@ -1354,6 +1360,7 @@ public final class CraftServer implements Server {
+ 
+     @Override
+     public boolean unloadWorld(World world, boolean save) {
++        //Preconditions.checkState(!this.console.isIteratingOverLevels, "Cannot unload a world while worlds are being ticked"); // Paper - Cat - Temp disable. We'll see how this goes.
+         if (world == null) {
+             return false;
+         }