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, 31 insertions, 0 deletions

diff --git a/patches/server/0283-Server-Tick-Events.patch b/patches/server/0283-Server-Tick-Events.patch
new file mode 100644
index 0000000000..5c5d419332
--- /dev/null
+++ b/patches/server/0283-Server-Tick-Events.patch
@@ -0,0 +1,31 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Aikar <[email protected]>
+Date: Wed, 27 Mar 2019 22:48:45 -0400
+Subject: [PATCH] Server Tick Events
+
+Fires event at start and end of a server tick
+
+diff --git a/src/main/java/net/minecraft/server/MinecraftServer.java b/src/main/java/net/minecraft/server/MinecraftServer.java
+index 8ecfd7a3daa99dabe796d28d27790fb8b45c628b..ddc3da84c5a55d2cd977fcdb18121351606a6b3c 100644
+--- a/src/main/java/net/minecraft/server/MinecraftServer.java
++++ b/src/main/java/net/minecraft/server/MinecraftServer.java
+@@ -1433,6 +1433,7 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
+         });
+         isOversleep = false;MinecraftTimings.serverOversleep.stopTiming();
+         // Paper end
++        new com.destroystokyo.paper.event.server.ServerTickStartEvent(this.tickCount+1).callEvent(); // Paper - Server Tick Events
+ 
+         ++this.tickCount;
+         this.tickRateManager.tick();
+@@ -1458,6 +1459,11 @@ public abstract class MinecraftServer extends ReentrantBlockableEventLoop<TickTa
+             this.runAllTasks();
+         }
+         // Paper end
++        // Paper start - Server Tick Events
++        long endTime = System.nanoTime();
++        long remaining = (TICK_TIME - (endTime - lastTick)) - catchupTime;
++        new com.destroystokyo.paper.event.server.ServerTickEndEvent(this.tickCount, ((double)(endTime - lastTick) / 1000000D), remaining).callEvent();
++        // Paper end - Server Tick Events
+         this.profiler.push("tallying");
+         long j = Util.getNanos() - i;
+         int k = this.tickCount % 100;