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, 85 insertions, 0 deletions

diff --git a/patches/server/0641-Fix-sticky-pistons-and-BlockPistonRetractEvent.patch b/patches/server/0641-Fix-sticky-pistons-and-BlockPistonRetractEvent.patch
new file mode 100644
index 0000000000..2ee001a77a
--- /dev/null
+++ b/patches/server/0641-Fix-sticky-pistons-and-BlockPistonRetractEvent.patch
@@ -0,0 +1,85 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Jake Potrebic <[email protected]>
+Date: Wed, 22 Dec 2021 09:51:48 -0800
+Subject: [PATCH] Fix sticky pistons and BlockPistonRetractEvent
+
+There is an explicit check in the handling code for empty pistons that
+prevents sticky pistons from firing the event. However when we look back
+at the history we see that this check was originally added so that ONLY
+sticky pistons would fire the retract event. I'm not sure why.
+https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/commits/1092acbddf07edfa4100bc6824504ac75088e913
+
+Over the course of several updates, the meaning of that field appears to
+have changed from "is NOT sticky" to "is sticky". So now its having the
+opposite effect. Only normal pistons fire the retraction event. And like
+all things in CB, it's just been carried around since.
+
+If we are to believe the history, the correct fix for this issue is to
+flip it so it only fires for sticky pistons, but that puts us in a
+bind. It's already firing for non-sticky pistons, changing it now would
+likely result in breakage. Furthermore, there is little documentation as
+to WHY that was ever intended to be the case.
+
+Instead we opt to remove the check entirely so that the event fires for
+all piston types.
+
+Co-authored-by: Zach Brown <[email protected]>
+Co-authored-by: Madeline Miller <[email protected]>
+
+diff --git a/src/main/java/net/minecraft/world/level/block/piston/PistonBaseBlock.java b/src/main/java/net/minecraft/world/level/block/piston/PistonBaseBlock.java
+index 2f2c9fb65d4cc8bd40303216e03c5c1956305ff4..e6bfbe2588e0c2a1be14e38d654e889d392ad4db 100644
+--- a/src/main/java/net/minecraft/world/level/block/piston/PistonBaseBlock.java
++++ b/src/main/java/net/minecraft/world/level/block/piston/PistonBaseBlock.java
+@@ -160,15 +160,15 @@ public class PistonBaseBlock extends DirectionalBlock {
+             }
+ 
+             // CraftBukkit start
+-            if (!this.isSticky) {
+-                org.bukkit.block.Block block = world.getWorld().getBlockAt(pos.getX(), pos.getY(), pos.getZ());
+-                BlockPistonRetractEvent event = new BlockPistonRetractEvent(block, ImmutableList.<org.bukkit.block.Block>of(), CraftBlock.notchToBlockFace(enumdirection));
+-                world.getCraftServer().getPluginManager().callEvent(event);
+-
+-                if (event.isCancelled()) {
+-                    return;
+-                }
+-            }
++            // if (!this.isSticky) { // Paper - Fix sticky pistons and BlockPistonRetractEvent; Move further down
++            //     org.bukkit.block.Block block = world.getWorld().getBlockAt(pos.getX(), pos.getY(), pos.getZ());
++            //     BlockPistonRetractEvent event = new BlockPistonRetractEvent(block, ImmutableList.<org.bukkit.block.Block>of(), CraftBlock.notchToBlockFace(enumdirection));
++            //     world.getCraftServer().getPluginManager().callEvent(event);
++            //
++            //     if (event.isCancelled()) {
++            //         return;
++            //     }
++            // }
+             // PAIL: checkME - what happened to setTypeAndData?
+             // CraftBukkit end
+             world.blockEvent(pos, this, b0, enumdirection.get3DDataValue());
+@@ -245,6 +245,13 @@ public class PistonBaseBlock extends DirectionalBlock {
+ 
+             BlockState iblockdata2 = (BlockState) ((BlockState) Blocks.MOVING_PISTON.defaultBlockState().setValue(MovingPistonBlock.FACING, enumdirection)).setValue(MovingPistonBlock.TYPE, this.isSticky ? PistonType.STICKY : PistonType.DEFAULT);
+ 
++            // Paper start - Fix sticky pistons and BlockPistonRetractEvent; Move empty piston retract call to fix multiple event fires
++            if (!this.isSticky) {
++                if (!new BlockPistonRetractEvent(CraftBlock.at(world, pos), java.util.Collections.emptyList(), CraftBlock.notchToBlockFace(enumdirection)).callEvent()) {
++                    return false;
++                }
++            }
++            // Paper end - Fix sticky pistons and BlockPistonRetractEvent
+             world.setBlock(pos, iblockdata2, 20);
+             world.setBlockEntity(MovingPistonBlock.newMovingBlockEntity(pos, iblockdata2, (BlockState) this.defaultBlockState().setValue(PistonBaseBlock.FACING, Direction.from3DDataValue(data & 7)), enumdirection, false, true));
+             world.blockUpdated(pos, iblockdata2.getBlock());
+@@ -271,6 +278,13 @@ public class PistonBaseBlock extends DirectionalBlock {
+                     if (type == 1 && !iblockdata3.isAir() && PistonBaseBlock.isPushable(iblockdata3, world, blockposition1, enumdirection.getOpposite(), false, enumdirection) && (iblockdata3.getPistonPushReaction() == PushReaction.NORMAL || iblockdata3.is(Blocks.PISTON) || iblockdata3.is(Blocks.STICKY_PISTON))) {
+                         this.moveBlocks(world, pos, enumdirection, false);
+                     } else {
++                        // Paper start - Fix sticky pistons and BlockPistonRetractEvent; fire BlockPistonRetractEvent for sticky pistons retracting nothing (air)
++                        if (type == TRIGGER_CONTRACT && iblockdata2.isAir()) {
++                            if (!new BlockPistonRetractEvent(CraftBlock.at(world, pos), java.util.Collections.emptyList(), CraftBlock.notchToBlockFace(enumdirection)).callEvent()) {
++                                return false;
++                            }
++                        }
++                        // Paper end - Fix sticky pistons and BlockPistonRetractEvent
+                         world.removeBlock(pos.relative(enumdirection), false);
+                     }
+                 }