From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Jake Potrebic <jake.m.potrebic@gmail.com>
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 <zach@zachbr.io>
Co-authored-by: Madeline Miller <mnmiller1@me.com>

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 24a822ade17849a083161216c184f02c30b5cb1f..f456ad8a74464414f69b616a48ee9a2c1cee4d90 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
@@ -146,15 +146,15 @@ 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 - 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());
@@ -236,6 +236,13 @@ public class PistonBaseBlock extends DirectionalBlock {
 
             BlockState iblockdata1 = (BlockState) ((BlockState) Blocks.MOVING_PISTON.defaultBlockState().setValue(MovingPistonBlock.FACING, enumdirection)).setValue(MovingPistonBlock.TYPE, this.isSticky ? PistonType.STICKY : PistonType.DEFAULT);
 
+            // Paper start - 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
             world.setBlock(pos, iblockdata1, 20);
             world.setBlockEntity(MovingPistonBlock.newMovingBlockEntity(pos, iblockdata1, (BlockState) this.defaultBlockState().setValue(PistonBaseBlock.FACING, Direction.from3DDataValue(data & 7)), enumdirection, false, true)); // Paper - diff on change
             world.blockUpdated(pos, iblockdata1.getBlock());
@@ -262,6 +269,13 @@ public class PistonBaseBlock extends DirectionalBlock {
                     if (type == 1 && !iblockdata2.isAir() && PistonBaseBlock.isPushable(iblockdata2, world, blockposition1, enumdirection.getOpposite(), false, enumdirection) && (iblockdata2.getPistonPushReaction() == PushReaction.NORMAL || iblockdata2.is(Blocks.PISTON) || iblockdata2.is(Blocks.STICKY_PISTON))) {
                         this.moveBlocks(world, pos, enumdirection, false);
                     } else {
+                        // Paper start - 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
                         world.removeBlock(pos.relative(enumdirection), false);
                     }
                 }
