From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Zach Brown <zach@zachbr.io>
Date: Thu, 31 Jan 2019 16:33:36 -0500
Subject: [PATCH] Fire BlockPistonRetractEvent for all empty pistons

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.

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 ca9e94b92d26eb6cd3ca6ff4b8cb1cd98aabf4e0..c345bd7542f3ffa09719864887e1516f1182e7e3 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
@@ -147,7 +147,7 @@ public class PistonBaseBlock extends DirectionalBlock {
             }
 
             // CraftBukkit start
-            if (!this.isSticky) {
+            //if (!this.sticky) { // Paper - Prevents empty sticky pistons from firing retract - history behind is odd
                 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);
@@ -155,7 +155,7 @@ public class PistonBaseBlock extends DirectionalBlock {
                 if (event.isCancelled()) {
                     return;
                 }
-            }
+            //} // Paper
             // PAIL: checkME - what happened to setTypeAndData?
             // CraftBukkit end
             world.blockEvent(pos, this, b0, enumdirection.get3DDataValue());