From 09f0f1e10341b6b3ff895713bb2f67cceec79e7d 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/server/BlockPiston.java b/src/main/java/net/minecraft/server/BlockPiston.java
index 141006e36e..78603d2cb5 100644
--- a/src/main/java/net/minecraft/server/BlockPiston.java
+++ b/src/main/java/net/minecraft/server/BlockPiston.java
@@ -121,7 +121,7 @@ public class BlockPiston extends BlockDirectional {
             }
 
             // CraftBukkit start
-            if (!this.sticky) {
+            //if (!this.sticky) { // Paper - Prevents empty sticky pistons from firing retract - history behind is odd
                 org.bukkit.block.Block block = world.getWorld().getBlockAt(blockposition.getX(), blockposition.getY(), blockposition.getZ());
                 BlockPistonRetractEvent event = new BlockPistonRetractEvent(block, ImmutableList.<org.bukkit.block.Block>of(), CraftBlock.notchToBlockFace(enumdirection));
                 world.getServer().getPluginManager().callEvent(event);
@@ -129,7 +129,7 @@ public class BlockPiston extends BlockDirectional {
                 if (event.isCancelled()) {
                     return;
                 }
-            }
+            //} // Paper
             // PAIL: checkME - what happened to setTypeAndData?
             // CraftBukkit end
             world.playBlockAction(blockposition, this, b0, enumdirection.a());
-- 
2.21.0