From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Spottedleaf <Spottedleaf@users.noreply.github.com>
Date: Thu, 11 Jun 2020 17:29:42 -0700
Subject: [PATCH] Fix piston physics inconsistency - MC-188840

Pistons invoke physics when they move blocks. The physics can cause
tnt blocks to ignite. However, pistons (when storing the blocks they "moved")
don't actually go back to the world state sometimes to check if something
like that happened. As a result they end up moving the tnt like it was
never ignited. This resulted in the ability to create machines
that can duplicate tnt, called "world eaters".
This patch makes the piston logic retrieve the block state from the world
prevent this from occuring.

This patch also sets the moved pos to air immediately after creating
the moving piston TE. This prevents the block from being updated from
other physics calls by the piston.

Tested against the following tnt duper design:
https://www.youtube.com/watch?v=mS7xxNGhjxs

This patch also affects every type of machine that utilises
this mechanic. For example, dead coral is removed by a physics
update when being moved while it is attached to slimeblocks.

Standard piston machines that don't destroy or modify the
blocks they move by physics updates should be entirely
unaffected.

This patch fixes https://bugs.mojang.com/browse/MC-188840

This patch also fixes rail duping and carpet duping.

diff --git a/src/main/java/com/destroystokyo/paper/PaperConfig.java b/src/main/java/com/destroystokyo/paper/PaperConfig.java
index 06adb1fa670e7d755560abae67d46447d63370f0..3b0f3127bcee8e9290b4640bcd4ec0d17fb43f43 100644
--- a/src/main/java/com/destroystokyo/paper/PaperConfig.java
+++ b/src/main/java/com/destroystokyo/paper/PaperConfig.java
@@ -422,4 +422,9 @@ public class PaperConfig {
         consoleHasAllPermissions = getBoolean("settings.console-has-all-permissions", consoleHasAllPermissions);
     }
 
+    public static boolean allowPistonDuplication;
+    private static void allowPistonDuplication() {
+        allowPistonDuplication = getBoolean("settings.unsupported-settings.allow-piston-duplication", config.getBoolean("settings.unsupported-settings.allow-tnt-duplication", false));
+        set("settings.unsupported-settings.allow-tnt-duplication", null);
+    }
 }
diff --git a/src/main/java/net/minecraft/server/BlockPiston.java b/src/main/java/net/minecraft/server/BlockPiston.java
index 6b1253fe7e8d4bb71d4d19c063d3e84c167c4d7b..0fb6debc0cd5f522614f32f3df1b5f37a415a0f5 100644
--- a/src/main/java/net/minecraft/server/BlockPiston.java
+++ b/src/main/java/net/minecraft/server/BlockPiston.java
@@ -368,12 +368,24 @@ public class BlockPiston extends BlockDirectional {
             }
 
             for (k = list.size() - 1; k >= 0; --k) {
-                blockposition3 = (BlockPosition) list.get(k);
-                iblockdata1 = world.getType(blockposition3);
+                // Paper start - fix a variety of piston desync dupes
+                boolean allowDesync = com.destroystokyo.paper.PaperConfig.allowPistonDuplication;
+                BlockPosition oldPos = blockposition3 = (BlockPosition) list.get(k);
+                iblockdata1 = allowDesync ? world.getType(oldPos) : null;
+                // Paper end - fix a variety of piston desync dupes
                 blockposition3 = blockposition3.shift(enumdirection1);
                 map.remove(blockposition3);
                 world.setTypeAndData(blockposition3, (IBlockData) Blocks.MOVING_PISTON.getBlockData().set(BlockPiston.FACING, enumdirection), 68);
+                // Paper start - fix a variety of piston desync dupes
+                if (!allowDesync) {
+                    iblockdata1 = world.getType(oldPos);
+                    map.replace(oldPos, iblockdata1);
+                }
                 world.setTileEntity(blockposition3, BlockPistonMoving.a((IBlockData) list1.get(k), enumdirection, flag, false));
+                if (!allowDesync) {
+                    world.setTypeAndData(oldPos, Blocks.AIR.getBlockData(), 2 | 4 | 16 | 1024); // set air to prevent later physics updates from seeing this block
+                }
+                // Paper end - fix a variety of piston desync dupes
                 aiblockdata[j++] = iblockdata1;
             }
 
diff --git a/src/main/java/net/minecraft/server/TileEntityPiston.java b/src/main/java/net/minecraft/server/TileEntityPiston.java
index 5b941321a7fdc561e6b794a1dce5d600083c505d..e7b7e468fc874b0fbcd43f87d816a42420f9b05e 100644
--- a/src/main/java/net/minecraft/server/TileEntityPiston.java
+++ b/src/main/java/net/minecraft/server/TileEntityPiston.java
@@ -256,7 +256,7 @@ public class TileEntityPiston extends TileEntity implements ITickable {
                 IBlockData iblockdata = Block.b(this.a, (GeneratorAccess) this.world, this.position);
 
                 if (iblockdata.isAir()) {
-                    this.world.setTypeAndData(this.position, this.a, 84);
+                    this.world.setTypeAndData(this.position, this.a, com.destroystokyo.paper.PaperConfig.allowPistonDuplication ? 84 : (84 | 2)); // Paper - force notify (flag 2), it's possible the set type by the piston block (which doesn't notify) set this block to air
                     Block.a(this.a, iblockdata, this.world, this.position, 3);
                 } else {
                     if (iblockdata.b(BlockProperties.C) && (Boolean) iblockdata.get(BlockProperties.C)) {