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 7f140333c2e62012fa572c1a061d84432426997f..b67ba8f75e4a3358d7c2462918b85b0bf9b5a922 100644
--- a/src/main/java/com/destroystokyo/paper/PaperConfig.java
+++ b/src/main/java/com/destroystokyo/paper/PaperConfig.java
@@ -434,4 +434,10 @@ public class PaperConfig {
         consoleHasAllPermissions = getBoolean("settings.console-has-all-permissions", consoleHasAllPermissions);
     }
 
+    public static boolean allowPistonDuplication;
+    private static void allowPistonDuplication() {
+        config.set("settings.unsupported-settings.allow-piston-duplication-readme", "This setting controls if player should be able to use TNT duplication, but this also allows duplicating carpet, rails and potentially other items");
+        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/world/level/block/piston/BlockPiston.java b/src/main/java/net/minecraft/world/level/block/piston/BlockPiston.java
index e062fd288098127fae22a55562e0207ceaf50163..8aa51fb207820a7629d50b80ea821ec6cccf8b54 100644
--- a/src/main/java/net/minecraft/world/level/block/piston/BlockPiston.java
+++ b/src/main/java/net/minecraft/world/level/block/piston/BlockPiston.java
@@ -399,12 +399,24 @@ 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);
-                world.setTileEntity(blockposition3, BlockPistonMoving.a((IBlockData) list1.get(k), enumdirection, flag, false));
+                // Paper start - fix a variety of piston desync dupes
+                if (!allowDesync) {
+                    iblockdata1 = world.getType(oldPos);
+                    map.replace(oldPos, iblockdata1);
+                }
+                world.setTileEntity(blockposition3, BlockPistonMoving.a(allowDesync ? list1.get(k) : iblockdata1, 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/world/level/block/piston/TileEntityPiston.java b/src/main/java/net/minecraft/world/level/block/piston/TileEntityPiston.java
index 8d13e60f40e1b760e9e69969dc3f37bc6c70dbe9..e70c3a8c9075b6c0bc73e6488d784dfe3b86e58d 100644
--- a/src/main/java/net/minecraft/world/level/block/piston/TileEntityPiston.java
+++ b/src/main/java/net/minecraft/world/level/block/piston/TileEntityPiston.java
@@ -279,7 +279,7 @@ 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)) {