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 f0284e81db3ab7c45018de2b446f2d8296df15c3..8444819f071b13e98ba07032520016a664b7b9bc 100644
--- a/src/main/java/com/destroystokyo/paper/PaperConfig.java
+++ b/src/main/java/com/destroystokyo/paper/PaperConfig.java
@@ -442,4 +442,9 @@ 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 39cd8ab5925ceb9494e0ac910c73338c24ecda2c..73ac83832121e0390b7c4649681fc247e3d8c30b 100644
--- a/src/main/java/net/minecraft/server/BlockPiston.java
+++ b/src/main/java/net/minecraft/server/BlockPiston.java
@@ -376,12 +376,24 @@ 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
                 --j;
                 aiblockdata[j] = iblockdata1;
             }
diff --git a/src/main/java/net/minecraft/server/TileEntityPiston.java b/src/main/java/net/minecraft/server/TileEntityPiston.java
index 489175abd8e582a3c082364fec357c4f061a22d7..d700e8281fe50b1c4131ac260ff6c0d0dd8412f0 100644
--- a/src/main/java/net/minecraft/server/TileEntityPiston.java
+++ b/src/main/java/net/minecraft/server/TileEntityPiston.java
@@ -275,7 +275,7 @@ 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((IBlockState) BlockProperties.C) && (Boolean) iblockdata.get(BlockProperties.C)) {