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/net/minecraft/world/level/block/piston/PistonBaseBlock.java b/src/main/java/net/minecraft/world/level/block/piston/PistonBaseBlock.java
index 530b6af1ca2e0ac81c0a8a55dbc7cf5c796c93ce..4aa34b7df734bb755906b228e0df9eb629569ea0 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
@@ -414,14 +414,26 @@ public class PistonBaseBlock extends DirectionalBlock {
             }
 
             for (j = list.size() - 1; j >= 0; --j) {
-                blockposition3 = (BlockPos) list.get(j);
-                iblockdata1 = world.getBlockState(blockposition3);
+                // Paper start - fix a variety of piston desync dupes
+                boolean allowDesync = io.papermc.paper.configuration.GlobalConfiguration.get().unsupportedSettings.allowPistonDuplication;
+                BlockPos oldPos = blockposition3 = (BlockPos) list.get(j);
+                iblockdata1 = allowDesync ? world.getBlockState(oldPos) : null;
+                // Paper end - fix a variety of piston desync dupes
                 blockposition3 = blockposition3.relative(enumdirection1);
                 map.remove(blockposition3);
                 BlockState iblockdata2 = (BlockState) Blocks.MOVING_PISTON.defaultBlockState().setValue(PistonBaseBlock.FACING, dir);
 
                 world.setBlock(blockposition3, iblockdata2, 68);
-                world.setBlockEntity(MovingPistonBlock.newMovingBlockEntity(blockposition3, iblockdata2, (BlockState) list1.get(j), dir, retract, false));
+                // Paper start - fix a variety of piston desync dupes
+                if (!allowDesync) {
+                    iblockdata1 = world.getBlockState(oldPos);
+                    map.replace(oldPos, iblockdata1);
+                }
+                world.setBlockEntity(MovingPistonBlock.newMovingBlockEntity(blockposition3, iblockdata2, allowDesync ? list1.get(j) : iblockdata1, dir, retract, false));
+                if (!allowDesync) {
+                    world.setBlock(oldPos, Blocks.AIR.defaultBlockState(), Block.UPDATE_CLIENTS | Block.UPDATE_KNOWN_SHAPE | Block.UPDATE_MOVE_BY_PISTON | 1024); // set air to prevent later physics updates from seeing this block
+                }
+                // Paper end - fix a variety of piston desync dupes
                 aiblockdata[i++] = iblockdata1;
             }
 
diff --git a/src/main/java/net/minecraft/world/level/block/piston/PistonMovingBlockEntity.java b/src/main/java/net/minecraft/world/level/block/piston/PistonMovingBlockEntity.java
index 8ea7d7da26e7d5644a8ee1a8d9e4c828c6b70a00..d7b963571c900f0f68005d6954bcd9ef1d9e0b7c 100644
--- a/src/main/java/net/minecraft/world/level/block/piston/PistonMovingBlockEntity.java
+++ b/src/main/java/net/minecraft/world/level/block/piston/PistonMovingBlockEntity.java
@@ -297,7 +297,7 @@ public class PistonMovingBlockEntity extends BlockEntity {
                 if (world.getBlockState(pos).is(Blocks.MOVING_PISTON)) {
                     BlockState blockState = Block.updateFromNeighbourShapes(blockEntity.movedState, world, pos);
                     if (blockState.isAir()) {
-                        world.setBlock(pos, blockEntity.movedState, 84);
+                        world.setBlock(pos, blockEntity.movedState, io.papermc.paper.configuration.GlobalConfiguration.get().unsupportedSettings.allowPistonDuplication ? 84 : (84 | Block.UPDATE_CLIENTS)); // Paper - fix a variety of piston desync dupes; force notify (flag 2), it's possible the set type by the piston block (which doesn't notify) set this block to air
                         Block.updateOrDestroy(blockEntity.movedState, blockState, world, pos, 3);
                     } else {
                         if (blockState.hasProperty(BlockStateProperties.WATERLOGGED) && blockState.getValue(BlockStateProperties.WATERLOGGED)) {