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 e4a3c937950b26e81b89d6ddcf3d6c2794ed934d..c34b34a6be594c502d1cb59703ace8ad8dd96915 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
@@ -426,14 +426,26 @@ 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 17a6327ab7b26dfab38881bbc0689b0b25f8f025..c71690dbc3dc52803945f1608f0ee3ba94146354 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
@@ -288,7 +288,7 @@ 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)) {