Paper/patches/server/0437-Fix-piston-physics-inconsistency-MC-188840.patch

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 374cb5a2fb8c44b7d914beff5688cf36fc08640c..ebbbffd209c6796bc608992e293035141a122d1f 100644
--- a/src/main/java/com/destroystokyo/paper/PaperConfig.java
+++ b/src/main/java/com/destroystokyo/paper/PaperConfig.java
@@ -450,4 +450,12 @@ public class PaperConfig {
     private static void consoleHasAllPermissions() {
         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/PistonBaseBlock.java b/src/main/java/net/minecraft/world/level/block/piston/PistonBaseBlock.java
index e9b315fb5a7b466e2ac65ae4ae69e893dd992739..da9ae487799e58b196ebf219a62020d0c28adc70 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
@@ -410,14 +410,26 @@ public class PistonBaseBlock extends DirectionalBlock {
             }
 
             for (k = list.size() - 1; k >= 0; --k) {
-                blockposition3 = (BlockPos) list.get(k);
-                iblockdata1 = world.getBlockState(blockposition3);
+                // Paper start - fix a variety of piston desync dupes
+                boolean allowDesync = com.destroystokyo.paper.PaperConfig.allowPistonDuplication;
+                BlockPos oldPos = blockposition3 = (BlockPos) list.get(k);
+                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(k), 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(k) : iblockdata1, dir, retract, false));
+                if (!allowDesync) {
+                    world.setBlock(oldPos, Blocks.AIR.defaultBlockState(), 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/PistonMovingBlockEntity.java b/src/main/java/net/minecraft/world/level/block/piston/PistonMovingBlockEntity.java
index 78d252b829e5c1f19532656a728620852403760c..613db573cef142e0ab1b24dc994677105a253042 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
@@ -285,7 +285,7 @@ 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, 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.updateOrDestroy(blockEntity.movedState, blockState, world, pos, 3);
                     } else {
                         if (blockState.hasProperty(BlockStateProperties.WATERLOGGED) && blockState.getValue(BlockStateProperties.WATERLOGGED)) {