Comparators do not update correctly in some cases

This is a compilation of instances were comparators do not update their power level correctly when a block with a comparator input override changed.

Affected blocks

• beehive / bee nest

  • placed (behind block)

  • broken (behind block)

• brewing stand

  • placed (behind block) using command /setblock x y z brewing_stand{Items:[{Slot:0b,id:"potion",Count:1b}]}

  • broken (behind block)

• cake

  • placed (behind block) using command /setblock x y z cake (Does update if placed manually)

  • broken (behind block)

  • completely eaten up (behind block)

• cauldron

  • placed (behind block) using command /setblock x y z cauldron[level=3]

  • removed (behind block)

  • filled by rain

• chest

  • placed (behind block) using command /setblock x y z chest{Items:[{Slot:0b,id:"stone",Count:1b}]}

• command block

  • placed (behind block) using command /setblock x y z command_block{SuccessCount:1}

  • broken (behind block)

• compost

  • removed (behind block)

• detector rail

  • broken (behind block)

• dispenser

  • placed (behind block) using command /setblock x y z dispenser{Items:[{Slot:0b,id:"stone",Count:1b}]}

• dropper

  • placed (behind block) using command /setblock x y z dropper{Items:[{Slot:0b,id:"stone",Count:1b}]}

• end portal frame

  • placed (behind block) using command /setblock x y z end_portal_frame[eye=true]

  • broken (behind block)

• furnace

  • placed (behind block) using command /setblock x y z furnace{Items:[{Slot:0b,id:"iron_ore",Count:1b}]}

• hopper

  • placed (behind block) using command /setblock x y z hopper{Items:[{Slot:0b,id:"stone",Count:1b}]}

• item frame

  • broken (by piston or command, left click would first remove the item)

  • In case MC-45619 is intended: If a block inside of the item frame is removed

• jukebox

  • broken (behind block)

• respawn anchors

  • explodes (behind block)

• sculk sensors

  • broken (behind block) → MC-208715

• cats -> MC-273741

  • standing up

  • sitting down

Explanations:

• (behind block): The comparator must detect the affected block through a solid block to not update in the described situation.

• placed: Comparator does not update when the affected block is placed.

• broken: Comparator does not update when the affected block is broken. Also applies for being moved away by a piston.

• changed: Comparator does not update when the affected block changes its comparator input override.

Here is a video with a few examples when this bug happens. Note that it doesn't make any difference whether a block is moved by a piston or removed by a player. Some blocks however cannot be moved by pistons.

https://youtu.be/OZ_P68JvPQQ

That's every block and issue I could find related to this. If I missed something, please let me know down below.

Code Analysis

This is based on a decompiled version of Minecraft 1.12 using mcp 980.

Breaking/Moving block

In net.minecraft.world.World.setBlockState(BlockPos, IBlockState, int) there is some code that is meant to update comparators when a block with a comparator input override is placed and the flag for block updates is set. In there, you could also cause updates if a block with comparator input override is removed, however there are some cases where the update flag is not set and instead the net.minecraft.world.World.notifyNeighborsOfStateChange(BlockPos, Block, boolean) method is called instead.
So my proposed fix would be to remove the following

net.minecraft.world.World.setBlockState(BlockPos, IBlockState, int)

if (!this.isRemote && (flags & 1) != 0)
{
    this.notifyNeighborsRespectDebug(pos, iblockstate.getBlock(), true);

    //remove this
    if (newState.hasComparatorInputOverride())
    {
        this.updateComparatorOutputLevel(pos, block);
    }
}

and instead add this to net.minecraft.world.World.notifyNeighborsOfStateChange(BlockPos, Block, boolean)

net.minecraft.world.World.notifyNeighborsOfStateChange(BlockPos, Block, boolean)

IBlockState newState = getBlockState(pos);
if(blockType.getDefaultState().hasComparatorInputOverride() || newState.hasComparatorInputOverride())
{
	updateComparatorOutputLevel(pos, newState.getBlock());
}

Some code that becomes redundant after applying that fix also should be removed. That would be to remove worldIn.updateComparatorOutputLevel(pos, this); :in the following methodes

• net.minecraft.block.BlockChest.breakBlock(World, BlockPos, IBlockState)

• net.minecraft.block.BlockDispenser.breakBlock(World, BlockPos, IBlockState)

• net.minecraft.block.BlockFurnace.breakBlock(World, BlockPos, IBlockState)

• net.minecraft.block.BlockHopper.breakBlock(World, BlockPos, IBlockState)

• net.minecraft.block.BlockShulkerBox.breakBlock(World, BlockPos, IBlockState)

An alternative would be to override the net.minecraft.block.Block.breakBlock(World, BlockPos, IBlockState) in all the classes that have a comparator input override as proposed by @unknown in this comment. But it might be cleaner to apply the fix above since you don't need to remember to override the method when adding a new block with a comparator override.

Another alternative might be to add a class that all blocks with a comparator input override need to extend in which the net.minecraft.block.Block.breakBlock(World, BlockPos, IBlockState) method is already overridden.

Cauldron being filled by rain

Code analysis by @unknown can be found in this comment