The bug
As grindstones have no inventory (or similar), and do not involve obsidian, I would have expected pistons to move them like crafting tables and the like.
To reproduce
Place a grindstone and place a piston facing it
Power the piston
Actual behavior
The piston cannot push the grindstone.
Code analysis
Code analysis by @unknown can be found in this comment.
Linked issues
Attachments
Comments 22
I think this is WAI. Grindstones have a GUI that you can put items in, but go into the player's inventory after exiting the GUI, just like anvils, which also can't be pushed by pistons.
Crafting tables are technically full blocks.
Another argument would be that Anvils and Grindstones aren't solid blocks, and thus can't be pushed, although slabs can be pushed by pistons. However, there are other block entities that don't take up a full block space, such as hoppers, chests, and brewing stands. You obviously can't store items inside a slab.
Anvils and grindstones are not block entities. Anvils are intentionally immovable, as they are too heavy to be pushed. I figure they applied the same idea to the grindstone (if this is intended).
Hoppers, chests, and brewing stands are all block entities, which cannot be pushed due to technical limitations.
Whether a block is full or not has nothing to do with it being movable.
Unfortunately this is not a bug, and tile entities in java cannot be moved by pistons. Hopefully mojang changes this one day.
Grindstones aren't block entities, and this has priority set, so it has been triaged (and will likely get fixed).
It will never be fixed. It's enough to change just one word in their codebase to resolve this issue (did it myself and it works perfectly fine). If it hasn't been done in 4 years, it's likely not to get fixed ever ¯_(ツ)_/¯
Can confirm in 1.20.5
Either grindstones should be movable like crafting benches or they should have an inventory to interact with hoppers like furnaces do.
May relate to MC-140322 (Grindstone resistant to any explosion)