Crash if calling minetest.pointed_thing_to_face_pos and pointed_thing.above and pointed_thing.below are equal

[Wuzzy2]

Tested in MT 0.4.16.

Just call minetest.pointed_thing_to_face_pos with a pointed_thing of type="node", and above and below having equal positions.

There will be crash with this error:

ERROR: /usr/share/minetest/builtin/common/misc_helpers.lua:717: Attempt to perform arithmetic on a nil value

Example:
minetest.pointed_thing_to_face_pos(player, {type="node", above={x=0,y=0,z=0}, below={x=0,y=0,z=0})

How can above and below have the same positions, you ask? Easy: Just place a node when your head is inside a non-walkable and buildable_to node (e.g. jungle grass). Then you will get a pointed_thing in which both above and below have equal positions. At least in the on_place function of the item you place.

In Minetest Game 0.4.16, you can reproduce this if you stand inside jungle grass, fly and noclip a bit down (so you are inside the same node), then place a slab inside that jungle grass.

[paramat]

Was reported again (see below), should be an easy fix, if they're the same return a certain fixed position on the face.

2017-12-14 20:20:59: ERROR[Main]: ServerError: AsyncErr: ServerThread::run Lua: Runtime error from mod 'stairs' in callback item_OnPlace(): /usr/local/share/minetest/builtin/common/misc_helpers.lua:717: attempt to perform arithmetic on a nil value
2017-12-14 20:20:59: ERROR[Main]: Stack Traceback
2017-12-14 20:20:59: ERROR[Main]: ===============
2017-12-14 20:20:59: ERROR[Main]: (2) Lua field 'pointed_thing_to_face_pos' at file '/usr/local/share/minetest/builtin/common/misc_helpers.lua:717'
2017-12-14 20:20:59: ERROR[Main]: 	Local variables:
2017-12-14 20:20:59: ERROR[Main]: 	 placer = userdata: 0x00ee3698
2017-12-14 20:20:59: ERROR[Main]: 	 pointed_thing = table: 0x01a65678  {type:node, above:table: 0x01a65768, under:table: 0x01a656a0}
2017-12-14 20:20:59: ERROR[Main]: 	 eye_offset_first = table: 0x01a65980  {y:0, x:0, z:0}
2017-12-14 20:20:59: ERROR[Main]: 	 node_pos = table: 0x01a656a0  {y:26, x:-9948, z:7638}
2017-12-14 20:20:59: ERROR[Main]: 	 camera_pos = table: 0x01a65aa0  {y:24.121999740601, x:-9948.0546875, z:7638.46875}
2017-12-14 20:20:59: ERROR[Main]: 	 pos_off = table: 0x01a65bc0  {y:0, x:0, z:0}
2017-12-14 20:20:59: ERROR[Main]: 	 look_dir = table: 0x01a65c50  {y:0.47101190686226, x:-0.50647139549255, z:-0.72224271297455}
2017-12-14 20:20:59: ERROR[Main]: 	 offset = nil
2017-12-14 20:20:59: ERROR[Main]: 	 nc = nil
2017-12-14 20:20:59: ERROR[Main]: 	 oc = table: 0x01a65ce0  {1:y, 2:x, 3:z}
2017-12-14 20:20:59: ERROR[Main]: 	 (*temporary) = table: 0x01a65d08  {}
2017-12-14 20:20:59: ERROR[Main]: 	 (*temporary) = nil
2017-12-14 20:20:59: ERROR[Main]: 	 (*temporary) = number: nan
2017-12-14 20:20:59: ERROR[Main]: 	 (*temporary) = string: "z"
2017-12-14 20:20:59: ERROR[Main]: 	 (*temporary) = number: 0
2017-12-14 20:20:59: ERROR[Main]: 	 (*temporary) = number: 3
2017-12-14 20:20:59: ERROR[Main]: 	 (*temporary) = number: nan
2017-12-14 20:20:59: ERROR[Main]: 	 (*temporary) = string: "jump"
2017-12-14 20:20:59: ERROR[Main]: 	 (*temporary) = boolean: false
2017-12-14 20:20:59: ERROR[Main]: 	 (*temporary) = string: "attempt to perform arithmetic on a nil value"
2017-12-14 20:20:59: ERROR[Main]: (3) Lua function 'rotate' at file '/home/UGX/.minetest/games/UGXrealms/mods/minetest_game/stairs/init.lua:30' (best guess)
2017-12-14 20:20:59: ERROR[Main]: 	Local variables:
2017-12-14 20:20:59: ERROR[Main]: 	 itemstack = userdata: 0x01a65668
2017-12-14 20:20:59: ERROR[Main]: 	 placer = userdata: 0x00ee3698
2017-12-14 20:20:59: ERROR[Main]: 	 pointed_thing = table: 0x01a65678  {type:node, above:table: 0x01a65768, under:table: 0x01a656a0}
2017-12-14 20:20:59: ERROR[Main]: 	 p0 = table: 0x01a656a0  {y:26, x:-9948, z:7638}
2017-12-14 20:20:59: ERROR[Main]: 	 p1 = table: 0x01a65768  {y:26, x:-9948, z:7638}
2017-12-14 20:20:59: ERROR[Main]: 	 param2 = number: 2
2017-12-14 20:20:59: ERROR[Main]: 	 placer_pos = table: 0x01a65860  {y:24.121999740601, x:-9948.0546875, z:7638.46875}

[SmallJoker]

@MarkuBu
As you wrote this function - could you please check whether the next face position in the direction of view can be returned here?

[MarkuBu]

If you stand inside jungle grass you can't place nodes at all because you are inside the hitbox and you point to this hitbox and not to the node the crosshair points to.

All you can do is check above and below if they are equal.

Off topic: I think anyway that the hitbox of jungle grass is too big. Every time you walk through a jungle and try to punch or dig something you often hit jungle grass first. Will open an issue soon

[MarkuBu]

Just place a node when your head is inside a non-walkable and buildable_to node (e.g. jungle grass)

To be honest I can't confirm this bug. I placed jungle grass, step inside and try to place a slab somewhere. Nothing happened.

[SmallJoker]

You can reproduce it by standing inside a full node with a dry shrub or jungle grass above. That's also what Wuzzy described in the first post.

[MarkuBu]

How can I stand in a full node in a regular game? As long as "noclip" is disabled or you "glitch" into a node somehow

However, best would be to add a check if above and below are equal. I won't try to find a solution to make such a rare and uncommon situation work.

[SmallJoker]

I won't try to find a solution to make such a rare and uncommon situation work.

minetest/minetest.conf.example

Lines 19 to 22 in f1d2bc0

	# If enabled, you can place blocks at the position (feet + eye level) where you stand.
	# This is helpful when working with nodeboxes in small areas.
	# type: bool
	# enable_build_where_you_stand = false

Players can crash servers by turning this setting off and placing the nodes to get this condition. Is that important enough to fix it?

[MarkuBu]

I already posted a solution. Check if above and below are equal. If yes, return and do nothing.

I'm not at home and can try it myself

And I didn't develop this function. The original function was from HybridDog, if I'm not wrong. I just adopted it to make it work for this situation

[MarkuBu]

Two suggestions to solve this issue:

function core.pointed_thing_to_face_pos(placer, pointed_thing)
	if vector.equals(pointed_thing.above, pointed_thing.under) then
		return vector.new()
	end

If you return vector.new() a slab or stair will be placed on top of the node the player currently stands in. The jungle grass or whatever node is on top will be replaced

instead of return a new vector yo can return nil. But than you need to catch the nil in init.lua from the stairs mod

		local finepos = minetest.pointed_thing_to_face_pos(placer, pointed_thing)
		if not finepos then
			return nil
		end
		local fpos = finepos.y % 1

In this case only the place sound is played, but nothing else happened.

[paramat]

Still crashing, will fix.

[paramat]

PR #7342
Best to return a vector similar to what would normally be returned, so returned 'under'.

[paramat]

#7342 merged.