Add the group attached_node

Nodes in this group will be dropped as items if the node under them or the node in the wallmounted direction is not walkable.

builtin/falling.lua

@@ -91,6 +91,48 @@ function spawn_falling_node(p, nodename)
 	obj:get_luaentity():set_node(nodename)
 end
 
+function drop_attached_node(p)
+	local nn = minetest.env:get_node(p).name
+	minetest.env:remove_node(p)
+	for _,item in ipairs(minetest.get_node_drops(nn, "")) do
+		local pos = {
+			x = p.x + math.random(60)/60-0.3,
+			y = p.y + math.random(60)/60-0.3,
+			z = p.z + math.random(60)/60-0.3,
+		}
+		minetest.env:add_item(pos, item)
+	end
+end
+
+function check_attached_node(p, n)
+	local def = minetest.registered_nodes[n.name]
+	local d = {x=0, y=0, z=0}
+	if def.paramtype2 == "wallmounted" then
+		if n.param2 == 0 then
+			d.y = 1
+		elseif n.param2 == 1 then
+			d.y = -1
+		elseif n.param2 == 2 then
+			d.x = 1
+		elseif n.param2 == 3 then
+			d.x = -1
+		elseif n.param2 == 4 then
+			d.z = 1
+		elseif n.param2 == 5 then
+			d.z = -1
+		end
+	else
+		d.y = -1
+	end
+	local p2 = {x=p.x+d.x, y=p.y+d.y, z=p.z+d.z}
+	local nn = minetest.env:get_node(p2).name
+	local def2 = minetest.registered_nodes[nn]
+	if def2 and not def2.walkable then
+		return false
+	end
+	return true
+end
+
 --
 -- Some common functions
 --
@@ -108,6 +150,13 @@ function nodeupdate_single(p)
 			nodeupdate(p)
 		end
 	end
+	
+	if minetest.get_node_group(n.name, "attached_node") ~= 0 then
+		if not check_attached_node(p, n) then
+			drop_attached_node(p)
+			nodeupdate(p)
+		end
+	end
 end
 
 function nodeupdate(p)

builtin/item.lua

@@ -181,6 +181,13 @@ function minetest.item_place_node(itemstack, placer, pointed_thing)
 		end
 	end
 
+	-- Check if the node is attached and if it can be placed there
+	if not check_attached_node(place_to, newnode) then
+		minetest.log("action", "attached node " .. def.name ..
+			" can not be placed at " .. minetest.pos_to_string(place_to))
+		return
+	end
+
 	-- Add node and update
 	minetest.env:add_node(place_to, newnode)
 

doc/lua_api.txt

@@ -457,6 +457,9 @@ Special groups
 - fall_damage_add_percent: damage speed = speed * (1 + value/100)
 - bouncy: value is bounce speed in percent
 - falling_node: if there is no walkable block under the node it will fall
+- attached_node: if the node under it is not a walkable block the node will be
+                  dropped as an item. If the node is wallmounted the
+                  wallmounted direction is checked.
 
 Known damage and digging time defining groups
 ----------------------------------------------

games/minimal/mods/default/init.lua

@@ -837,7 +837,7 @@ minetest.register_node("default:junglegrass", {
 	wield_image = "default_junglegrass.png",
 	paramtype = "light",
 	walkable = false,
-	groups = {snappy=3},
+	groups = {snappy=3,attached_node=1},
 	sounds = default.node_sound_leaves_defaults(),
 })
 
@@ -1106,7 +1106,7 @@ minetest.register_node("default:torch", {
 		wall_bottom = {-0.1, -0.5, -0.1, 0.1, -0.5+0.6, 0.1},
 		wall_side = {-0.5, -0.3, -0.1, -0.5+0.3, 0.3, 0.1},
 	},
-	groups = {choppy=2,dig_immediate=3},
+	groups = {choppy=2,dig_immediate=3,attached_node=1},
 	legacy_wallmounted = true,
 	sounds = default.node_sound_defaults(),
 })
@@ -1127,7 +1127,7 @@ minetest.register_node("default:sign_wall", {
 		--wall_bottom = <default>
 		--wall_side = <default>
 	},
-	groups = {choppy=2,dig_immediate=2},
+	groups = {choppy=2,dig_immediate=2,attached_node=1},
 	legacy_wallmounted = true,
 	sounds = default.node_sound_defaults(),
 	on_construct = function(pos)
@@ -1495,7 +1495,7 @@ minetest.register_node("default:sapling", {
 	wield_image = "default_sapling.png",
 	paramtype = "light",
 	walkable = false,
-	groups = {snappy=2,dig_immediate=3},
+	groups = {snappy=2,dig_immediate=3,attached_node=1},
 	sounds = default.node_sound_defaults(),
 })