Add support for `group:supported_node`

* Add support for `group:supported_node` to CORE/mcl_attached.
  Supported nodes are nodes that can be placed on any node that does not
  have the `drawtype = "airlike"` attribute.
* Copy the `drop_attached_node()` function from minetest/builtin, so
  that the override function provides the same behavior when nodes drop.
* Add comments to CORE/mcl_attached and to the functions defined in it.
* Add more local aliases for global minetest.* functions.
* If the original function returns true, it is not necessary anymore
  to perform more tests and the override function can simply return true
  immediately.

mods/CORE/mcl_attached/init.lua

@@ -1,29 +1,93 @@
+-- Overrides the builtin minetest.check_single_for_falling.
+-- We need to do this in order to handle nodes in mineclone specific groups
+-- "supported_node" and "attached_node_facedir".
+--
+-- Nodes in group "supported_node" can be placed on any node that does not
+-- have the "airlike" drawtype.  Carpets are an example of this type.
+
 local vector = vector
 
 local facedir_to_dir = minetest.facedir_to_dir
 local get_item_group = minetest.get_item_group
 local remove_node = minetest.remove_node
 local get_node = minetest.get_node
+local get_meta = minetest.get_meta
+local registered_nodes = minetest.registered_nodes
+local get_node_drops = minetest.get_node_drops
+local add_item = minetest.add_item
 
+-- drop_attached_node(p)
+--
+-- This function is copied verbatim from minetest/builtin/game/falling.lua
+-- We need this to do the exact same dropping node handling in our override
+-- minetest.check_single_for_falling() function as in the builtin function.
+--
+local function drop_attached_node(p)
+	local n = get_node(p)
+	local drops = get_node_drops(n, "")
+	local def = registered_nodes[n.name]
+	if def and def.preserve_metadata then
+		local oldmeta = get_meta(p):to_table().fields
+		-- Copy pos and node because the callback can modify them.
+		local pos_copy = vector.new(p)
+		local node_copy = {name=n.name, param1=n.param1, param2=n.param2}
+		local drop_stacks = {}
+		for k, v in pairs(drops) do
+			drop_stacks[k] = ItemStack(v)
+		end
+		drops = drop_stacks
+		def.preserve_metadata(pos_copy, node_copy, oldmeta, drops)
+	end
+	if def and def.sounds and def.sounds.fall then
+		core.sound_play(def.sounds.fall, {pos = p}, true)
+	end
+	remove_node(p)
+	for _, item in pairs(drops) do
+		local pos = {
+			x = p.x + math.random()/2 - 0.25,
+			y = p.y + math.random()/2 - 0.25,
+			z = p.z + math.random()/2 - 0.25,
+		}
+		add_item(pos, item)
+	end
+end
+
+-- minetest.check_single_for_falling(pos)
+--
+-- * causes an unsupported `group:falling_node` node to fall and causes an
+--   unattached `group:attached_node` or `group:attached_node_facedir` node
+--   or unsupported `group:supported_node` to drop.
+-- * does not spread these updates to neighbours.
+--
+-- Returns true if the node at <pos> has spawned a falling node or has been
+-- dropped as item(s).
+--
 local original_function = minetest.check_single_for_falling
 
 function minetest.check_single_for_falling(pos)
-	local ret_o = original_function(pos)
-	local ret = false
-	local node = minetest.get_node(pos)
+	if original_function(pos) then
+		return true
+	end
+
+	local node = get_node(pos)
 	if get_item_group(node.name, "attached_node_facedir") ~= 0 then
 		local dir = facedir_to_dir(node.param2)
 		if dir then
 			if get_item_group(get_node(vector.add(pos, dir)).name, "solid") == 0 then
-				remove_node(pos)
-				local drops = minetest.get_node_drops(node.name, "")
-				for dr=1, #drops do
-					minetest.add_item(pos, drops[dr])
-				end
-				ret = true
+				drop_attached_node(pos)
+				return true
 			end
 		end
 	end
-	return ret_o or ret
+
+	if get_item_group(node.name, "supported_node") ~= 0 then
+		local def = registered_nodes[get_node(vector.offset(pos, 0, -1, 0)).name]
+		if def and def.drawtype == "airlike" then
+			drop_attached_node(pos)
+			return true
+		end
+	end
+
+	return false
 end