Add "orphaned" leaves nodes.

* Add "orphaned" leaves nodes, these are copies of the regular leaves
  nodes whose sole purpose is to replace leaves nodes when no more tree
  trunks are present nearby.  The orphaned nodes are swapped in by a
  callback on destruction of tree trunk nodes and will be decayed by a
  dedicated abm that processes all nodes in `group:leaves_orphan`.
* Add `mcl_core.update_leaves()` function, a `after_destruct` callback
  handler for tree trunk nodes.  The function finds leaves nodes nearby
  destroyed tree trunk nodes and swaps these for orphaned leaves nodes
  if no other tree trunk nodes remain near them.
* Add `after_destruct` callbacks to tree trunk node registrations in
  mcl_core/nodes_trees.lua and mcl_mangrove/init.lua to update orphaned
  leaves.
* Add entry for group `leaves_orphan` to GROUPS.md
* Also add entry for group `leaves` to GROUPS.md (it was missing).

GROUPS.md

@@ -74,6 +74,8 @@ Please read <http://minecraft.gamepedia.com/Breaking> to learn how digging times
 * `coral_species=X`: Specifies the species of a coral; equal X means equal species
 * `set_on_fire=X`: Sets any (not fire-resistant) mob or player on fire for X seconds when touching
 * `compostability=X`: Item can be used on a composter block; X (1-100) is the % chance of adding a level of compost
+* `leaves=X`: Node will spotaneously decay if no tree trunk nodes remain within 6 blocks distance.
+* `leaves_orphan`: See above, these nodes are in the process of decayed.
 
 #### Footnotes
 

mods/ITEMS/mcl_core/nodes_trees.lua

@@ -8,6 +8,40 @@ if mod_screwdriver then
 	on_rotate = screwdriver.rotate_3way
 end
 
+-- Check dug/destroyed tree trunks for orphaned leaves.
+--
+-- This function is meant to be called by the `after_destruct` handler of
+-- treetrunk nodes.
+--
+-- Whenever a trunk node is removed, all `group:leaves` nodes in a sphere
+-- with radius 6 are checked.  Every such node that does not have a trunk
+-- node within a distance of 6 blocks is converted into a orphan leaf node.
+-- An ABM will gradually decay these nodes.
+--
+-- If param2 of the node is set to a nonzero value, the node will always
+-- be preserved.  This is set automatically when leaves are placed manually.
+--
+-- @param pos the position of the removed trunk node.
+-- @param oldnode the node table of the removed trunk node.
+function mcl_core.update_leaves(pos, oldnode)
+	local pos1, pos2 = vector.offset(pos, -6, -6, -6), vector.offset(pos, 6, 6, 6)
+	local lnode
+	local leaves = minetest.find_nodes_in_area(pos1, pos2, "group:leaves")
+	for _, lpos in pairs(leaves) do
+		lnode = minetest.get_node(lpos)
+		-- skip already decaying leaf nodes
+		if minetest.get_item_group(lnode.name, "orphan_leaves") ~= 1 then
+			if not minetest.find_node_near(lpos, 6, "group:tree") then
+				-- manually placed leaf nodes have param2
+				-- set and will never decay automatically
+				if lnode.param2 == 0 then
+					minetest.swap_node(lpos, {name = lnode.name .. "_orphan"})
+				end
+			end
+		end
+	end
+end
+
 -- Register tree trunk (wood) and bark
 local function register_tree_trunk(subname, description_trunk, description_bark, longdesc, tile_inner, tile_bark, stripped_variant)
 	minetest.register_node("mcl_core:"..subname, {
@@ -17,6 +51,7 @@ local function register_tree_trunk(subname, description_trunk, description_bark,
 		tiles = {tile_inner, tile_inner, tile_bark},
 		paramtype2 = "facedir",
 		on_place = mcl_util.rotate_axis,
+		after_destruct = mcl_core.update_leaves,
 		stack_max = 64,
 		groups = {handy=1,axey=1, tree=1, flammable=2, building_block=1, material_wood=1, fire_encouragement=5, fire_flammability=5},
 		sounds = mcl_sounds.node_sound_wood_defaults(),
@@ -141,7 +176,7 @@ local function register_leaves(subname, description, longdesc, tiles, sapling, d
 		return drop
 	end
 
-	minetest.register_node("mcl_core:"..subname, {
+	local l_def = {
 		description = description,
 		_doc_items_longdesc = longdesc,
 		_doc_items_hidden = false,
@@ -164,7 +199,19 @@ local function register_leaves(subname, description, longdesc, tiles, sapling, d
 		_mcl_hardness = 0.2,
 		_mcl_silk_touch_drop = true,
 		_mcl_fortune_drop = { get_drops(1), get_drops(2), get_drops(3), get_drops(4) },
-	})
+	}
+
+	minetest.register_node("mcl_core:" .. subname, l_def)
+
+	local o_def = table.copy(l_def)
+	o_def._doc_items_create_entry = false
+	o_def.place_param2 = nil
+	o_def.groups.not_in_creative_inventory = 1
+	o_def.groups.orphan_leaves = 1
+	o_def._mcl_shears_drop = {"mcl_core:" .. subname}
+	o_def._mcl_silk_touch_drop = {"mcl_core:" .. subname}
+
+	minetest.register_node("mcl_core:" .. subname .. "_orphan", o_def)
 end
 
 local function register_sapling(subname, description, longdesc, tt_help, texture, selbox)

mods/ITEMS/mcl_mangrove/init.lua

@@ -52,6 +52,7 @@ minetest.register_node("mcl_mangrove:mangrove_tree", {
 	tiles = {"mcl_mangrove_log_top.png", "mcl_mangrove_log_top.png", "mcl_mangrove_log.png"},
 	paramtype2 = "facedir",
 	on_place = mcl_util.rotate_axis,
+	after_destruct = mcl_core.update_leaves,
 	groups = {handy=1,axey=1, tree=1, flammable=2, building_block=1, material_wood=1, fire_encouragement=5, fire_flammability=5},
 	sounds = mcl_sounds.node_sound_wood_defaults(),
 	on_place = mcl_util.rotate_axis,
@@ -86,7 +87,7 @@ minetest.register_node("mcl_mangrove:mangrove_wood", {
 	_mcl_hardness = 2,
 })
 
-minetest.register_node("mcl_mangrove:mangroveleaves", {
+local l_def = {
 	description = S("Mangrove Leaves"),
 	_doc_items_longdesc = S("mangrove leaves are grown from mangrove trees."),
 	_doc_items_hidden = false,
@@ -103,7 +104,19 @@ minetest.register_node("mcl_mangrove:mangroveleaves", {
 	_mcl_hardness = 0.2,
 	_mcl_silk_touch_drop = true,
 	_mcl_fortune_drop = { get_drops(1), get_drops(2), get_drops(3), get_drops(4) },
-})
+}
+
+minetest.register_node("mcl_mangrove:mangroveleaves", l_def)
+
+local o_def = table.copy(l_def)
+o_def._doc_items_create_entry = false
+o_def.place_param2 = nil
+o_def.groups.not_in_creative_inventory = 1
+o_def.groups.orphan_leaves = 1
+o_def._mcl_shears_drop = {"mcl_mangrove:mangroveleaves"}
+o_def._mcl_silk_touch_drop = {"mcl_mangrove:mangroveleaves"}
+
+minetest.register_node("mcl_mangrove:mangroveleaves_orphan", o_def)
 
 minetest.register_node("mcl_mangrove:mangrove_stripped_trunk", {
 	description = S("Stripped Mangrove Wood"),