Add callback to preserve node metadata as item metadata

builtin/game/falling.lua

@@ -150,8 +150,22 @@ end
 
 local function drop_attached_node(p)
 	local n = core.get_node(p)
+	local drops = core.get_node_drops(n, "")
+	local def = core.registered_items[n.name]
+	if def and def.preserve_metadata then
+		local oldmeta = core.get_meta(p):to_table().fields
+		-- Copy pos and node because the callback can modify them.
+		local pos_copy = {x=p.x, y=p.y, z=p.z}
+		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
 	core.remove_node(p)
-	for _, item in pairs(core.get_node_drops(n, "")) do
+	for _, item in pairs(drops) do
 		local pos = {
 			x = p.x + math.random()/2 - 0.25,
 			y = p.y + math.random()/2 - 0.25,

builtin/game/item.lua

@@ -579,6 +579,20 @@ function core.node_dig(pos, node, digger)
 		digger:set_wielded_item(wielded)
 	end
 
+	-- Check to see if metadata should be preserved.
+	if def and def.preserve_metadata then
+		local oldmeta = core.get_meta(pos):to_table().fields
+		-- Copy pos and node because the callback can modify them.
+		local pos_copy = {x=pos.x, y=pos.y, z=pos.z}
+		local node_copy = {name=node.name, param1=node.param1, param2=node.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
+
 	-- Handle drops
 	core.handle_node_drops(pos, drops, digger)
 

doc/lua_api.txt

@@ -4710,6 +4710,13 @@ Definition tables
         ^ interval. Default: nil.
         ^ Warning: making a liquid node 'floodable' does not work and may cause problems. ]]
 
+	preserve_metadata = func(pos, oldnode, oldmeta, drops) --[[
+	^ Called when oldnode is about be converted to an item, but before the
+	  node is deleted from the world or the drops are added.  This is generally
+	  the result of either the node being dug or an attached node becoming detached.
+	^ drops is a table of ItemStacks, so any metadata to be preserved can be
+	  added directly to one or more of the dropped items.  See "ItemStackMetaRef".
+	^ default: nil ]]
         after_place_node = func(pos, placer, itemstack, pointed_thing) --[[
         ^ Called after constructing node when node was placed using
           minetest.item_place_node / minetest.place_node