Move node itemstacks from meta inv to field

Node inventories are sent to clients, while metadata
fields are automatically set private by the server
and not sent to clients.  Not using node meta
inventories may help reduce the amount of data
sent to clients, especially in storage-heavy areas
where we were duplicating the work of sending the
inventory data not used on the client, as well as the
visible item entities.

Automatically convert old-format data to new upon
reading.

Also, needed to provide some APIs for sane access
to data in serialized format, and another way to
signal that a node can accept a stack other than
the size of inventory[solo]

mods/nc_api/util_stack.lua

@@ -46,12 +46,29 @@ function nodecore.stack_merge(dest, src)
 	return src
 end
 
-function nodecore.node_inv(pos)
-	return minetest.get_meta(pos):get_inventory()
-end
+local metakey = "ncitem"
 
 function nodecore.stack_get(pos)
-	return nodecore.node_inv(pos):get_stack("solo", 1)
+	local meta = minetest.get_meta(pos)
+	local str = meta:get_string(metakey)
+	if str and str ~= "" then return ItemStack(str) end
+	local inv = meta:get_inventory()
+	local stack = inv:get_stack("solo", 1)
+	if stack:is_empty() then return stack end
+	meta:set_string(metakey, stack:to_string())
+	inv:set_size("solo", 0)
+	return stack
+end
+
+function nodecore.stack_get_serial(metatable)
+	local str = metatable
+	str = str and str.fields
+	str = str and str[metakey]
+	if str and str ~= "" then return ItemStack(str) end
+	local inv = metatable.inventory
+	inv = inv and inv.solo
+	inv = inv and inv[1]
+	if inv and inv ~= "" then return ItemStack(inv) end
 end
 
 local function update(pos, ...)
@@ -64,12 +81,14 @@ function nodecore.stack_set(pos, stack, player)
 		nodecore.log("action", string_format("%s sets stack %q at %s",
 				player:get_player_name(), shortdesc(stack), minetest.pos_to_string(pos)))
 	end
-	return update(pos, nodecore.node_inv(pos):set_stack("solo", 1, ItemStack(stack)))
+	return update(pos, minetest.get_meta(pos):set_string(metakey,
+			ItemStack(stack):to_string()))
 end
 
 function nodecore.stack_add(pos, stack, player)
 	local node = minetest.get_node(pos)
 	local def = minetest.registered_items[node.name] or {}
+	if not def.can_have_itemstack then return end
 	if def.stack_allow then
 		local ret = def.stack_allow(pos, node, stack)
 		if ret == false then return stack end
@@ -79,13 +98,8 @@ function nodecore.stack_add(pos, stack, player)
 	local donate = stack:get_count()
 	local item = nodecore.stack_get(pos)
 	local exist = item:get_count()
-	local left
-	if item:is_empty() then
-		left = nodecore.node_inv(pos):add_item("solo", stack)
-	else
-		left = nodecore.stack_merge(item, stack)
-		nodecore.stack_set(pos, item)
-	end
+	local left = nodecore.stack_merge(item, stack)
+	nodecore.stack_set(pos, item)
 	local remain = left:get_count()
 	if donate ~= remain then
 		if player then

mods/nc_api_visinv/init.lua

@@ -148,15 +148,12 @@ nodecore.register_globalstep("visinv check", function()
 -- NODE REGISTRATION HELPERS
 
 function nodecore.visinv_on_construct(pos)
-	local meta = minetest.get_meta(pos)
-	local inv = meta:get_inventory()
-	inv:set_size("solo", 1)
-	nodecore.visinv_update_ents(pos)
+	return nodecore.visinv_update_ents(pos)
 end
 
 function nodecore.visinv_after_destruct(pos)
 	nodecore.visinv_update_ents(pos)
-	nodecore.fallcheck(pos)
+	return nodecore.fallcheck(pos)
 end
 
 nodecore.register_on_register_item(function(_, def)
@@ -165,6 +162,7 @@ nodecore.register_on_register_item(function(_, def)
 		def.groups = def.groups or {}
 
 		if def.groups.visinv then
+			def.can_have_itemstack = true
 			def.on_construct = def.on_construct or nodecore.visinv_on_construct
 			def.after_destruct = def.after_destruct or nodecore.visinv_after_destruct
 		end

mods/nc_items/ent_falling.lua

@@ -1,21 +1,19 @@
 -- LUALOCALS < ---------------------------------------------------------
-local ItemStack, minetest, nodecore
-    = ItemStack, minetest, nodecore
+local minetest, nodecore
+    = minetest, nodecore
 -- LUALOCALS > ---------------------------------------------------------
 
 local modname = minetest.get_current_modname()
 
 nodecore.register_falling_node_on_setnode(function(self, node, meta)
-		if node and node.name == modname .. ":stack"
-		and meta and meta.inventory and meta.inventory.solo then
-			local stack = ItemStack(meta.inventory.solo[1] or "")
-			if not stack:is_empty() then
-				local pos = self.object:get_pos()
-				if not pos then return end
-				local ent = minetest.add_item(pos, stack)
-				if ent then ent:set_velocity(self.object:get_velocity()) end
-				self.object:remove()
-				return true
-			end
+		if not (node and node.name == modname .. ":stack") then return end
+		local stack = nodecore.stack_get_serial(meta)
+		if not stack:is_empty() then
+			local pos = self.object:get_pos()
+			if not pos then return end
+			local ent = minetest.add_item(pos, stack)
+			if ent then ent:set_velocity(self.object:get_velocity()) end
+			self.object:remove()
+			return true
 		end
 	end)