Merge pull request 'ENTITIES/mcl_item_entity: Fix non-serializable item entity unload crash' (#132) from fix-overlong-meta-item-crash-2 into master

Reviewed-on: https://git.minetest.land/Mineclonia/Mineclonia/pulls/132
Reviewed-by: cora <cora@noreply.git.minetest.land>

mods/ENTITIES/mcl_item_entity/init.lua

@@ -1,3 +1,4 @@
+local S = minetest.get_translator("mcl_item_entity")
 --basic settings
 local item_drop_settings                 = {} --settings table
 item_drop_settings.age                   = 1.0 --how old a dropped item (_insta_collect==false) has to be before collecting
@@ -448,7 +449,7 @@ minetest.register_entity(":__builtin:item", {
 	end,
 
 	get_staticdata = function(self)
-		return minetest.serialize({
+		local data = minetest.serialize({
 			itemstring = self.itemstring,
 			always_collect = self.always_collect,
 			age = self.age,
@@ -456,6 +457,39 @@ minetest.register_entity(":__builtin:item", {
 			_flowing = self._flowing,
 			_removed = self._removed,
 		})
+		-- sfan5 guessed that the biggest serializable item
+		-- entity would have a size of 65530 bytes. This has
+		-- been experimentally verified to be still too large.
+		--
+		-- anon5 has calculated that the biggest serializable
+		-- item entity has a size of exactly 65487 bytes:
+		--
+		-- 1. serializeString16 can handle max. 65535 bytes.
+		-- 2. The following engine metadata is always saved:
+		--    • 1 byte (version)
+		--    • 2 byte (length prefix)
+		--    • 14 byte “__builtin:item”
+		--    • 4 byte (length prefix)
+		--    • 2 byte (health)
+		--    • 3 × 4 byte = 12 byte (position)
+		--    • 4 byte (yaw)
+		--    • 1 byte (version 2)
+		--    • 2 × 4 byte = 8 byte (pitch and roll)
+		-- 3. This leaves 65487 bytes for the serialization.
+		if #data > 65487 then -- would crash the engine
+			local stack = ItemStack(self.itemstring)
+			stack:get_meta():from_table(nil)
+			self.itemstring = stack:to_string()
+			minetest.log(
+				"warning",
+				"Overlong item entity metadata removed: “" ..
+				self.itemstring ..
+				"” had serialized length of " ..
+				#data
+			)
+			return self:get_staticdata()
+		end
+		return data
 	end,
 
 	on_activate = function(self, staticdata, dtime_s)
@@ -748,3 +782,29 @@ minetest.register_entity(":__builtin:item", {
 
 	-- Note: on_punch intentionally left out. The player should *not* be able to collect items by punching
 })
+
+-- The “getwrittenbook” command was added as a debug aid. It can help
+-- reproducing situations in which items with lots of metadata trigger
+-- issues like heavy lag or server crashes. Do not remove this command
+-- unless another easy way of getting items with overlong meta exists!
+--
+-- “/getwrittenbook 65323” creates an item that creates the largest
+-- possible serializable written book item entity when dropped.
+--
+-- “/getwrittenbook 65324” creates an item that creates the smallest
+-- possible non-serializable written book item entity when dropped.
+minetest.register_chatcommand("getwrittenbook", {
+	params = S("<Count>"),
+	description = S("Get a written book with a configurable amount of characters."),
+	privs = {debug=true},
+	func = function(name, param)
+		local count = tonumber(param)
+		local itemstack = ItemStack("mcl_books:written_book")
+		local meta = itemstack:get_meta()
+		meta:set_string("description", "")
+		meta:set_string("text", string.rep("x", count))
+		local player = minetest.get_player_by_name(name)
+		local inv = player:get_inventory()
+		inv:add_item("main", itemstack)
+	end
+})