Prevent another crash if minetest crashes during startup

Fix crash when lua tubes are DoSed.

See merge request VanessaE/pipeworks!8

depends.txt

@@ -1,5 +1,6 @@
 default
 basic_materials
-mesecons
+screwdriver
-mesecons_mvps
+mesecons?
+mesecons_mvps?
 digilines?

filter-injector.lua

@@ -193,6 +193,8 @@ local function punch_filter(data, filtpos, filtnode, msg)
 		["technic:mv_grinder"] = "dst",
 		["technic:mv_grinder_active"] = "dst",
 		["technic:tool_workshop"] = "src",
+		["technic:mv_freezer"] = "dst",
+		["technic:mv_freezer_active"] = "dst"
 	}
 
 	-- make sure there's something appropriate to inject the item into

init.lua

@@ -133,15 +133,16 @@ dofile(pipeworks.modpath..logicdir.."flowable_node_registry_install.lua")
 if pipeworks.enable_pipes then dofile(pipeworks.modpath.."/pipes.lua") end
 if pipeworks.enable_teleport_tube then dofile(pipeworks.modpath.."/teleport_tube.lua") end
 if pipeworks.enable_pipe_devices then dofile(pipeworks.modpath.."/devices.lua") end
-
 if pipeworks.enable_redefines then
 	dofile(pipeworks.modpath.."/compat-chests.lua")
 	dofile(pipeworks.modpath.."/compat-furnaces.lua")
 end
 if pipeworks.enable_autocrafter then dofile(pipeworks.modpath.."/autocrafter.lua") end
-if pipeworks.enable_lua_tube then dofile(pipeworks.modpath.."/lua_tube.lua") end
+if pipeworks.enable_lua_tube and
+		(minetest.get_modpath("mesecons") or minetest.get_modpath("digilines")) then
+	dofile(pipeworks.modpath.."/lua_tube.lua")
+end
 
 minetest.register_alias("pipeworks:pipe", "pipeworks:pipe_110000_empty")
 
 print("Pipeworks loaded!")
-

lua_tube.lua

@@ -333,8 +333,11 @@ local safe_globals = {
 }
 
 local function create_environment(pos, mem, event)
-	-- Gather variables for the environment
+	-- Make sure the tube hasn't broken.
 	local vports = minetest.registered_nodes[minetest.get_node(pos).name].virtual_portstates
+	if not vports then return {} end
+
+	-- Gather variables for the environment
 	local vports_copy = {}
 	for k, v in pairs(vports) do vports_copy[k] = v end
 	local rports = get_real_port_states(pos)

luaentity.lua

@@ -46,6 +46,11 @@ local function read_entities()
 end
 
 local function write_entities()
+	if not luaentity.entities then
+		-- This can happen if crashing on startup, causing another error that
+		-- masks the original one. Return gracefully in that case instead.
+		return
+	end
 	for _, entity in pairs(luaentity.entities) do
 		setmetatable(entity, nil)
 		for _, attached in pairs(entity._attached_entities) do