let tubes break if they're "over-pressure"

that is, if there are more than X number of items in a tube.  Default is
40 in a tube, but breaking is also disabled by default.

(original framework by Novatux, with changes by VanessaE)

item_transport.lua

@@ -1,4 +1,8 @@
 local luaentity = pipeworks.luaentity
+local enable_max_limit = minetest.setting_get("pipeworks_enable_items_per_tube_limit")
+local max_tube_limit = minetest.setting_get("pipeworks_max_items_per_tube") or 40
+
+pipeworks.tube_item_count = {}
 
 function pipeworks.tube_item(pos, item)
 	error("obsolete pipeworks.tube_item() called; change caller to use pipeworks.tube_inject_item() instead")
@@ -77,6 +81,21 @@ local function go_next(pos, velocity, stack)
 		end
 	end
 
+	if enable_max_limit then
+		local itemcount = #minetest.get_objects_inside_radius(pos, 0.5)
+
+		local h = minetest.hash_node_position(pos)
+		if itemcount > max_tube_limit then
+			cmeta:set_string("the_tube_was", minetest.serialize(cnode))
+			print("[Pipeworks] Warning - a tube at "..minetest.pos_to_string(pos).." broke due to too many items ("..itemcount..")")
+			minetest.swap_node(pos, {name = "pipeworks:broken_tube_1"})
+			pipeworks.scan_for_tube_objects(pos)
+			pipeworks.tube_item_count[h] = 0
+		else
+			pipeworks.tube_item_count[h] = itemcount
+		end
+	end
+
 	if not next_positions[1] then
 		return false, nil
 	end

routing_tubes.lua

@@ -1,3 +1,4 @@
+
 -- the default tube and default textures
 pipeworks.register_tube("pipeworks:tube", "Pneumatic tube segment")
 minetest.register_craft( {
@@ -9,6 +10,28 @@ minetest.register_craft( {
 	},
 })
 
+pipeworks.register_tube("pipeworks:broken_tube", {
+	description = "Broken Tube (you hacker you)",
+	plain = { { name = "pipeworks_broken_tube_plain.png", backface_culling = false, color = nodecolor } },
+	noctr = { { name = "pipeworks_broken_tube_plain.png", backface_culling = false, color = nodecolor } },
+	ends  = { { name = "pipeworks_broken_tube_end.png",   color = nodecolor } },
+	short =   { name = "pipeworks_broken_tube_short.png", color = nodecolor },
+	node_def = {
+		drop = "pipeworks:tube_1",
+		groups = {not_in_creative_inventory = 1, tubedevice_receiver = 1},
+		tube = {
+			insert_object = function(pos, node, stack, direction)
+				minetest.item_drop(stack, nil, pos)
+				return ItemStack("")
+			end,
+			can_insert = function(pos,node,stack,direction)
+				return true
+			end,
+			priority = 50,
+		}
+	}
+})
+
 -- the high priority tube is a low-cpu replacement for sorting tubes in situations
 -- where players would use them for simple routing (turning off paths)
 -- without doing actual sorting, like at outputs of tubedevices that might both accept and eject items