Prevent shops from trading tools deceptively

This limits the tools a shop will accept or give to those whose wear is less
than or equal to the tool shown in the Owner needs or Owner gives lists. This
protects both owners and customers from unfair trades. Owners are protected
from receiving worn out tools when a new tool is placed in the needs list.
Customers are protected from receiving worn out tools when a new one is placed
in the give list.

shop.lua

@@ -5,6 +5,29 @@
 
 local exchange_shop = {}
 
+-- Tool wear aware replacement for contains_item.
+local function list_contains_item(inv, listname, stack)
+	local list = inv:get_list(listname)
+	for i, list_stack in pairs(list) do
+		if list_stack:get_name()  == stack:get_name()  and
+		   list_stack:get_count() >= stack:get_count() and
+		   list_stack:get_wear()  <= stack:get_wear() then
+			return i
+		end
+	end
+end
+
+-- Tool wear aware replacement for remove_item.
+local function list_remove_item(inv, listname, stack)
+	local index = list_contains_item(inv, listname, stack)
+	if index then
+		local list_stack = inv:get_stack(listname, index)
+		local removed_stack = list_stack:take_item(stack:get_count())
+		inv:set_stack(listname, index, list_stack)
+		return removed_stack
+	end
+end
+
 local function get_exchange_shop_formspec(number,pos,title)
 	local formspec = ""
 	local name = "nodemeta:"..pos.x..","..pos.y..","..pos.z
@@ -200,7 +223,7 @@ minetest.register_on_player_receive_fields(function(sender, formname, fields)
 		-- Check availability of the shop's items
 		if err_msg == "" then
 			for i, item in pairs(cust_og) do
-				if not shop_inv:contains_item("stock", item) then
+				if not list_contains_item(shop_inv, "stock", item) then
 					err_msg = "This shop is sold out."
 					break
 				end
@@ -220,7 +243,7 @@ minetest.register_on_player_receive_fields(function(sender, formname, fields)
 		-- Check availability of the player's items
 		if err_msg == "" then
 			for i, item in pairs(cust_ow) do
-				if not player_inv:contains_item("main", item) then
+				if not list_contains_item(player_inv, "main", item) then
 					err_msg = "You do not have the required items."
 					break
 				end
@@ -231,7 +254,7 @@ minetest.register_on_player_receive_fields(function(sender, formname, fields)
 		if err_msg == "" then
 			local fully_exchanged = true
 			for i, item in pairs(cust_ow) do
-				local stack = player_inv:remove_item("main", item)
+				local stack = list_remove_item(player_inv, "main", item)
 				if shop_inv:room_for_item("custm", stack) then
 					shop_inv:add_item("custm", stack)
 				else
@@ -241,7 +264,7 @@ minetest.register_on_player_receive_fields(function(sender, formname, fields)
 				end
 			end
 			for i, item in pairs(cust_og) do
-				local stack = shop_inv:remove_item("stock", item)
+				local stack = list_remove_item(shop_inv, "stock", item)
 				if player_inv:room_for_item("main", stack) then
 					player_inv:add_item("main", stack)
 				else