Make sure #160 cannot be exploited to make servers crash.

This is not exactly a fix for the issue, because extremely
large circuits (3000+ conductors) still won't work with this applied.
This simply aborts any execution if there is the danger of a stack overflow.

mesecons/internal.lua

@@ -367,6 +367,7 @@ end
 
 function mesecon:turnon(pos, rulename, recdepth)
 	recdepth = recdepth or 2
+	if (recdepth > STACK_SIZE) then return end
 	local node = minetest.get_node(pos)
 
 	if(node.name == "ignore") then
@@ -417,6 +418,7 @@ end)
 
 function mesecon:turnoff(pos, rulename, recdepth)
 	recdepth = recdepth or 2
+	if (recdepth > STACK_SIZE) then return end
 	local node = minetest.get_node(pos)
 
 	if(node.name == "ignore") then
@@ -477,7 +479,9 @@ function mesecon:connected_to_receptor(pos, rulename)
 	return false
 end
 
-function mesecon:find_receptor_on(pos, checked, rulename)
+function mesecon:find_receptor_on(pos, checked, rulename, recdepth)
+	recdepth = recdepth or 2
+	if (recdepth > STACK_SIZE) then return true end -- ignore request
 	local node = minetest.get_node(pos)
 
 	if mesecon:is_receptor_on(node.name) then
@@ -501,7 +505,7 @@ function mesecon:find_receptor_on(pos, checked, rulename)
 			local rulenames = mesecon:rules_link_rule_all_inverted(pos, rule)
 			for _, rname in ipairs(rulenames) do
 				local np = mesecon:addPosRule(pos, rname)
-				if mesecon:find_receptor_on(np, checked, mesecon:invertRule(rname)) then
+				if mesecon:find_receptor_on(np, checked, mesecon:invertRule(rname), recdepth + 1) then
 					return true
 				end
 			end

mesecons/settings.lua

@@ -10,3 +10,5 @@ MESECONS_RESUMETIME = 4		-- time to wait when starting the server before
 OVERHEAT_MAX = 20		-- maximum heat of any component that directly sends an output
 				-- signal when the input changes (e.g. luacontroller, gates)
 				-- Unit: actions per second, checks are every 1 second
+STACK_SIZE = 3000		-- Recursive functions will abort when this is reached. Therefore,
+				-- this is also limits the maximum circuit size.