prevent double execution of on_event.lua

makes it safe to dofile("on_event.lua")

(cherry-picked from commit 6010ffe)

data/lua/on_event.lua

@@ -3,19 +3,25 @@ local utils = wesnoth.require "wml-utils"
 -- so you have to call this function from a toplevel lua tag or from a preload event.
 -- It is also not possible to use this for first_time_only=yes events.
 
+if rawget(_G, "core_on_event") then
+	return rawget(_G, "core_on_event")  -- prevent double execution
+end
+
 local event_handlers = {}
+
 local old_on_event = wesnoth.game_events.on_event or function(eventname) end
 wesnoth.game_events.on_event = function(eventname)
 	old_on_event(eventname)
 	local context = nil
-	for k,v in pairs(event_handlers[eventname] or {}) do
+	for _, entry in pairs(event_handlers[eventname] or {}) do
 		if context == nil then
 			context = wesnoth.current.event_context
 		end
-		v.h(context)
+		entry.h(context)
 	end
 end
 
+
 local function on_event(eventname, arg1, arg2)
 	if string.match(eventname, ",") then
 		for elem in utils.split(eventname or "") do
@@ -35,12 +41,13 @@ local function on_event(eventname, arg1, arg2)
 	event_handlers[eventname] = event_handlers[eventname] or {}
 	local eh = event_handlers[eventname]
 	table.insert(eh, { h = handler, p = priority})
-	-- sort it.
+	-- prioritize last entry
 	for i = #eh - 1, 1, -1 do
 		if eh[i].p < eh[i + 1].p then
 			eh[i], eh[i + 1] = eh[i + 1], eh[i]
 		end
 	end
 end
 
+core_on_event = on_event
 return on_event