# Pastebin 21mr1uUB
function callback.spin()
	local state			= nil
	local callbacks	= {}

	function callback.register_async(name, fn)
		if callbacks[name] ~= nil then
			return ':error', 'callback already exists'
		end

		callbacks[name] = { fn = fn }
		return ':ok'
	end

	function callback.register_sync(name, fn)
		local ok, errmsg = callback.register_async(name, fn)
		if ok == ':error' then return ok, errmsg end
		callbacks[name][sync] = true
		return ':ok'
	end

	while true do
		local msg = scheduler.recv()
		if type(msg) == 'table' then
			local hint     = msg[1]
			local callback = msg[2]
			local args     = msg[3] or {}
			local source   = msg[4]
			local tag      = msg[5]

			if hint == ':callback' then
				-- This is probably a callback. Proceed:
				if callbacks[callback] ~= nil then
					local retvals = table.pack( callbacks[callback]['fn'](state, table.unpack(args)) )

					-- Syncronous callbacks need return values:
					if source ~= nil and callbacks[callback['sync']] ~= nil then
						scheduler.send( source, {':callback-return', self(), callback, tag, retvals} )
					end
				end
			end
		end
	end

end