resolves ytdl for urls in playlist when the demuxer reaches near the end of the file

ytdl prefetch resolver.lua

local utils = require 'mp.utils'
local ytdlPath = mp.find_config_file("youtube-dl.exe")
local fileDuration = 0
local function check_position()
	if fileDuration==0 then
		mp.unobserve_property(check_position)
		return
	end
	local demuxEndPosition = mp.get_property("demuxer-cache-time")
	if demuxEndPosition and 
		fileDuration > 0 and 
		(fileDuration - demuxEndPosition < 10) and 
		(mp.get_property("playlist-pos-1")~=mp.get_property("playlist-count")) then
		local next = tonumber(mp.get_property("playlist-pos")) + 1
		local nextFile = mp.get_property("playlist/"..tostring(next).."/filename")
		if nextFile then
			local ytdl = {}
			ytdl.args = {ytdlPath, "-f "..mp.get_property("ytdl-format"), "-e", "-g", nextFile}
			local res = utils.subprocess(ytdl)
			local lines = {}
			for s in res.stdout:gmatch("[^\r\n]+") do
				table.insert(lines, s)
			end
			local audioURL = ""
			if lines[3] then 
				audioURL = ',audio-file=['..lines[3]..']'
			end
			if lines[1] and lines[2] then
				mp.commandv("loadfile", lines[2], "append", 'force-media-title=['..lines[1]..']'..audioURL)
				mp.commandv("playlist_move", mp.get_property("playlist-count") - 1 , next)
				mp.commandv("playlist_remove", next + 1)
			end
			mp.unobserve_property(check_position)
		end
	end
end
local function observe()
	if mp.get_property("path"):find("://", 0, false) then
		fileDuration = mp.get_property("duration", 0)
		fileDuration = tonumber(fileDuration)
		mp.observe_property("time-pos", "native", check_position)
	end
end
mp.register_event("file-loaded", observe)

One limitation of this script is that because the urls are resolved in this way, --resume-playback will almost certainly not work.

On Linux change line 2:
local ytdlPath = "youtube-dl"

I've discovered that a lot of usefulness of this script doesn't actually work :(
It appears --prefetch-playlist doesn't actually fill the cache with subsequent data and merely opens the url and keeps it active, downloading what appears to be header and metadata and mpv just waits for that file to actually be playing before downloading any more.