Thursday, November 29, 2012

WebMote part Tornado

So that went pretty well.

Except that github still thinks this project is 20% Perl. If I were mean, I'd make a joke about binary files being improperly recognized.

Granted, the re-write encompassed player.py and not just main.py, but that's because I never stopped to sit down and think through the threading model. Because web.py does a thread-per-request, it technically worked anyway, but that module was due for a proper tear-down and re-build whether I moved servers or not.

## player.py

from subprocess import Popen, PIPE, call
from threading import Thread
from Queue import Queue
import os
import util, conf
from main import ServerStatus

############################################################
### MASSIVE AMOUNTS OF CONFIG (this should probably be in a DB somewhere)
############################################################
defaultPlayer = ["mplayer"]

### If `omxplayer` is available, use it for `mp4`s and `ogv`s (with audio output to the HDMI port)
### If not, use the default player for everything
try:
    call(["omxplayer"])
    playerTable = { 
        'mp4': ["omxplayer", "-o", "hdmi"], 
        'ogv': ["omxplayer", "-o", "hdmi"] }
except:
    playerTable = {}

commandTable = {
    'mplayer':
        {'step-backward': "\x1B[B", 'backward': "\x1B[D", 'forward': "\x1B[C", 'step-forward': "\x1B[A",
         ## down | left | right | up
         'volume-down': "9", 'volume-off': "m", 'volume-up': "0",
         'stop': "q", 'pause': " ", 'play': " "},
    'omxplayer':
        {'step-backward': "\x1B[B", 'backward': "\x1B[D", 'forward': "\x1B[C", 'step-forward': "\x1B[A",
         'volume-off': " ", #oxmplayer doesn't have a mute, so we pause instead
         'volume-down': "+", 'volume-up': "-", 
         'stop': "q", 'pause': " ", 'play': " "}
    }
### END THE MASSIVE CONFIG
############################################################
try:
    commandQueue ## Global multi-process queue to accept player commands
    playQ        ## Global multi-process queue to accept files to play
except:
    commandQueue = Queue()
    playQ = Queue()

def listen():
    while True:
        aFile = playQ.get()
        if util.isInRoot(aFile):
            ServerStatus.write_message_to_all(aFile, event='playing')
            playerCmd = __getPlayerCommand(aFile)
            cmdTable = commandTable[playerCmd[0]]
            playFile(playerCmd, aFile, cmdTable)

def playFile(playerCmd, fileName, cmdTable):
    __clearQueue(commandQueue)
    activePlayer = Popen(playerCmd + [fileName], stdin=PIPE)
    while activePlayer.poll() == None:
        try:
            res = commandQueue.get(timeout=1)
            activePlayer.stdin.write(cmdTable[res])
            ServerStatus.write_message_to_all(res, event="command")
            if unicode(res) == unicode("stop"):
                __clearQueue(playQ)
                activePlayer.terminate()
                return False
        except:
            None
    ServerStatus.write_message_to_all(fileName, event="finished")
    activePlayer = False
    return True

### Local Utility
def __getPlayerCommand(filename):
    global playerTable, defaultPlayer
    name, ext = os.path.splitext(filename)
    return playerTable.get(ext[1:], defaultPlayer)

def __clearQueue(q):
    while not q.empty():
        q.get()
    return True

### Start the player process
playerThread = Thread(target=listen, args=())
playerThread.start()

The relevant parts are actually just those two functions in the center. I'll assume you know what all the imports mean, and that we can just gloss over the MASSIVE CONFIG. The utility functions are self-explanatory-ish. __getPlayerCommand takes a file name, and figures out which player that file is going to be using by looking its extension up in the command table. By default, that's mplayer, but as you can see by that try block in the config section, if omxplayer is available, we use it for mp4s and ogvs[1]. __clearQueue takes a Queue and pulls from it while it's not empty, resulting in an empty queue.

Like I said, the real meat here is

def listen():
    while True:
        aFile = playQ.get()
        if util.isInRoot(aFile):
            ServerStatus.write_message_to_all(aFile, event='playing')
            playerCmd = __getPlayerCommand(aFile)
            cmdTable = commandTable[playerCmd[0]]
            playFile(playerCmd, aFile, cmdTable)

def playFile(playerCmd, fileName, cmdTable):
    __clearQueue(commandQueue)
    activePlayer = Popen(playerCmd + [fileName], stdin=PIPE)
    while activePlayer.poll() == None:
        try:
            res = commandQueue.get(timeout=1)
            activePlayer.stdin.write(cmdTable[res])
            ServerStatus.write_message_to_all(res, event="command")
            if unicode(res) == unicode("stop"):
                __clearQueue(playQ)
                activePlayer.terminate()
                return False
        except:
            None
    ServerStatus.write_message_to_all(fileName, event="finished")
    activePlayer = False
    return True

listen pulls stuff out of the playQ[2], checks that the thing it got is a file it should be able to play and if so, pulls the relevant metadata and passes it on to playFile.

playFile is probably the oddest function I've ever had to write. It has to be blocking, because we don't want its caller to think it can play another file before the last one is done[3], but it also has to launch its player in an asynchronous subprocess, because it needs to be able to receive input from the user, but it can't wait for input because that means that it would have to receive some before it returned[4]. The result is what you see there. The first thing we do is clear the commandQueue[5] and launch the player and retain a handle to it. Then, until playback finishes, we poll commandQueue for user input. We have to leave a timeout for that input check, because we'd otherwise wait here even after the file finished playing, and that's not fun. ServerStatus.write_message_to_all write out an SSE notifying the front-end of

  • playing a file
  • receiving a user command
  • finishing the file

respectively. Hmm. I should probably notify the front end that I've finished playback even when a stop command is received. Just for completeness. I'll make a note of it.

Those changes essentially make the player into an actor, except that it reaches into surrounding state in order to send notifications. If I really felt strongly about it, I could instead give it an output queue that other processes could pull from in order to communicate, rather than have it send messages into ServerStatus directly. I don't today, but you knever no.

Now that we've got that out of the way, here's what the new main.py looks like using tornado

import tornado.ioloop, tornado.web, os, json, random, time
import util, conf, sse, player

class ShowDirectory(tornado.web.RequestHandler):
    def post(self):
        try:
            dir = self.get_argument("dir")
            assert util.isInRoot(dir)
            self.write(util.dirToJSON(dir))
        except:
            self.write(util.entriesToJSON(conf.root))

class Play(tornado.web.RequestHandler):
    def post(self):
        t = self.get_argument('target')
        player.commandQueue.put('stop')
        if os.path.isfile(t):
            fileList = [t]
        elif os.path.isdir(t):
            fileList = util.deepListDir(t)
        else:
            fileList = json.loads(t)
        if self.get_argument('shuffle', False):
            random.shuffle(fileList)
        self.write(json.dumps(fileList))
        time.sleep(1)
        [player.playQ.put(f) for f in fileList]

class ServerStatus(sse.SSEHandler): 
    def on_open(self):
        self.write_message(self.connection_id, event='connection_id')
    def on_close(self):
        self.write_message_to_all(self.connection_id, event='left')

class Command(tornado.web.RequestHandler):
    def post(self):
        cmd = self.get_argument('command')
        player.commandQueue.put(cmd)

class Index(tornado.web.RequestHandler):
    def get(self):
        self.redirect("/static/web-mote.html", permanent=True)

urls = [(r"/", Index),
        (r"/show-directory", ShowDirectory),
        (r"/play", Play),
        (r"/command", Command),
        (r"/status", ServerStatus),
        (r".*", Index)]

settings = {
    "static_path": os.path.join(os.path.dirname(__file__), "static")
    }

app = tornado.web.Application(urls, **settings)

if __name__ == "__main__":
    app.listen(8080)
    tornado.ioloop.IOLoop.instance().start()

As you can tell, it's not significantly different. The handler classes now need lowercase POST/GET methods, we use self.write and self.redirect instead of return and raise, handlers now subclass tornado.web.RequestHandler, the routing table has slightly different syntax, and that's basically it. We also communicate with the player slightly differently, but that's due to the rewrite in the player. The only really significant difference (which I actually prefer the tornado approach for) is

You can specify your own static directory.

This bugged the ever-loving crap out of me in web.py, where doing the same required non-trivially subclassing StaticMiddleware. It's also not obvious based on the documentation, but the default was a static folder relative to cwd, rather than relative to __file__, which meant that running python a/long/path/to/my-app.py 4141 was needlessly tricky. tornado just takes a path, and you get to decide how complete/relative it is.

Oh, I should mention, ServerStatus is not actually a default tornado class. I didn't have to write it myself, but the sse.py file is derived from this. The diff is minimal; I added the capability to specify event fields, and made the id auto-increment by default. The class itself implements Server Sent Events; an asynchronous handler which assumes it isn't getting closed by the other end; a message written to it is going to be sent over to the client without a new request coming your way.

That's an essentially working, non-blocking media server.

Now, it's not done yet. I still have to re-write the Play handler, because I'm currently doing something fishy with time.sleep and the stop command instead of formalizing new-queue as a separate directive, and I still have to make mild edits to the front-end to actually use all this data that's being SSEd over, and setting up a play queue makes it almost trivial to implement skip forward/backward functionality so you bet I'll fucking do it, and it would be really nice to be able to make config changes through the front-end somehow.

But you should be able to use it in its current state.


Footnotes

1 - [back] - That's the RasPi video player; it's more primitive than mplayer, works specifically on the RasPi hardware, and can only really play a few different kinds of video, but the upside is that it can do surprisingly smooth HD output. So we really want to use it if at all possible.

2 - [back] - Which is where we keep the files we still need to play.

3 - [back] - There's only one TV.

4 - [back] - And it won't necessarily; a file might get played with no further intervention from me at all. I don't want to put my music on shuffle and still have to press a button after each song.

5 - [back] - Any commands still there were meant for the previous file, not this one.

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