p.wait() doesn't block

[runeksvendsen]

Take a look at this example:

from sh import mpv
import multiprocessing, time

class MpvPlay:
    CACHE_SIZE = 4096

    def __init__(self, url, exit_callback=None):
        self.url = url
        self.exit_callback = exit_callback

    def play(self):
        self.p = mpv(self.url, cache=self.CACHE_SIZE, no_video=True, _bg=True)

        self.process = multiprocessing.Process(target=self.process_thread)
        self.process.start()

    def process_thread(self):
        try:
            print "MpvPlay thread: waiting"
            self.p.wait()
        except Exception, e:
            pass

        print "MpvPlay thread: executing callback"
        if self.exit_callback:
            self.exit_callback(self.p)

def exit_callback(p):
    print "MPV exited. stdout:\n\t%s" % (p.stdout)

if __name__ == "__main__":
    url="https://www.youtube.com/watch?v=nKAVpoPjEJ8"
    playobj = MpvPlay(url, exit_callback)
    playobj.play()

    while True:
        time.sleep(1)

As far as I can see, this should start the program "mpv" (you can substitute with "mplayer" is you don't have mpv installed), and after the program exits it should execute the exit_callback function. But the exit_callback function is executed immediately, while mpv is still running. Shouldn't self.p.wait() block in process_thread(), so exit_callback isn't called until mpv exits?

[miohtama]

If it's a GUI application some applications tend to return to command line immediately and not block (some kind of daemonization). In this case, you should check from the application itself what it is doing and if there is a command-line flag to prevent this behavior.

[runeksvendsen]

@miohtama With the --no-video (no_video=True) option it's not a GUI application, and even when it is it doesn't return from the command line until you close the video window.

[amoffat]

this is super old, but I'll attempt to answer it...

the problem was concealed by:

        try:
            print "MpvPlay thread: waiting"
            self.p.wait()
        except Exception, e:
            pass

a valid exception was being thrown, but it was ignored. that exception was OSError: [Errno 10] No child processes. the reason for this was because you ran a multiprocessing.Process and tried to wait() on a process that wasn't it's child process (it was a sibling process at this point). so that exception was thrown and your callback was called, while the process happily played the video.

it seems like you did this in order to use an exit callback when the spawned process finishes. good news is, exit callbacks have been added in the release-1.10 branch which should go out before the end of the year