how to make sure bus messages expected/received?

[Sergei Vorobyov]

Thanks, Tim!

Imagine your code is a part of a bigger loop (as I said):

for (;;) {
  msg = gst_bus_timed_pop_filtered(...)
  while (msg) {
    handle_message (msg);
    gst_message_unref (msg);
    msg = gst_bus_pop (bus);
  }
}

Suppose while you are handling msg in the while loop the other thread posts
a message on the bus. It turns out that this message is not expected nor
handled (the blocking msg =  gst_bus_timed_pop_filtered is not active).
[This is bad, especially in real-time applications.]

How could I figure in the other thread that the message is being expected,
i.e., stays in the blocking msg = gst_bus_timed_pop_filtered(...)?

Of course, this is a general synchronization problem, but I though
GStreamer had special APIs or idioms for this common problem. This would be
possible, if, e.g.,  gst_bus_post (bus, ...) returned more meaningful (than
just boolean) return codes.

I suggested surrounding by flush OFF/ON since this (almost) allows the
other thread to check whether its posting is expected or not (by boolean
result), and repeat (and/or taking other actions) until it is.

BTW, if you post a message on the bus no one is listening to, does it
disappear or remains until someone starts to listen for this kind of
messages on the bus?


On Fri, Jun 13, 2014 at 2:15 PM, Tim Müller <tim at centricular.com> wrote:

> On Fri, 2014-06-13 at 13:47 +0200, Sergei Vorobyov wrote:
>
> Hi Sergei,
>
> > Sorry for a simple question, I could do it by using mutexes, of
> > course, but maybe there is a canonical way of doing it using the
> > GStreamer APIs?
> >
> >
> > The problem is this.
> >
> >
> > 1. In one thread, periodically, in the loop, I am waiting for a
> > message on the bus:
> >
> >
> > msg = gst_bus_timed_pop_filtered (bus, duration,
> >
> > (GstMessageType)(GST_MESSAGE_EOS |
> >
> >  GST_MESSAGE_SEGMENT_DONE |
> >
> >  GST_MESSAGE_ERROR |
> >
> >  GST_MESSAGE_APPLICATION));
> >
> >
> > and this waiting occupies only part of the whole loop:
> >
> >
> > for(;;) {
> >   // do smth else...
> >
> >
> >   msg = gst_bus_timed_pop_filtered...
> >
> >
> >   // do smth else ...
> > }
> >
> >
> > In the other thread I am posting (using the GStreamer's gift):
> >
> >
> > gst_bus_post (bus, gst_message_new_application (NULL, NULL));
> >
> >
> >
> > But how can I make sure, before posting, that it is being expected
> > (and received) by the blocking
> >
> >
> > msg = gst_bus_timed_pop_filtered... in the other thread,
> >
> >
> >
> > and not consumed by the loop doing smth else?
> >
> >
> > Actually, the boolean TRUE returned by gst_bus_post does not guarantee
> > that the posting was expected and received.
> >
> >
> > One might naively think that the following is a solution (simply
> > surround with gst_bus_set_flushing):
> >
> >
> > for(;;) {
> >   // do smth else...
> >
> >
> >   gst_bus_set_flushing (bus, FALSE);
> >   msg = gst_bus_timed_pop_filtered...
> >   gst_bus_set_flushing (bus, TRUE);
> >
> >
> >   // do smth else ...
> > }
> >
> >
> > However, it isn't. Imagine gst_bus_timed_pop_filtered returns because
> > duration expired, but before the next gst_bus_set_flushing (bus, TRUE)
> > the other thread makes gst_bus_post (bus, gst_message_new_application
> > (NULL, NULL)). Then this posting is acknowledged (returns TRUE) but
> > the message is actually flushed.
> >
> >
> > Some real acknowledge mechanism is desired. Does it exist in the
> > GStreamer API?
>
> It's not entirely clear to me what you're trying to achieve, if you want
> to make sure in your element that the message was actually processed
> (and maybe block until then), or if you're having trouble picking up the
> message in your other thread.
>
> In you sample code you would probably want to do something like:
>
>   msg = gst_bus_timed_pop_filtered(...)
>   while (msg) {
>     handle_message (msg);
>     gst_message_unref (msg);
>     msg = gst_bus_pop (bus);
>   }
>
> to make sure that if there's a message you get all pending messages off
> in one go before you do something else.
>
> Flushing the bus is a bit weird IMHO and should probably only be done in
> special circumstances.
>
>  Cheers
>   -Tim
>
> --
> Tim Müller, Centricular Ltd - http://www.centricular.com
>
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>
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