[Gstreamer-bugs] gstqueue.c: on a full queue, writer thread not unblocked if upstream queue flushed

[Martin Janzen]

[Not sure this belongs on gstreamer-devel; but let me know if that would 
be a better place for this than gstreamer-maint...]

I ran into a small problem the other day, and I believe it was related 
to a bug in the GStreamer queue.  It's pretty trivial, and I have a 
patch; but before I post this to Bugzilla I want to check that I am 
using the framework correctly.

Background:  I have an element which extracts PTS (Presentation Time 
Stamp) values from an MPEG-2 transport stream.  If a large discontinuity 
in the PTS values occurs, the element needs to adjust its 
PTS-to-GST-clock mapping, and then to flush either the upstream 
(transport buffer) or downstream (frame buffer) queue (or maybe both; 
still under construction!).

Flushing the downstream queue is easy, of course:

   gst_pad_push (myelement->srcpad,
     GST_BUFFER (gst_event_new_flush ()));

And if I understand correctly, flushing the upstream one is not much 
more difficult; something like this:

   GstPad *peer = gst_pad_get_peer (myelement->sink_pad);
   GstElement *src = gst_pad_get_parent (src);
   if (GST_IS_QUEUE (src)) {
     gst_element_set_state (src, GST_STATE_PAUSED);
     gst_pad_send_event (peer, gst_event_new_flush ());
     gst_element_set_state (src, GST_STATE_PLAYING);
   }

(Arguably this code shouldn't be in my element itself; instead, the 
element could generate a discontinuity notification/signal/event of some 
sort, which  causes, um, something else to flush the queue.  But for 
testing...)

Anyway, the problem I found is that, if the upstream queue is full and a 
writer thread is blocked waiting for the queue to empty, that thread 
will stay blocked forever.

It seems clear that the gst_queue_handle_src_event() event function 
which flushes the queue should also wake up the writer thread.  It's a 
trivial fix -- see attached patch -- and I've tested it successfully; 
but I just want to make sure that sending events upstream as I've done 
above is acceptable practice, and that the GStreamer framework is in 
fact supposed to support this sort of thing.  Comments, anyone...?

-- 
Martin Janzen
janzen at pixelmetrix dot com

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