[gstreamer-bugs] [Bug 543444] New: gst_base_sink_get_position can return 0 unexpectedly

[GStreamer (bugzilla.gnome.org)]

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  GStreamer | gstreamer (core) | Ver: HEAD CVS
           Summary: gst_base_sink_get_position can return 0 unexpectedly
           Product: GStreamer
           Version: HEAD CVS
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gstreamer (core)
        AssignedTo: gstreamer-bugs at lists.sourceforge.net
        ReportedBy: wingo at pobox.com
         QAContact: gstreamer-bugs at lists.sourceforge.net
     GNOME version: Unspecified
   GNOME milestone: Unspecified


Not sure if this is a bug, but it's certainly unexpected. Let's say I'm sending
50 flushing seeks per second to a pipeline while scrubbing. I make sure that
the pipeline is prerolled before I seek, so I don't overload it with flushing
seeks. Let's say at the same time, I'm querying the pipeline's position 100
times a second. I disregard failed queries, taking the last value that I saw.

The problem is that e.g. when scrubbing around second 200 of a MOV file, that
basesink sometimes returns 0. I haven't investigated too much, but things like
this creep me out, in gstbasesink.c:3243:

      /* collect all data we need holding the lock */
      if (GST_CLOCK_TIME_IS_VALID (basesink->segment.time))
        time = basesink->segment.time;
      else
        time = 0;

and in gstbasesink.c:2546:

    case GST_EVENT_FLUSH_STOP:
      ...
      /* we need new segment info after the flush. */
      gst_segment_init (&basesink->segment, GST_FORMAT_UNDEFINED);
      gst_segment_init (basesink->abidata.ABI.clip_segment,
          GST_FORMAT_UNDEFINED);
      basesink->have_newsegment = FALSE;

So in this case after getting the flush_stop but before getting the first
buffer, get_position will report 0. (gst_segment_init() sets segment.time to
0.) But we actually don't know the position then.

I guess the right thing to do is to protect have_newsegment with the object
lock (currently it's just the stream lock), and to make get_position return -1
or fail when we don't actually have the new segment. Or does this diagnostic
look wrong?


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