[Bug 796900] New: avdec: loses the last (num_cores + 1) frames

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=796900

            Bug ID: 796900
           Summary: avdec: loses the last (num_cores + 1) frames
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-libav
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: aboya at igalia.com
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

How to reproduce:

1. Update to the latest master. The problem reproduces with ffmpeg 4.0.

2. Create a test file with 3 + (num_cores + 1) frames:

NUM_CORES=$(getconf _NPROCESSORS_ONLN)
gst-launch-1.0 videotestsrc num-buffers=$((3 + ($NUM_CORES + 1))) !
video/x-raw, framerate=1/1 ! timeoverlay font-desc="Monospace, 64" ! x264enc !
qtmux ! filesink location=/tmp/movie.mp4

3. Play it with gst-play-1.0 or any other GStreamer based player in the same
machine, making sure that avdec_h264 is being used as decoder.

Expected result:

`3 + (num_cores + 1)` frames should be played, during `3 + (num_cores + 1)`
seconds.

Actual result:

Only the first 3 frames are emited by avdec_h264. Then the playback ends. If a
file with only `(num_cores + 1)` or fewer frames is open, playback fails
completely.

This can also be checked with gst-launch:

$ gst-launch-1.0 -v filesrc location=/tmp/movie.mp4 ! qtdemux ! avdec_h264 !
fakesink silent=false | grep -c chain
3

It can also be reproduced in a single pipeline if desired:

$ gst-launch-1.0 -v videotestsrc num-buffers=$((3 + ($NUM_CORES + 1))) \
  ! video/x-raw, framerate=1/1 ! timeoverlay font-desc="Monospace, 64" \
  ! x264enc ! avdec_h264 ! fakesink silent=false | grep -c chain
3

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