[Libav-user] avcodec_receive_frame returns duplicate buffers.

[Hugh Waite]

Hello,
I have an application that decodes an input stream into AVFrames and stores
these in a ring buffer. On a trigger, I can create an output file
containing video from the past N seconds until I stop.
I am seeing a problem with the output video that it sometimes contains
duplicate frames and sometimes contains frames from "the future" (i.e. the
wrong point in the ringbuffer). I have added debug and it appears that
avcodec_receive_frame() sometimes returns the same data buffer for multiple
decoded frames, even though I have not unref'ed the previous AVFrame.
Specifically I am checking that the returned av_frame->data[0] is not being
used by any other AVFrames and this is failing.

This is on Ubuntu bionic (18.04) using the bionic backports of v4.1.3 and
v4.2.0 from https://launchpad.net/~jonathonf/+archive/ubuntu/ffmpeg-4.
The source of the video stream is a v4l2 device.

Cut-down example of the frame retrieve function:
void *reader_run()
{
  AVFrame *ff = NULL;
  while (is_running)
  {
    AVPacket;
    av_init_packet(&pkt);
    ff = av_alloc_frame();

    /* Get packet from input */
    ret = av_read_frame(reader_fctx, &pkt);
    /* error checking ... */
    ret = avcodec_send_packet(decoder_cctx, &pkt);
    /* error checking ... */
    ret = avcodec_receive_frame(decoder_cctx, ff);
    /* error checking ... */

    /* Debug to check buffers in other frames */
    for (int i = 0; i< ringbuffer->size; i++)
    {
      if ringbuffer->items[i]->data[0] == ff->data[0]
      {
        ERROR("The buffer at %u is a duplicate when inserting %u %p", i,
ringbuffer->next, ff->data[0]);
      }
    } /* End of debug */

    av_packet_unref(&pkt);
    rb_push(ringbuffer, ff); // Push onto head of ringbuffer and free
oldest frame
  }
  /* Exiting. Flush decoder ... */
}

The buffer at 0 is a duplicate when inserting 1 0x7f1eb881e000
The buffer at 0 is a duplicate when inserting 2 0x7f1eb881e000
The buffer at 1 is a duplicate when inserting 2 0x7f1eb881e000
The buffer at 3 is a duplicate when inserting 301 0x7f1eb8a55000

The AVFrame's are not duplicated (new one alloc'ed each time), but they are
being populated with the same "data[0]" as other AVFrames. Is there
anything I am misunderstanding about these buffers?
This usually happens 2-3 times as soon as the decoder starts, and then
again when the buffer is full and these frames are being freed. It is not
the same buffer pointers each time.

This code is being run in a thread, but it is the only thread using this
decoder. It happens even when not passing the frames to an encoder.

Can anyone see something that would cause this?

I am going to build my own copy of the libraries, but if this exhibits the
same issue what can I check further to find the source of this problem
(e.g. any data that would be useful in a bug report)

Many thanks,
Hugh
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