int ff_read_packet(AVFormatContext *s, AVPacket *pkt)
{
    int ret, i, err;
    AVStream *st;
    for (;;) {
        AVPacketList *pktl = s->internal->raw_packet_buffer;
        if (pktl) {
            *pkt = pktl->pkt;
            st   = s->streams[pkt->stream_index];
            if (s->internal->raw_packet_buffer_remaining_size <= 0)
                if ((err = probe_codec(s, st, NULL)) < 0)
                    return err;
            if (st->request_probe <= 0) {
                s->internal->raw_packet_buffer                 = pktl->next;
                s->internal->raw_packet_buffer_remaining_size += pkt->size;
                av_free(pktl);
                return 0;
        pkt->data = NULL;
        pkt->size = 0;
        av_init_packet(pkt);
        ret = s->iformat->read_packet(s, pkt);
        if (ret < 0) {
            /* Some demuxers return FFERROR_REDO when they consume
               data and discard it (ignored streams, junk, extradata).
               We must re-call the demuxer to get the real packet. */
            if (ret == FFERROR_REDO)
                continue;
            if (!pktl || ret == AVERROR(EAGAIN))
                return ret;
            for (i = 0; i < s->nb_streams; i++) {
                st = s->streams[i];
                if (st->probe_packets || st->request_probe > 0)
                    if ((err = probe_codec(s, st, NULL)) < 0)
                        return err;
                av_assert0(st->request_probe <= 0);
            continue;
        if (!pkt->buf) {
            AVPacket tmp = { 0 };
            ret = av_packet_ref(&tmp, pkt);
            if (ret < 0)
                return ret;
            *pkt = tmp;
        if ((s->flags & AVFMT_FLAG_DISCARD_CORRUPT) &&
            (pkt->flags & AV_PKT_FLAG_CORRUPT)) {
            av_log(s, AV_LOG_WARNING,
                   "Dropped corrupted packet (stream = %d)\n",
                   pkt->stream_index);
            av_packet_unref(pkt);
            continue;
        if (pkt->stream_index >= (unsigned)s->nb_streams) {
            av_log(s, AV_LOG_ERROR, "Invalid stream index %d\n", pkt->stream_index);
            continue;
        st = s->streams[pkt->stream_index];
        if (update_wrap_reference(s, st, pkt->stream_index, pkt) && st->pts_wrap_behavior == AV_PTS_WRAP_SUB_OFFSET) {
            // correct first time stamps to negative values
            if (!is_relative(st->first_dts))
                st->first_dts = wrap_timestamp(st, st->first_dts);
            if (!is_relative(st->start_time))
                st->start_time = wrap_timestamp(st, st->start_time);
            if (!is_relative(st->cur_dts))
                st->cur_dts = wrap_timestamp(st, st->cur_dts);
        pkt->dts = wrap_timestamp(st, pkt->dts);
        pkt->pts = wrap_timestamp(st, pkt->pts);
        force_codec_ids(s, st);
        /* TODO: audio: time filter; video: frame reordering (pts != dts) */
        if (s->use_wallclock_as_timestamps)
            pkt->dts = pkt->pts = av_rescale_q(av_gettime(), AV_TIME_BASE_Q, st->time_base);
        if (!pktl && st->request_probe <= 0)
            return ret;
        err = add_to_pktbuf(&s->internal->raw_packet_buffer, pkt,
                            &s->internal->raw_packet_buffer_end, 0);
        if (err)
            return err;
        s->internal->raw_packet_buffer_remaining_size -= pkt->size;
        if ((err = probe_codec(s, st, pkt)) < 0)
            return err;