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h265parse: Messes up timestamps for byte-stream

Submitted by Matej @Knopp

Link to original bug (#791495)

Description

It happens when previous frame has trailing zero. Parser split frame too early, leaving trailing zeros as prefix for new frame, which results in messed up timestamps.

Caused by following code in gst_h265_parser_identify_nalu

/* Mini performance improvement:

  • We could have a way to store how many 0s were skipped to avoid
  • parsing them again on the next NAL */
    while (off2 > 0 && data[nalu->offset + off2 - 1] == 00)
    off2--;

Annex B does says that there should be zero byte before AU startcode, however it also says that during parsing the zero byte should be discarded. So maybe determining the timestamp from zero byte is not a very good idea.

The stream already parsed by FFMPEG HEVC parser, which seems to leave zero byte trailing, but that doesn't mean such files can't be encountered in the wild.

Btw. Why's there loop? Why skipping more than 1 zero byte? Doesn't that just increases chance of messed up timestamps?