[FFmpeg-devel] mp4 demuxer only set video stream start_time

[vacingfang at qq.com]

Hi, 
    Recently, I met a case of wrong audio duration when ffprobe a mp4 file use FFmpeg 4.0, but right in FFmpeg 4.1.3. 
    In moov, the audio duration was right, the format duration (in mvhd) was wrong. But the probe output showed that the audio duration was set to the wrong format duration.

    I root cause this case and found out two patches can fix the problem:
commit 28503c5aea5f5ecaab95e9d0c56064132e4796d3, This fix the problem of wrong duration caculation. [https://github.com/FFmpeg/FFmpeg/commit/28503c5aea5f5ecaab95e9d0c56064132e4796d3]
commit 12673bb25342c772e1bf5fd2adebd85f97dc616e, This fix the problem of audio stream start_time not set. [https://github.com/FFmpeg/FFmpeg/commit/12673bb25342c772e1bf5fd2adebd85f97dc616e]
    The first patch was resonable while the second patch puzzed me a lot. Why I need this one? Because if the stream start_time is not set, it will be filled in avformat_find_stream_info() -> estimate_timings() -> fill_all_stream_timings() [libavformat/utils.c], AND at the same time, this function will replace the stream duration with the Format Duration, so stream start_time must be set so that I can get the right audio duration. 
    
    I have two questions now:
Why should fill_all_stream_timings() update stream duration when stream start_time is not set? 
In mp4 demuxer, codes related to stream start_time setting only used by video stream before patch 12673bb25342c772e1bf5fd2adebd85f97dc616e. Now in mov_build_index() still as it is, should this be fixed too?
    


vacingfang at qq.com