ffmpeg fails to get the duration of some webm files

[Thirumalaisamy K]

FFmpeg fails to get the duration of some WebM files. Works fine with VLC and MediaInfo.

$ ffprobe.git -v trace /Users/Shared/Lydian\'s\ Two-Piano\ Medley\ for\ \$1M\ Prize\ -\ The\ World\'s\ Best\ Finale.webm
ffprobe version N-93384-g0dda0f3bdb-tessus  https://evermeet.cx/ffmpeg/  Copyright (c) 2007-2019 the FFmpeg developers
  built with Apple LLVM version 10.0.0 (clang-1000.11.45.5)
  configuration: --cc=/usr/bin/clang --prefix=/opt/ffmpeg --extra-version=tessus --enable-avisynth --enable-fontconfig --enable-gpl --enable-libaom --enable-libass --enable-libbluray --enable-libdav1d --enable-libfreetype --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libmysofa --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264 --enable-libopenjpeg --enable-libopus --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-libzmq --enable-libzvbi --enable-version3 --pkg-config-flags=--static --disable-ffplay
  libavutil      56. 26.100 / 56. 26.100
  libavcodec     58. 47.103 / 58. 47.103
  libavformat    58. 26.101 / 58. 26.101
  libavdevice    58.  6.101 / 58.  6.101
  libavfilter     7. 48.100 /  7. 48.100
  libswscale      5.  4.100 /  5.  4.100
  libswresample   3.  4.100 /  3.  4.100
  libpostproc    55.  4.100 / 55.  4.100
[NULL @ 0x7fbdb1000000] Opening '/Users/Shared/Lydian's Two-Piano Medley for $1M Prize - The World's Best Finale.webm' for reading
[file @ 0x7fbdb0500240] Setting default whitelist 'file,crypto'
Probing matroska,webm score:100 size:2048
Probing mp3 score:1 size:2048
[matroska,webm @ 0x7fbdb1000000] Format matroska,webm probed with size=2048 and score=100
st:0 removing common factor 1000000 from timebase
st:1 removing common factor 1000000 from timebase
[matroska,webm @ 0x7fbdb1000000] Before avformat_find_stream_info() pos: 4722 bytes read:32768 seeks:0 nb_streams:2
[vp9 @ 0x7fbdb0808000] Format yuv420p chosen by get_format().
[matroska,webm @ 0x7fbdb1000000] All info found
[matroska,webm @ 0x7fbdb1000000] stream 0: start_time: 0.000 duration: -9223372036854776.000
[matroska,webm @ 0x7fbdb1000000] stream 1: start_time: 0.000 duration: -9223372036854776.000
[matroska,webm @ 0x7fbdb1000000] format: start_time: 0.000 duration: -9223372036854.775 bitrate=0 kb/s
[matroska,webm @ 0x7fbdb1000000] After avformat_find_stream_info() pos: 5084 bytes read:32768 seeks:0 frames:2
Input #0, matroska,webm, from '/Users/Shared/Lydian's Two-Piano Medley for $1M Prize - The World's Best Finale.webm':
  Metadata:
    encoder         : Lavf56.40.101
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0(eng), 1, 1/1000: Video: vp9 (Profile 0), 1 reference frame, yuv420p(tv, bt709/unknown/unknown), 1280x720, 0/1, SAR 1:1 DAR 16:9, 29.97 fps, 29.97 tbr, 1k tbn, 1k tbc (default)
    Stream #0:1(eng), 1, 1/1000: Audio: vorbis, 44100 Hz, stereo, fltp (default)
[AVIOContext @ 0x7fbdb07000c0] Statistics: 32768 bytes read, 0 seeks

[Thirumalaisamy K]

The sample file can be found here
​https://www.dropbox.com/s/bqfe8efhwxwene1/Lydian%27s%20Two-Piano%20Medley%20for%20%241M%20Prize%20-%20The%20World%27s%20Best%20Finale.webm?dl=1