HLS muxer: inital fmp4 segment has non-zero duration even if `hls_init_time` is set to zero

[kebl]

The HLS muxer supports fragmented mp4, which by default produces an initial segment called init.mp4. With ffmpeg from the Arch repo (version n4.4) the command below produces an initial segment 6.08 seconds long, but with static builds of ffmpeg 4.4.1 and 4.2.2 the initial segment has no duration. hls_init_time is set to zero so the inital segment should have no duration.

The effect the non-zero duration has is concatenating segments doesn't work properly anymore. For example with a no-duration initial segment, $ cat init.mp4 out2.m4s out3.m4s | ffplay - will play the segments starting from out2.m4s (12 seconds in from the input video). But with a non-zero duration it'll play the first six seconds of the input video, freeze-frame for six seconds, then play out2.m4s. (It looks like the initial segment has the same video content as the 0th segment because $ cat init.mp4 | ffplay - behaves identically to $ cat init.mp4 out0.m4s | ffplay -.

% ffmpeg -i in.webm -g 24 -sc_threshold 0 -f hls -hls_init_time 0 -hls_time 6 -hls_segment_type fmp4 out.m3u8

in.webm is ​https://upload.wikimedia.org/wikipedia/commons/e/e7/Slow_motion_drop.webm.

---

Initial segment produced by Arch ffmpeg (Duration: 00:00:06.08):

% ffprobe init.mp4
ffprobe version n4.4 Copyright (c) 2007-2021 the FFmpeg developers
  built with gcc 11.1.0 (GCC)
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-amf --enable-avisynth --enable-cuda-llvm --enable-lto --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --enable-libmfx --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-librav1e --enable-librsvg --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-libzimg --enable-nvdec --enable-nvenc --enable-shared --enable-version3
  libavutil      56. 70.100 / 56. 70.100
  libavcodec     58.134.100 / 58.134.100
  libavformat    58. 76.100 / 58. 76.100
  libavdevice    58. 13.100 / 58. 13.100
  libavfilter     7.110.100 /  7.110.100
  libswscale      5.  9.100 /  5.  9.100
  libswresample   3.  9.100 /  3.  9.100
  libpostproc    55.  9.100 / 55.  9.100
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'init.mp4':
  Metadata:
    major_brand     : iso5
    minor_version   : 512
    compatible_brands: iso5iso6mp41
    encoder         : Lavf58.76.100
  Duration: 00:00:06.08, start: 0.166341, bitrate: 381 kb/s
  Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], 378 kb/s, 24 fps, 24 tbr, 12288 tbn, 48 tbc (default)
    Metadata:
      handler_name    : VideoHandler
      vendor_id       : [0][0][0][0]

---

Initial segment produced by ffmpeg 4.4.1-static (Duration: N/A):

ffprobe version n4.4 Copyright (c) 2007-2021 the FFmpeg developers
  built with gcc 11.1.0 (GCC)
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-amf --enable-avisynth --enable-cuda-llvm --enable-lto --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --enable-libmfx --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-librav1e --enable-librsvg --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-libzimg --enable-nvdec --enable-nvenc --enable-shared --enable-version3
  libavutil      56. 70.100 / 56. 70.100
  libavcodec     58.134.100 / 58.134.100
  libavformat    58. 76.100 / 58. 76.100
  libavdevice    58. 13.100 / 58. 13.100
  libavfilter     7.110.100 /  7.110.100
  libswscale      5.  9.100 /  5.  9.100
  libswresample   3.  9.100 /  3.  9.100
  libpostproc    55.  9.100 / 55.  9.100
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x560481b4f240] Could not find codec parameters for stream 0 (Video: h264 (avc1 / 0x31637661), none, 1280x720): unspecified pixel format
Consider increasing the value for the 'analyzeduration' (0) and 'probesize' (5000000) options
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'init.mp4':
  Metadata:
    major_brand     : iso5
    minor_version   : 512
    compatible_brands: iso5iso6mp41
    encoder         : Lavf58.76.100
  Duration: N/A, bitrate: N/A
  Stream #0:0(und): Video: h264 (avc1 / 0x31637661), none, 1280x720, SAR 1:1 DAR 16:9, 12288 tbr, 12288 tbn, 24576 tbc (default)
    Metadata:
      handler_name    : VideoHandler
      vendor_id       : [0][0][0][0]

[Balling]

Lets close this one as duplicate instead. #9484