piped input partial file error

[Mark]

Summary of the bug:

ffmpeg failes to read file from the piped input

ffmpeg -i - out.mp3 < in.mp4 # it works perfectly

cat in.mp4 | ffmpeg -i - out.mp3 # fails with the error below.

Why do I need it:
I'm trying to convert to pcm_s16le file while downloading it from the internet. And this is the simplest example to reproduce the issue. cat is the replacement of wget/curl.

How to reproduce:

% cat in.mp4 | ffmpeg -i - out.mp3
ffmpeg version 6.0 Copyright (c) 2000-2023 the FFmpeg developers
  built with Apple clang version 14.0.3 (clang-1403.0.22.14.1)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/6.0_1 --enable-shared --enable-pthreads --enable-version3 --cc=clang --host-cflags= --host-ldflags= --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libaribb24 --enable-libbluray --enable-libdav1d --enable-libmp3lame --enable-libopus --enable-librav1e --enable-librist --enable-librubberband --enable-libsnappy --enable-libsrt --enable-libsvtav1 --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libspeex --enable-libsoxr --enable-libzmq --enable-libzimg --disable-libjack --disable-indev=jack --enable-videotoolbox --enable-audiotoolbox
  libavutil      58.  2.100 / 58.  2.100
  libavcodec     60.  3.100 / 60.  3.100
  libavformat    60.  3.100 / 60.  3.100
  libavdevice    60.  1.100 / 60.  1.100
  libavfilter     9.  3.100 /  9.  3.100
  libswscale      7.  1.100 /  7.  1.100
  libswresample   4. 10.100 /  4. 10.100
  libpostproc    57.  1.100 / 57.  1.100
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fe6f5704b80] stream 0, offset 0x2c: partial file
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'fd:':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2mp41
    encoder         : Lavf60.3.100
  Duration: 00:00:08.00, start: 0.000000, bitrate: N/A
  Stream #0:0[0x1](eng): Audio: aac (mp4a / 0x6134706D), 48000 Hz, mono, fltp, 69 kb/s (default)
    Metadata:
      handler_name    : SoundHandle
      vendor_id       : [0][0][0][0]
Stream mapping:
  Stream #0:0 -> #0:0 (aac (native) -> mp3 (libmp3lame))
[mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fe6f5704b80] stream 0, offset 0x2c: partial file
Error demuxing input file 0: Invalid data found when processing input
fd:: Invalid data found when processing input
Output #0, mp3, to 'out.mp3':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2mp41
    TSSE            : Lavf60.3.100
  Stream #0:0(eng): Audio: mp3, 48000 Hz, mono, fltp (default)
    Metadata:
      handler_name    : SoundHandle
      vendor_id       : [0][0][0][0]
      encoder         : Lavc60.3.100 libmp3lame
size=       0kB time=-577014:32:22.77 bitrate=N/A speed=N/A    ed=N/A
video:0kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Output file is empty, nothing was encoded (check -ss / -t / -frames parameters if used)

[Elon Musk]

for mp4 one usually needs to seek to end and than to beginning and that can not work via pipe.
Not really ffmpeg fault but of mp4 file.