-err_detect crccheck and explode+crccheck are broken for flac files

[Balling]

Summary of the bug: use ​https://files.catbox.moe/kvvrac.flac it was printing "CRC error at PTS 29583360" but it no longer warns about it. Regression of #2266 probably because of #9621. It is problematic that line ​https://github.com/FFmpeg/FFmpeg/blob/56085e057ff2d2da180a5d56bdd1efd70d53e662/libavcodec/flacdec.c#L772 is dead code now.
How to reproduce:

% ffmpeg -err_detect crccheck -i https://files.catbox.moe/kvvrac.flac -f null -

Patches should be submitted to the ffmpeg-devel mailing list and not this bug tracker.

[Elon Musk]

Not proof of anything. Just some statements that have no hard evidence.
If you knew how to check you would know that code at line 772 is still always executed.

[Balling]

Before:

ffmpeg version git-2020-08-31-4a11a6f Copyright (c) 2000-2020 the FFmpeg developers
  built with gcc 10.2.1 (GCC) 20200805
  configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libdav1d --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libsrt --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libgsm --enable-librav1e --enable-libsvtav1 --disable-w32threads --enable-libmfx --enable-ffnvcodec --enable-cuda-llvm --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth --enable-libopenmpt --enable-amf
  libavutil      56. 58.100 / 56. 58.100
  libavcodec     58.101.101 / 58.101.101
  libavformat    58. 51.101 / 58. 51.101
  libavdevice    58. 11.101 / 58. 11.101
  libavfilter     7. 87.100 /  7. 87.100
  libswscale      5.  8.100 /  5.  8.100
  libswresample   3.  8.100 /  3.  8.100
  libpostproc    55.  8.100 / 55.  8.100
Input #0, flac, from 'kvvrac.flac':
  Metadata:
    ARTIST          : Van Morrison
    Title           : See Me Through > Soldier Of Fortune > Thank You Falletin Me Be Mice Elf Agin > Burning Ground
    ALBUM           : Playhouse Theatre, Edinburgh, Scotland (1999-04-02)
    Date            : 1999-04-02
    track           : 07
    comment         : Encoded by FLAC v1.1.2a with FLAC Frontend v1.7.1; CD 2
  Duration: 00:17:55.21, start: 0.000000, bitrate: 589 kb/s
    Stream #0:0: Audio: flac, 44100 Hz, stereo, s16
Stream mapping:
  Stream #0:0 -> #0:0 (flac (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
Output #0, null, to 'pipe:':
  Metadata:
    ARTIST          : Van Morrison
    Title           : See Me Through > Soldier Of Fortune > Thank You Falletin Me Be Mice Elf Agin > Burning Ground
    ALBUM           : Playhouse Theatre, Edinburgh, Scotland (1999-04-02)
    Date            : 1999-04-02
    track           : 07
    comment         : Encoded by FLAC v1.1.2a with FLAC Frontend v1.7.1; CD 2
    encoder         : Lavf58.51.101
    Stream #0:0: Audio: pcm_s16le, 44100 Hz, stereo, s16, 1411 kb/s
    Metadata:
      encoder         : Lavc58.101.101 pcm_s16le
[flac @ 000001de24b1a380] CRC error at PTS 29583360
size=N/A time=00:17:55.21 bitrate=N/A speed=2.17e+03x
video:0kB audio:185222kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown

After:

ffmpeg version N-111995-g5f8c113b75-20230909 Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 13.1.0 (crosstool-NG 1.25.0.196_227d99d)
  configuration: --prefix=/ffbuild/prefix --pkg-config-flags=--static --pkg-config=pkg-config --cross-prefix=x86_64-w64-mingw32- --arch=x86_64 --target-os=mingw32 --enable-gpl --enable-version3 --disable-debug --enable-shared --disable-static --disable-w32threads --enable-pthreads --enable-iconv --enable-libxml2 --enable-zlib --enable-libfreetype --enable-libfribidi --enable-gmp --enable-lzma --enable-fontconfig --enable-libvorbis --enable-opencl --disable-libpulse --enable-libvmaf --disable-libxcb --disable-xlib --enable-amf --enable-libaom --enable-libaribb24 --enable-avisynth --enable-chromaprint --enable-libdav1d --enable-libdavs2 --disable-libfdk-aac --enable-ffnvcodec --enable-cuda-llvm --enable-frei0r --enable-libgme --enable-libkvazaar --enable-libass --enable-libbluray --enable-libjxl --enable-libmp3lame --enable-libopus --enable-librist --enable-libssh --enable-libtheora --enable-libvpx --enable-libwebp --enable-lv2 --enable-libvpl --enable-openal --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenh264 --enable-libopenjpeg --enable-libopenmpt --enable-librav1e --enable-librubberband --enable-schannel --enable-sdl2 --enable-libsoxr --enable-libsrt --enable-libsvtav1 --enable-libtwolame --enable-libuavs3d --disable-libdrm --disable-vaapi --enable-libvidstab --enable-vulkan --enable-libshaderc --enable-libplacebo --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxvid --enable-libzimg --enable-libzvbi --extra-cflags=-DLIBTWOLAME_STATIC --extra-cxxflags= --extra-ldflags=-pthread --extra-ldexeflags= --extra-libs=-lgomp --extra-version=20230909
  libavutil      58. 19.100 / 58. 19.100
  libavcodec     60. 26.100 / 60. 26.100
  libavformat    60. 11.100 / 60. 11.100
  libavdevice    60.  2.101 / 60.  2.101
  libavfilter     9. 11.100 /  9. 11.100
  libswscale      7.  3.100 /  7.  3.100
  libswresample   4. 11.100 /  4. 11.100
  libpostproc    57.  2.100 / 57.  2.100
Input #0, flac, from 'kvvrac.flac':
  Metadata:
    ARTIST          : Van Morrison
    Title           : See Me Through > Soldier Of Fortune > Thank You Falletin Me Be Mice Elf Agin > Burning Ground
    ALBUM           : Playhouse Theatre, Edinburgh, Scotland (1999-04-02)
    Date            : 1999-04-02
    track           : 07
    comment         : Encoded by FLAC v1.1.2a with FLAC Frontend v1.7.1; CD 2
  Duration: 00:17:55.21, start: 0.000000, bitrate: 589 kb/s
  Stream #0:0: Audio: flac, 44100 Hz, stereo, s16
Stream mapping:
  Stream #0:0 -> #0:0 (flac (native) -> pcm_s16le (native))
Press [q] to stop, [?] for help
Output #0, null, to 'pipe:':
  Metadata:
    ARTIST          : Van Morrison
    Title           : See Me Through > Soldier Of Fortune > Thank You Falletin Me Be Mice Elf Agin > Burning Ground
    ALBUM           : Playhouse Theatre, Edinburgh, Scotland (1999-04-02)
    Date            : 1999-04-02
    track           : 07
    comment         : Encoded by FLAC v1.1.2a with FLAC Frontend v1.7.1; CD 2
    encoder         : Lavf60.11.100
  Stream #0:0: Audio: pcm_s16le, 44100 Hz, stereo, s16, 1411 kb/s
    Metadata:
      encoder         : Lavc60.26.100 pcm_s16le
[out#0/null @ 00000253f9edeb40] video:0kB audio:185168kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
size=N/A time=00:17:55.20 bitrate=N/A speed=1.2e+03x

[Balling]

I am really tired of this, here is a file where I changed one byte randomly, same happens on it: ​https://0x0.st/HVQc.flac

[Balling]

Why? Chrome uses explode, this means bad flac files will pass through. I did not notice that other issue, but that is worse.

Of course it is more or less cosmetic, but still. Do you like dead log code?

[Balling]

How do you trigger line 722 and line 774? Can you at least remove dead code and add a warning for underread.

[Balling]

This is fixed now

[flac @ 000001e2f63c1f40] CRC error at PTS -9223372036854775808

Though ffmpeg -i HVQc.flac -f md5 -

now has different md5 other runs, it is not reproducible anymore (it is caused video stream, -vn fixes it). But whatever.