Recording to FLAC results in undecodable file

[PatrickJosh]

Bug Description

Hi,

with the current commit on the main branch (05ee404), I cannot record to FLAC any more; the resulting file is not readable by any flac decoder that I have tried (ffmpeg, libflac). libflac tells me that there is an issue with the metadata of the file, which does seem to be the case; have a look at the metadata from a file attached below (output of metaflac --list test.flac):

METADATA block #0
  type: 0 (STREAMINFO)
  is last: false
  length: 0
  minimum blocksize: 65528 samples
  maximum blocksize: 47640 samples
  minimum framesize: 14 bytes
  maximum framesize: 11414 bytes
  sample_rate: 10 Hz
  channels: 3
  bits-per-sample: 32
  total samples: 452608
  MD5 signature: 8741c93c8c77b88572201850ae781805
METADATA block #1
  type: 4 (VORBIS_COMMENT)
  is last: true
  length: 40
  vendor string: reference libFLAC 1.5.0 20250211
  comments: 0

The length, sample rate and channels are not correct. Unfortunately, re-encoding the file is not possible (at least not with the tools mentioned above) as it is completely undecodable.

When compiling tag 2.5.6 and using the resulting binary, the resulting file is fine.

Here the mentioned example file (zipped because of file type restrictions): test.zip

Version

05ee404

OS

Fedora Linux 44

[ronso0]

Sounds like the regression I experienced with #13761
Details are here #13761 (comment)

Can you git revert 548094e4cb689ebdf1283b0716b64aa193907f83 and test again?

[PatrickJosh]

Thanks, I'll test this later today!

Were you able to make the broken files decodable somehow?

[ronso0]

No, I gave up after both Mixxx and VLC failed to play them.
I'm not into encoders/decoders and don't intend to debug this, so I simply reverted #13761 in my production branch.

[PatrickJosh]

After having reverted 548094e, it does work again indeed.

[PatrickJosh]

I have found the reason for the problem, I will submit a pull request.

[ronso0]

Great, a PR is very much appreciated!

[PatrickJosh]

Btw, I have been able to make my recording decodable by fixing the metadata according to the FLAC file format specification by directly editing the binary. Only a few fields in the first metadata block, the streaminfo block, are corrupted, actually. Hence, by setting these to correct values, the file becomes decodable again.

I case there is need, I can look into creating a script or providing a detailed explanation of what to do to fix such a file.

[ronso0]

I case there is need, I can look into creating a script or providing a detailed explanation of what to do to fix such a file.

Sure, that would be nice.
Fwiw I did only two short vinyl rips in a hurry just before a gig, and have already deleted them.
But idk who else is affected and needs to repair their mix recordings, so if it's not too much effort just drop the instructions here (or in #16526, or both 🤷 )

[PatrickJosh]

fix_flac.py

Took a little too long. 😓

Use it as python3 fix_flac.py <filename> <sample_rate> <bitrate> <channels>, so e.g. python fix_flac.py 2026-05-29_22h19m36s.flac 48000 16 2.

Make a backup of the file beforehand as the script modifies the file in place!

Edit: Updated the file because of a bug.

[PatrickJosh]

I forgot to say, it might be beneficial to re-encode the file using ffmpeg or flac afterwards, since in my tests, the last sample was broken sometimes, which lead to an error when checking the file with flac -t.