Convert plugin: Allow for variable output file extensions

[FibreTTP]

Issue

Currently, the file extension is fixed when using the Convert plugin in auto mode: the destination path will always end with the extension specified in convert.extension.

Background

My use for the Convert plugin is currently to convert input WAVE files to FLAC at import time. The problem here is that FLAC doesn't support floating-point audio, and thus the conversion to a 32-bit or below fixed-point sample format is necessary for those files. This cannot be simply solved with a single command, as the conversion would apply to all input files, even if they are in a supported sample format already. The solution is to use a shell script as the convert.command which handles all the logic of determining if a resample is necessary:

convert-flac-wav.sh

#!/usr/bin/bash

tmpfile="$(mktemp beets-convert-temp-file.XXXXXX)"

# $1 = $source
# $2 = $dest

if [ "$(/root/ffmpeg/git-main/ffprobe -v error -select_streams a:0 -show_entries stream=bits_per_sample -of default=noprint_wrappers=1:nokey=1 "$1")" -ge 32 ]; then
    if [ "$(/root/ffmpeg/git-main/ffprobe -v error -select_streams a:0 -show_entries stream=codec_name -of default=noprint_wrappers=1:nokey=1 "$1")" != "pcm_s32le" ]; then
        /root/ffmpeg/git-main/ffmpeg -hide_banner -loglevel error -i "$1" -map a:0 -c:a pcm_s32le -f wav -y "$tmpfile" && /usr/local/bin/flac --totally-silent -f -S 100x --best -e -p -o "$2" "$tmpfile"
    else
        /usr/local/bin/flac --totally-silent -f -S 100x --best -e -p -o "$2" "$1"
    fi
else
    /usr/local/bin/flac --totally-silent -f -S 100x --best -e -p -o "$2" "$1"
fi

rm "$tmpfile"

Unfortunately, a conversion from floating to fixed point is inherently lossy, though I didn't really care about that, as regardless of the method used to convert, the result will never be noticeable. Today I realised that since the reason I collect lossless tracks isn't for the quality, I should stop doing these lossy conversions anyway. I didn't want to store uncompressed WAVE files, as that would take up too much space, so I researched and found that WavPack can losslessly compress 32-bit float audio, though it is less compatible with everything.

I rewrote the shell script to include more cases for sample formats: transcoding to FLAC if it supports it, and WavPack if not:

convert-wav.sh

#!/usr/bin/bash

# $1 = $source
# $2 = $dest

readarray -t media_attributes < <(/root/ffmpeg/git-main/ffprobe -v error -select_streams a:0 -show_entries stream=codec_name -show_entries stream=bits_per_sample -of default=noprint_wrappers=1:nokey=1 "$1")
codec_name="${media_attributes[0]}"
bits_per_sample="${media_attributes[1]}"


if [[ "${bits_per_sample}" -ge 32 ]]; then
    if [[ "${codec_name}" == pcm_f32le ]]; then
        /usr/local/bin/wavpack -hh -x6 -m -r -v -q "$1" -o "$2"
    elif [[ "${codec_name}" == pcm_s32le ]]; then
        /usr/local/bin/flac --totally-silent -f -S 100x --best -e -p -o "$2" "$1"
    elif [[ "${codec_name}" == pcm_f64le ]]; then
        tmpfile="$(mktemp --suffix=.wav --tmpdir beets-convert-temp-file.XXXXXX)"
        /root/ffmpeg/git-main/ffmpeg -hide_banner -loglevel error -i "$1" -map a:0 -c:a pcm_f32le -f wav -y "$tmpfile" && /usr/local/bin/wavpack -hh -x6 -m -r -v -q "$tmpfile" -o "$2"
        rm "$tmpfile"
    elif [[ "${codec_name}" == pcm_s64le ]]; then
        tmpfile="$(mktemp --suffix=.wav --tmpdir beets-convert-temp-file.XXXXXX)"
        /root/ffmpeg/git-main/ffmpeg -hide_banner -loglevel error -i "$1" -map a:0 -c:a pcm_s32le -f wav -y "$tmpfile" && /usr/local/bin/flac --totally-silent -f -S 100x --best -e -p -o "$2" "$tmpfile"
        rm "$tmpfile"
    fi
else
    /usr/local/bin/flac --totally-silent -f -S 100x --best -e -p -o "$2" "$1"
fi

However, I didn't realise until editing my configuration that the file extension is fixed, and has to be known to Beets before the conversion even begins. You have to specify convert.extension. There is no logic in Beets to determine the file format. And you cannot implement this logic externally.

Ideal behaviour

I would like Beets to not need a specific file extension in the Convert plugin. This could be implemented by checking whether convert.extension is set. If it is, continue with the current behaviour. If it is not:

1. Set dest to some path prefix: dest = /tmp/beets/convert/tmp-song
2. Run convert.command. Users will have to manually specify the extension they want in the command: ffmpeg -i $source -c:a flac $dest.flac
3. When the command is complete, search the temp path for a file with prefix $dest: Find /tmp/beets/convert/tmp-song.flac
4. Set dest to the file found, and continue with linking, moving, etc.

For those using shell scripts, the extension would be determined and set within the script.

This should maintain compatibility with current behaviour, and will support more powerful transcoding rules.

Related: #5346