music-mirror/README.md

# music-mirror

Maintain a lossy MP3 mirror of a lossless music library.

Walks a source library and reproduces it, path for path, as MP3 in a separate
tree. Tags and cover art are carried across; sources that are already MP3 are
copied rather than re-encoded; mirror files whose source has been deleted are
removed. **The source library is never written to** — it is mounted read-only
in the supplied compose file, and nothing in the code opens it for writing.

The intended use is an iPod. Apple's Music app cannot read FLAC at all, so a
converted copy has to exist somewhere; this keeps that copy next to the
library on a NAS instead of on a laptop, and keeps it current without a human
remembering to do anything.

## How it decides what to do

| Situation                        | Action                                   |
| -------------------------------- | ---------------------------------------- |
| No mirror file                   | encode                                   |
| Source modified since the mirror | re-encode (a Lidarr quality upgrade)     |
| Mirror up to date                | skip                                     |
| Source is already MP3            | copy verbatim                            |
| Source gone                      | delete the mirror file, prune empty dirs |

Freshness is modification time: an encoded file is stamped with its source's
mtime, so a file is stale exactly when the two differ. There is no database to
fall out of step with the library, which matters when something else — Lidarr,
in this case — is the thing that owns and reorganises it.

Encodes are written to a temporary file and renamed into place, so an
interrupted run cannot leave a truncated MP3 that the next run mistakes for
finished work. A lock file in the mirror root stops two passes overlapping.

## Usage

```sh
music-mirror --source /music --mirror /music-mp3            # one pass
music-mirror --source /music --mirror /music-mp3 --interval 6h   # keep running
music-mirror --source /music --mirror /music-mp3 --dry-run  # report only
music-mirror --source /music --mirror /music-mp3 --subdir "Artist/Album"
```

| Option       | Environment variable    | Default   | Meaning                                        |
| ------------ | ----------------------- | --------- | ---------------------------------------------- |
| `--source`   | `MUSIC_MIRROR_SOURCE`   | —         | Root of the lossless library, read-only        |
| `--mirror`   | `MUSIC_MIRROR_MIRROR`   | —         | Root of the MP3 mirror                         |
| `--quality`  | `MUSIC_MIRROR_QUALITY`  | `V0`      | LAME VBR level `V0`–`V9`, or kbps e.g. `256`   |
| `--jobs`     | `MUSIC_MIRROR_JOBS`     | CPU count | Concurrent encodes                             |
| `--interval` | `MUSIC_MIRROR_INTERVAL` | unset     | Repeat forever, e.g. `45m`, `6h`, `1d`         |
| `--subdir`   | —                       | unset     | Limit the pass to one directory; skips pruning |
| `--no-prune` | —                       | off       | Keep mirror files whose source has gone        |
| `--dry-run`  | —                       | off       | Report what would change, write nothing        |

`--subdir` never prunes: a partial pass cannot tell an orphan from a file
outside its own scope.

Requires `ffmpeg` and `ffprobe` on `PATH`. The container and the Nix package
both provide them.

## Running it on TrueNAS Scale

`compose.yaml` is a Custom App definition. Build the image on the NAS, adjust
the two host paths and the `user:` to match your pool, then add it as a custom
app:

```sh
git clone https://code.emmathe.dev/lyrathorpe/music-mirror
cd music-mirror && docker build -t music-mirror:latest .
```

Point the mirror at its own dataset rather than a directory inside the music
dataset — it is derived data, so it wants its own snapshot policy, its own
quota, and its own SMB share. The tool refuses to run with a mirror inside the
source tree.

New Lidarr imports are picked up on the next pass. With `MUSIC_MIRROR_INTERVAL`
at `6h` that is the worst case; run `--subdir` by hand if you want an album
immediately.

## Nix

```sh
nix run .#music-mirror -- --source ./flac --mirror ./mp3
nix build .#music-mirror   # the test suite runs as part of the build
nix develop                # python, pytest and ffmpeg
```

`overlays.default` provides `pkgs.music-mirror`.

## Tests

```sh
pytest
```

The tests run real ffmpeg encodes rather than mocking them — the interesting
failures are in what ffmpeg actually does with tags, cover art and container
formats, and a mock cannot fail that way. They skip if ffmpeg is absent.

## Getting the result onto an iPod

The mirror is just a directory of MP3s, so any client will do:

- **macOS.** Add the mirror's SMB share to the Music app with _Copy files to
  Music Media folder_ and _Keep Media folder organised_ both **off**. The Mac
  then stores a library database and nothing else. Keep the share mounted at a
  stable path — if it is missing when Music opens, every track shows `!`.
- **Linux.** Rhythmbox links `libgpod` and handles iPod sync. An iPod Video
  (5th generation) predates the models whose database has to be signed, so no
  firmware-hash trickery is needed.

Neither client transcodes at sync time; they copy finished MP3s.

Two device-side details worth knowing: the iPod reads cover art from the file's
tags and ignores `folder.jpg`, which is why art is embedded here; and volume
levelling on the device uses iTunes' Soundcheck tag, not ReplayGain, so
ReplayGain tags in the source are not carried over as such.