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.

What that means for Lidarr

Lidarr does this	The mirror does this
Replaces a file with a better rip	Re-encodes in place. Same path in, same path out, so no duplicate
Upgrades MP3 to FLAC	Both map to the same .mp3 mirror path, so the old one is overwritten
Renames a track, album or artist folder	Old path pruned, new path encoded. Correct, but it re-encodes rather than moving
Deletes an album or artist	Every orphaned mirror file is deleted and the emptied directories go too

Pruning is driven by what the pass actually found, not by guessing source filenames from mirror ones: a .FLAC source would not be found by a search for .flac, and the mirror file would be deleted and rebuilt on alternate passes for ever.

If two sources want the same mirror path — a 01 Song.flac next to a leftover 01 Song.mp3, which is what an interrupted upgrade leaves — the better format wins, ties break on path, and the loser is logged. Without that rule both encode to the same destination and every pass finds one of them stale.

The mtime is read before encoding rather than after. A file still being written when the pass reaches it would otherwise be stamped with its final mtime while holding truncated audio, and never be revisited.

Both encodes and copies 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. Copies need it as much as encodes do: the mtime comes across with the bytes, so a half-written copy would look current for ever. A lock file in the mirror root stops two passes overlapping.

Permissions

Everything written into the mirror is made group-readable, and its directories group-traversable, so the mirror can be read back by whatever serves it. Neither writer does that unaided: the temporary file an encode renames into place is created 0600 regardless of the umask, and a straight copy of an existing MP3 inherits the mode of a source file in a library this tool does not own.

Directories are handled by clearing the owner and group read/execute bits from the process umask, once, at startup. Owner as well as group, because a umask carrying 0400 produces directories of mode 0300 — writable and enterable, unreadable to the very run that created them. The other bits are left where the umask puts them: whether the mirror is world-readable is a genuine policy question, and so is its ownership.

Mirror files written before this existed are topped up on the next pass. Their mtimes are correct, so nothing else would revisit them — and they are not re-encoded, only chmod'ed.

Usage

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.

Concurrency

LAME is single-threaded — ffmpeg reports Threading capabilities: none for libmp3lame — so throughput comes entirely from running several encoders at once, one process per file. --jobs defaults to the CPUs the process may actually use, which inside a container means the cpus: allowance rather than the host's core count. Each pass logs the number it settled on.

As a rough guide, a Zen 3 core encodes about 40–60× realtime at V0 depending on clock, so six cores clear roughly 250 hours of audio per hour of wall clock. The first full pass is the expensive one; after that only new and changed files are touched. Lower MUSIC_MIRROR_JOBS if you would rather the NAS stayed responsive than finished sooner.

Requires ffmpeg and ffprobe on PATH. The container image provides both.

Running it on TrueNAS Scale

compose.yaml is a Custom App definition. Adjust the two host paths and the user: to match your pool, then add it as a custom app. The image is published to this Gitea's registry on every release:

code.emmathe.dev/lyrathorpe/music-mirror:latest

Tags are latest, the full version, and the truncated major.minor and major forms; builds that are not releases are published as sha-<short>.

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.

Tests

docker build --target test .   # what CI runs
pytest                         # needs ffmpeg and pytest on PATH

The suite runs 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 — which is also why CI runs the tests inside the image, against the ffmpeg that ships, rather than against whatever the build runner provides. The published image is the runtime stage and carries neither the tests nor pytest.

Run them directly instead if you prefer; they skip when ffmpeg is absent. On a Nix machine:

nix shell nixpkgs#python3Packages.pytest nixpkgs#ffmpeg -c pytest

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.