fix: correct Rockbox's local wall-clock timestamps to UTC

Rockbox has no concept of a timezone. Its clock holds local time, and it
builds log timestamps with mktime(get_time()) -- but firmware/libc/mktime.c
is plain calendar arithmetic applying no offset, so the RTC's local fields
come out as though they were UTC. The number in the log is ahead of the real
instant by whatever the offset was, and Last.fm stores UTC, so every play
submitted during BST landed an hour in the future.

Rockbox states this itself: its scrobbler plugin writes #TZ/UNKNOWN, and the
AUDIOSCROBBLER spec allows #TZ/UTC only for a device that actually converted.
The correction belongs to the consumer.

Each timestamp is decoded back to its wall-clock fields and reinterpreted in
the player's zone, per play rather than as one offset over the whole log, so
a log spanning a daylight saving change converts each side correctly. A log
declaring #TZ/UTC is left alone rather than shifted twice.

The zone defaults to this machine's, overridable with --device-timezone or
ROCKBOX_TIMEZONE. Deriving it needs the whole IANA name: /etc/localtime
resolves into the tzdata tree, and taking only the final component yields
"London", which no database holds, silently falling back to a fixed offset
that is wrong for half the year.

Since Rockbox cannot adjust for daylight saving on its own, the player's
clock has to be changed by hand twice a year. Any play converting to a future
time is now reported, which is what a forgotten adjustment looks like.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-and-publish.yaml

@@ -45,7 +45,8 @@ jobs:
 
       # The suite runs inside the image, against the ffmpeg that ships, rather
       # than against whatever the runner happens to provide. A failing test
-      # fails the build. Layers are shared with the push build below.
+      # fails the build. The runtime stage below is built from the same daemon
+      # afterwards, so its layers are already in cache.
       - name: Run the test suite inside the image
         run: docker build --target test -t music-mirror:test .
 
@@ -124,9 +125,6 @@ jobs:
           echo "release=${release}" >> "$GITHUB_OUTPUT"
           echo "Computed bump=${bump}, release=${release}, base=${base}"
 
-      - name: Set up Buildx
-        uses: docker/setup-buildx-action@d7f5e7f509e45cec5c76c4d5afdd7de93d0b3df5 # v4
-
       - name: Log in to the Gitea container registry
         if: github.event_name != 'pull_request'
         uses: docker/login-action@650006c6eb7dba73a995cc03b0b2d7f5ca915bee # v4
@@ -135,21 +133,34 @@ jobs:
           username: ${{ github.repository_owner }}
           password: ${{ secrets.PACKAGES_TOKEN }}
 
-      - name: Build and push
+      # Plain `docker build` rather than buildx. buildx boots its own buildkit
-        uses: docker/build-push-action@f9f3042f7e2789586610d6e8b85c8f03e5195baf # v7
+      # in a container with a cache of its own, so it shared nothing with the
-        with:
+      # test build above and rebuilt the image from the base up -- installing
-          context: .
+      # ffmpeg and the package a second time, for nothing. It earns that cost
-          # Without this the last stage in the Dockerfile -- the test stage --
+      # when building for several platforms; this only ever targets the amd64
-          # would be what gets published.
+      # NAS, so it does not.
-          target: runtime
+      #
-          # The NAS is the only host this runs on. Building arm64 as well would
+      # `--target runtime` is a strict prefix of the test stage, so every layer
-          # mean emulating it under QEMU for no consumer.
+      # is already in the daemon's cache and this resolves in seconds.
-          platforms: linux/amd64
+      - name: Build the runtime image
-          push: ${{ github.event_name != 'pull_request' }}
+        run: |
-          tags: ${{ steps.version.outputs.tags }}
+          set -euo pipefail
-          labels: |
+          tags=()
-            org.opencontainers.image.source=${{ github.server_url }}/${{ github.repository }}
+          while IFS= read -r tag; do
-            org.opencontainers.image.revision=${{ github.sha }}
+            [ -n "$tag" ] && tags+=(-t "$tag")
+          done <<< "${{ steps.version.outputs.tags }}"
+          docker build --target runtime \
+            --label "org.opencontainers.image.source=${GITHUB_SERVER_URL}/${GITHUB_REPOSITORY}" \
+            --label "org.opencontainers.image.revision=${GITHUB_SHA}" \
+            "${tags[@]}" .
+
+      - name: Push
+        if: github.event_name != 'pull_request'
+        run: |
+          set -euo pipefail
+          while IFS= read -r tag; do
+            [ -n "$tag" ] && docker push "$tag"
+          done <<< "${{ steps.version.outputs.tags }}"
 
       # Record the release: write the computed version into pyproject.toml, then
       # commit and tag it, so the packaging metadata always matches the release

Dockerfile

@@ -26,6 +26,11 @@ ENTRYPOINT ["music-mirror"]
 FROM runtime AS test
 
 RUN pip install --no-cache-dir pytest
+# sync-to-ipod.sh and its tests need these; the runtime image deliberately does
+# not carry them, and neither does the base.
+RUN apk add --no-cache bash rsync findmnt
 COPY pytest.ini ./
+# Host-side tools; not in the runtime image, but the suite covers them.
+COPY tools ./tools
 COPY tests ./tests
 RUN python -m pytest

README.md

@@ -63,9 +63,14 @@ 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. Only the
+inherits the mode of a source file in a library this tool does not own.
-group bits are touched; whether the mirror is world-readable stays with the
+
-umask, as does the ownership.
+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
@@ -88,12 +93,27 @@ music-mirror --source /music --mirror /music-mp3 --subdir "Artist/Album"
 | `--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 |
+| `--fat32-safe` | `MUSIC_MIRROR_FAT32_SAFE` | off     | Name files so a FAT32 device accepts them      |
 | `--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
@@ -118,6 +138,249 @@ 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.
 
+## Tools
+
+Host-side scripts under `tools/`, not part of the container image.
+
+`sync-to-ipod.sh` does a whole transfer: submits the scrobbler log, checks the
+mirror, rsyncs, syncs and unmounts.
+
+```sh
+tools/sync-to-ipod.sh /mnt/tank/media/music-mp3 /media/IPOD/Music
+tools/sync-to-ipod.sh -n /mnt/tank/media/music-mp3 /media/IPOD/Music   # dry run
+```
+
+The destination is where the artist folders should end up — normally a
+subdirectory such as `/media/IPOD/Music`, not the card root. A subdirectory is
+the better target: `--delete` is confined to it, and the device path budget is
+derived from it rather than configured, so the two cannot disagree.
+
+It refuses to start unless the destination is on a mounted FAT filesystem. That
+check is also what catches an unmounted device — `/media/IPOD/Music` then
+resolves to the host's own root filesystem, and this refuses to empty that.
+`--help` says all of it. It also excludes `/.rockbox`, the scrobbler logs
+and the various filesystem metadata directories from deletion — the mirror does
+not contain them, and without the exclusion a sync to the card root would
+remove the Rockbox install.
+
+Progress is a single line that rewrites itself:
+
+```
+[ 24%] 12,345/49,600  3.2 GiB/13.1 GiB  4.4 MiB/s  ETA 38m12s  King Gizzard / Petro…
+```
+
+The estimate comes from rsync's `%l`, which gives each file's size as it
+completes. Bytes done over time elapsed is the same arithmetic rsync would do,
+and needs nothing it does not already print. The rate is measured over a
+trailing thirty seconds rather than the whole run, so it follows a device that
+slows down instead of averaging the slowdown away — and it is suppressed
+entirely for the first two seconds, where the window is microseconds wide and
+would report gigabytes per second.
+
+rsync says nothing at all while it builds its file list, which on fifty
+thousand files is minutes of apparent hang, and its own `progress2` percentage
+is computed against a list it has not finished discovering. So the script
+renders its own.
+
+**The percentage and the estimate are opt-in, via `-P`.** They need a total,
+the total needs a counting pass, and that pass walks and compares both trees in
+full exactly as the transfer does. Measured on a real card: read from the
+source at 35 MB/s and write to the card at 21 MB/s, yet the sync crawled —
+because the traversal, not the data, was the cost, and it was being paid twice.
+Without `-P` the line still shows the running count, the rate and the album in
+flight; only the two figures that needed the second walk are missing.
+
+Piped to a log it prints a plain line every thirty seconds instead, with no
+carriage returns, and a summary at the end either way.
+
+### The Rockbox database
+
+Point `MUSIC_MIRROR_DATABASE_TOOL` at Rockbox's host-side builder and the sync
+rebuilds the database itself, so it never has to happen on the device.
+
+```sh
+git clone --depth 1 https://github.com/Rockbox/rockbox.git
+cd rockbox && mkdir build-db && cd build-db
+../tools/configure --target=ipodvideo --type=d && make -j$(nproc)
+```
+
+It needs a native compiler and SDL2 development headers, not the ARM
+cross-toolchain, and `tools/configure` detects `__aarch64__` correctly. On a
+distribution without `/usr/bin/perl` or `gcc-ar` — NixOS, say — patch the
+shebangs in `tools/*.pl` and pass `AR=ar`.
+
+Building it here rather than on the device is not merely faster. The on-device
+commit sorts the whole index in whatever memory `core_alloc_maximum()` can
+scrape together; on a fifty-thousand-track library it runs for hours or aborts
+outright.
+
+The scan runs against a scratch root — a real `.rockbox` beside a symlink
+standing in for wherever the music lands on the device — so the paths recorded
+are the ones Rockbox will look up, while the **bytes are read from the mirror
+rather than over USB**. Only the dozen `.tcd` files cross to the card.
+
+Cost, measured: the parser makes about 49 reads and 43 seeks per file, probing
+the head for ID3v2 and the tail for ID3v1. On a local disk that is 2,000 files
+in half a second. Over SMB, readahead absorbs most of the reads but the opens
+and the head/tail split are real round trips, so a first full scan is minutes
+rather than seconds. It is a one-time cost: the builder is incremental, and the
+scratch root is kept between runs, so a later pass over unchanged files does no
+metadata reads at all.
+
+If minutes is still too many, run the builder where the mirror is local — on
+the NAS — and copy the `.tcd` files across. There is nothing to parallelise:
+the tool is single-threaded, and two instances cannot produce one database.
+
+### If the sync is interrupted
+
+No partially copied track is ever left under a name Rockbox would play. rsync
+writes to a hidden temporary file and only renames it into place once the file
+is complete, and *"by default, rsync will delete any partially transferred file
+if the transfer is interrupted"*. `--partial` is deliberately not used, and
+there is a test asserting it never will be.
+
+After an unclean kill or a power cut a hidden `.track.mp3.XXXXXX` can survive.
+It is not playable, it is not in the source, and the next run's `--delete`
+removes it.
+
+Interrupting does not, by itself, endanger the filesystem. The kernel flushes
+dirty pages within `dirty_expire_centisecs` — thirty seconds by default — and
+`umount` always syncs before it returns. Losing data needs you to interrupt,
+*and* pull the card inside that window, *and* skip the unmount.
+
+The script still traps `INT` and `TERM` and flushes and unmounts on the way
+out, exiting 130. Not because a Ctrl-C is dangerous, but because it removes the
+manual step and makes the exit deterministic — you get the same "safe to
+disconnect" either way, rather than having to remember which path you took.
+Both paths call the same function, so they cannot drift apart.
+
+What genuinely does lose data is pulling the cable or the card without
+unmounting at all, interrupted or not. FAT32 has no journal. Wait for the
+unmount line.
+
+### Making it faster over a network mount
+
+The transfer is metadata-bound, not throughput-bound: 49,600 files means 49,600
+round trips, and the counting pass doubles that. In rough order of what it is
+worth doing:
+
+| Lever | Why |
+| ----- | --- |
+| Mount the source with `actimeo=60,cache=loose` | SMB defaults to a **one second** attribute cache, so nearly every `stat` goes to the wire — twice, once per pass. This is the single biggest change and it is a mount option, not an rsync flag. |
+| Put the card in a reader for the first load | USB 2.0 through an iPod in disk mode is the floor for the destination. No amount of source tuning gets past it. |
+| Counting is off by default | The percentage costs a second full traversal of both trees. On a FAT card of fifty thousand files that is slower than the transfer. `-P` asks for it. |
+| `--whole-file`, `--omit-dir-times` | Already set. The first stops rsync checksumming destination files it is about to overwrite whole; the second drops a setattr per directory, 6,150 of them. |
+
+**NFS instead of SMB** is worth trying but is not the big win it looks like.
+Its attribute caching defaults are far more generous than SMB's — `acregmax` of
+sixty seconds against `actimeo=1` — which is precisely the gap that
+`actimeo=60` closes on the mount you already have. Bulk read throughput between
+the two is much of a muchness on a gigabit link. Try the mount option first; it
+is one line and needs no change on the NAS.
+
+And if the destination is the iPod rather than a card reader, none of this
+matters much: the source can feed data faster than USB 2.0 through an iPod will
+take it either way.
+
+The unmount is the point of doing this in a script. FAT32 has no journal and
+the device is reached through disk mode, so an interrupted write is corruption
+that needs `fsck.vfat` from another machine.
+
+`submit_scrobbles.py` sends what was played to Last.fm and sets the logs aside.
+
+It reads **Rockbox's own `playback.log`**, which core Rockbox writes whenever
+"play log" is enabled, with no plugin running. Each line is
+`timestamp:elapsed_ms:length_ms:path` — a path and nothing else, which is why
+the on-device scrobbler plugin exists at all: reading tags back off the player
+is slow. Off the mirror it is free, so `--mirror` lets the conversion happen
+here and the plugin never has to be run. A play counts as listened at half the
+track's length, the same fraction the plugin uses, so the two cannot disagree
+about what a play was.
+
+It still reads a `.scrobbler.log` if the plugin has been run and left one. Rockbox writes `/.scrobbler.log` in AUDIOSCROBBLER 1.1 format, one
+tab-separated line per track rated `L` for listened or `S` for skipped; only
+the listened ones are sent. It runs **before** the copy, since the plays
+already happened and a failed transfer is no reason to lose them.
+
+Two things are unlike every other Last.fm call in these projects. Scrobbling is
+a *write* method, so it needs `LASTFM_API_SECRET` and a session key obtained
+once through the browser, not just the read-only key. And on a target with no
+real-time clock Rockbox writes `/.scrobbler-timeless.log` with every timestamp
+set to zero; those are counted and reported but never submitted, because
+scrobbling them would mean inventing when they happened.
+
+### Timestamps are local wall clock, and are corrected here
+
+Rockbox has no concept of a timezone. Its clock is set to local time, and it
+builds log timestamps with `mktime(get_time())` — but
+[its `mktime`](https://git.rockbox.org/cgit/rockbox.git/tree/firmware/libc/mktime.c)
+is plain calendar arithmetic that applies no offset, so the RTC's local fields
+come out as if they were UTC. The number in the log is therefore ahead of the
+real instant by whatever the offset was. Last.fm stores UTC, so submitting it
+raw puts every play an hour into the future for the half of the year the UK is
+on BST.
+
+Rockbox is candid about this: its scrobbler plugin writes `#TZ/UNKNOWN` in the
+log header, and the AUDIOSCROBBLER spec says a device may claim `#TZ/UTC` only
+if it actually converted. The correction is the consumer's job.
+
+Each timestamp is decoded back into the wall-clock fields it came from and
+reinterpreted in the player's zone. Doing it **per play** rather than applying
+one offset to the whole log matters: a week's listening can straddle a daylight
+saving change, and the two sides need different offsets. A log that declares
+`#TZ/UTC` is left alone, so a client that already converted is not shifted
+twice.
+
+The zone defaults to this machine's. Set `ROCKBOX_TIMEZONE` (or pass
+`--device-timezone`) to an IANA name if the player's clock is keeping a
+different one.
+
+Because Rockbox cannot adjust for daylight saving itself, **you have to change
+the player's clock by hand twice a year**. If you forget, its times are an hour
+out and no amount of zone arithmetic recovers them. The submitter reports any
+play that converts to a time in the future, which is what a forgotten
+adjustment looks like:
+
+```
+  37 plays are timestamped up to 58 minutes in the future, converting from
+  Europe/London. Either the player's clock is wrong or that is not the zone
+  it is set to.
+```
+
+`--dry-run` prints each play's local time beside the epoch, so the conversion
+can be checked against when you actually remember listening.
+
+### Nothing played is thrown away
+
+Two separate obligations, because a play that happened and never reached
+Last.fm is gone for good.
+
+**The original is renamed, never deleted.** If Last.fm quietly dropped
+something, the evidence is still on the device as `playback.log.<ts>.submitted`.
+
+**Anything not submitted is written back** into a live log for the next run:
+
+| Outcome | What happens to it |
+| ------------------------------ | ----------------------------------------- |
+| Accepted by Last.fm | dropped from the live log |
+| Not in the mirror yet | written back, tried again next run |
+| In a batch that failed | written back, tried again next run |
+| Nothing accepted at all | logs left completely untouched |
+| A skip, or no usable timestamp | not retained — neither can ever be submitted, and the original still has it |
+
+The batch boundary matters: submission is recorded as each batch is accepted,
+so a failure partway through knows exactly what got through and writes back
+only the remainder. No duplicates, no losses.
+
+A file `ffprobe` cannot read costs one unidentified play, not the run.
+
+`check_fat32.py` reports paths a FAT32 device will not accept — reserved
+characters, trailing dots and spaces, over-long components and paths, and names
+colliding case-insensitively. Run it against the mirror **before** an rsync:
+rsync reports the failures too, but scattered through fifty thousand files where
+they are easy to miss. Exits non-zero when it finds anything, so it can gate a
+script.
+
 ## Tests
 
 ```sh
@@ -139,6 +402,93 @@ Nix machine:
 nix shell nixpkgs#python3Packages.pytest nixpkgs#ffmpeg -c pytest
 ```
 
+## FAT32 and Rockbox
+
+`--fat32-safe` names mirror files so a FAT32 device will accept them. Off by
+default, because turning it on renames files and that should be a decision
+rather than a surprise.
+
+What it handles, per path component:
+
+| Problem | Treatment |
+| ------------------------------- | ------------------------------ |
+| `< > : " \ \| ? *` and control characters | replaced with `_` |
+| trailing dots and spaces | stripped — FAT eats them silently, so the name round-trips as a different name |
+| a component left empty | becomes `_` |
+| names differing only in case | detected and reported; one wins, as with any other collision |
+
+`Dada Life - Kick Out the Epic Motherf**ker` is a real example from a real
+library. Without this it simply never arrives on the device.
+
+**Turning it on does not re-encode anything.** Every track whose name held a
+reserved character changes path, and re-encoding those would be hours of work
+producing files that already exist byte for byte. The run moves them instead,
+and says so. Prune then finds nothing to remove because nothing was left
+behind.
+
+Renames are counted apart from encodes in the pass summary, and `--dry-run`
+reports `would rename` rather than `would encode` — the difference between the
+two is a minute against an afternoon, so a preview that conflated them would be
+worse than no preview. A dry run also does not list the pre-rename files as
+orphans: nothing was moved, so they are still there, but they are what a real
+run would move rather than what it would delete.
+
+### Path length
+
+Rockbox's `MAX_PATH` is 260, from `firmware/include/fs_defines.h`, and it bounds
+the path *as the device sees it*. The directory the mirror is copied into comes
+out of the same budget, so `--device-prefix` (default `/Music`) is subtracted
+from `--max-path` to get what a mirror-relative path may spend:
+
+| Destination on the device | Mirror-relative budget |
+| ------------------------- | ---------------------- |
+| `/Music/` | 253 |
+| the card root | 259 |
+
+Worth being exact about, because a checker that measures mirror-relative paths
+against the flat 260 quietly passes everything from 253 to 260 — and those are
+the paths most likely to be near the edge in the first place.
+
+Over-budget paths are shortened from the **deepest component outward**: the
+track name carries the least navigational value and the artist directory the
+most, so the filename goes first and the artist is touched only if nothing else
+will do.
+
+A component is cut **from the middle**, not the end, because of how these names
+are built. Lidarr writes `Artist - Album - 07 - Flamethrower.mp3` inside a
+directory already named for that artist and album, so a long album title
+appears three times in one path and the informative part — the track number and
+title — is at the very end. Cutting from the end throws exactly that away:
+
+```
+before  King Gizzard & the Lizard Wizard - PetroDragonic Apocalypse; or, Dawn of Eternal
+        Night - An Annihilation of Planet Earth and the Beginning of Merciless
+        Damnation - 07 - Flamethrower.mp3
+after   King Gizzard & the Lizard~c526~ginning of Merciless Damnation - 07 - Flamethrower.mp3
+```
+
+Two thirds of the remaining room goes to the tail, since the head is usually a
+restatement of the directory it sits in. A shortened component gains four hex
+digits of the original name: two names sharing both a head and a tail would
+otherwise produce the same string, and a silent collision between two tracks is
+worse than an ugly filename.
+
+The result is stable: the same source always produces the same shortened name,
+so a pass does not rename what the previous pass wrote. A path too deeply
+nested to fit without reducing every component to nonsense is left alone and
+reported instead.
+
+### Album art
+
+Rockbox looks for cover art **on the filesystem** — `cover.jpg`, `folder.jpg`
+and friends beside the track or in its parent — and that search never touches
+the picture embedded in the tag. So a JPEG cover found beside the source is now
+copied into the mirror as `cover.jpg`, in addition to being embedded. The iPod
+firmware reads the embedded one; Rockbox reads the file. Both are satisfied.
+
+A cover left behind in a directory whose tracks have all gone is pruned, or the
+directory would never look empty and never be removed.
+
 ## Getting the result onto an iPod
 
 The mirror is just a directory of MP3s, so any client will do:
@@ -150,6 +500,30 @@ The mirror is just a directory of MP3s, so any client will do:
 - **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.
+- **Rockbox.** No database to write at all — it reads a plain directory tree
+  and builds its own index from tags. Copy the mirror across with rsync:
+
+  ```sh
+  python3 tools/check_fat32.py /mnt/tank/media/music-mp3   # before, not during
+  rsync -rtv --delete --modify-window=2 \
+      /mnt/tank/media/music-mp3/ /media/IPOD/Music/
+  ```
+
+  `--modify-window=2` because FAT stores modification times to two-second
+  resolution; without it rsync re-copies the entire library on every run. `-rt`
+  rather than `-a` because owners, groups and permissions mean nothing on FAT
+  and asking for them only produces errors.
+
+  Do not route this through Rhythmbox. Its `rb_ipod_helpers_is_ipod()` reads
+  `access-protocols` from media-player-info first and returns true without
+  looking at the filesystem at all, so an iPod in disk mode is identified by its
+  USB id and managed as an iPod — writing a database Rockbox does not want.
+  Deleting `iPod_Control` does not change this. Either use rsync, or untick
+  Preferences → Plugins → Portable Players - iPod.
+
+  Faster still for the first bulk copy: take the card out of the iFlash adapter
+  and use a card reader. Fifty thousand files over USB 2.0 through an iPod is a
+  long evening, and it avoids Rockbox's USB stack entirely.
 
 Neither client transcodes at sync time; they copy finished MP3s.
 

compose.yaml

@@ -22,6 +22,15 @@ services:
       # Concurrent encodes; defaults to the CPU count. Lower it to leave the
       # NAS responsive during the first full pass.
       # MUSIC_MIRROR_JOBS: "4"
+      # Name files so a FAT32 device will take them, for copying to a Rockbox
+      # player. Quoted, because an unquoted yes or true is a YAML boolean and
+      # compose rejects a boolean here. Accepts 1, true or yes.
+      #
+      # Turning this on renames every file whose name held a reserved
+      # character. They are moved, not re-encoded, so the first pass after
+      # enabling it is quick -- but it is a one-way change to every such path,
+      # so decide before running it rather than after.
+      MUSIC_MIRROR_FAT32_SAFE: "true"
     volumes:
       - /mnt/tank/media/music:/music:ro
       - /mnt/tank/media/music-mp3:/mirror

music_mirror.py

@@ -16,6 +16,8 @@ makes runs idempotent without a database to keep in step.
 import argparse
 import concurrent.futures
 import fcntl
+import functools
+import hashlib
 import logging
 import os
 import re
@@ -70,12 +72,34 @@ SOURCE_PRIORITY = [
 # Looked for in the source directory when a file has no embedded picture.
 COVER_NAMES = ("cover.jpg", "folder.jpg", "front.jpg", "cover.png", "folder.png")
 
+# What a copied cover is called in the mirror. Rockbox looks for album art on
+# the filesystem -- cover.jpg, folder.jpg and friends beside the track -- and
+# its search never touches the picture embedded in the tag, so a mirror that
+# only embeds art shows none of it on the device.
+MIRROR_COVER = "cover.jpg"
+
 # Files the mirror is allowed to contain, and therefore allowed to delete.
 MIRROR_SUFFIX = ".mp3"
 
 # Filesystems disagree about mtime precision; SMB in particular rounds.
 MTIME_TOLERANCE_SECONDS = 2
 
+# What FAT32 refuses in a filename, plus the control characters. The mirror is
+# copied onto a FAT32 device, and one of these in a path means a track that
+# never arrives -- "Kick Out the Epic Motherf**ker" is a real example.
+FAT32_RESERVED = re.compile(r'[<>:"/\\|?*\x00-\x1f]')
+
+# Rockbox's MAX_PATH, from firmware/include/fs_defines.h. It bounds the whole
+# path as the device sees it, so the budget for a mirror-relative path is this
+# less whatever directory the mirror is copied into.
+MAX_PATH = 260
+DEVICE_PREFIX = "/Music"
+
+# A component cut below this is no longer recognisable, and a path that cannot
+# be brought under the limit without going there is better reported than
+# mangled.
+MIN_COMPONENT = 12
+
 # The mirror exists to be read back by something else -- an SMB share, another
 # account on the box -- so everything written into it has to be group-readable.
 # Neither writer manages that unaided: tempfile.mkstemp forces 0600 whatever the
@@ -83,7 +107,12 @@ MTIME_TOLERANCE_SECONDS = 2
 # that may be tighter still. Directories need the execute bit too, or the group
 # cannot enter them to reach the readable files inside.
 GROUP_READ = 0o040
-GROUP_ENTER = 0o050
+
+# Cleared from the umask so directories this run creates can be listed and
+# entered. Owner as well as group: a umask carrying 0400 -- which is unusual but
+# not ours to assume away -- otherwise produces a mirror tree that not even the
+# process that built it can read back.
+DIRECTORY_ACCESS = 0o550
 
 
 @dataclass
@@ -120,9 +149,102 @@ def parse_interval(interval):
     return value * {"": 1, "s": 1, "m": 60, "h": 3600, "d": 86400}[match.group(2)]
 
 
-def mirror_path_for(source, source_root, mirror_root):
+def fat32_safe(component):
+    """Return a single path component FAT32 will accept.
+
+    The mirror exists to be copied onto a FAT32 device, and a name that device
+    will not take is a track that silently does not arrive. Cheaper to produce
+    an acceptable name here than to discover the problem partway through
+    copying fifty thousand files.
+
+    Handled: the reserved characters, control characters, and the trailing dots
+    and spaces that FAT quietly eats -- a name ending in one round-trips as a
+    different name, which is worse than being rejected outright.
+    """
+    cleaned = FAT32_RESERVED.sub("_", component).rstrip(". ")
+    # Stripping can empty a component outright: a directory named "..." is
+    # legal on ext4 and nothing at all on FAT.
+    return cleaned or "_"
+
+
+def device_prefix_length(prefix):
+    """Return the on-device prefix as it will actually appear, with slashes.
+
+    "/Music" costs seven characters -- the leading slash, the name, and the
+    separator before the mirror's own path -- while an empty prefix costs one.
+    Approximating that loses a character at the root, which is precisely where
+    the longest paths are.
+    """
+    cleaned = prefix.strip("/")
+    return f"/{cleaned}/" if cleaned else "/"
+
+
+def shorten_component(component, budget):
+    """Return a component of at most `budget` characters, cut from the middle.
+
+    From the middle, not the end, because of how these names are built. Lidarr
+    writes "Artist - Album - 07 - Flamethrower.mp3" inside a directory already
+    named for that artist and album, so the informative part -- the track
+    number and title -- is at the very end. Cutting from the end discards it
+    and leaves every track on the record with the same name.
+
+    The four hex digits are of the original component. Two names sharing both a
+    head and a tail would otherwise produce the same string, and a silent
+    collision between two tracks is worse than an ugly filename.
+    """
+    stem, dot, extension = component.rpartition(".")
+    if not dot or len(extension) > 4:
+        stem, extension = component, ""
+    else:
+        extension = dot + extension
+
+    digest = hashlib.blake2s(component.encode("utf-8"), digest_size=2).hexdigest()
+    marker = f"~{digest}~"
+    room = max(2, budget - len(extension) - len(marker))
+    if room >= len(stem):
+        return stem + extension
+
+    # Two thirds to the tail: the head is usually a restatement of the
+    # directory it sits in, and the tail is what tells two tracks apart.
+    keep_end = min(len(stem), room * 2 // 3)
+    keep_start = max(1, room - keep_end)
+    return stem[:keep_start].rstrip(". ") + marker + stem[len(stem) - keep_end :] + extension
+
+
+def fit_path(relative, budget):
+    """Return a relative path within `budget` characters, or the best available.
+
+    Shortened from the deepest component outward. The filename carries the least
+    navigational value and the artist directory the most, so the track name is
+    sacrificed before the album and the album before the artist.
+    """
+    parts = list(relative.parts)
+    for index in reversed(range(len(parts))):
+        overage = len(str(Path(*parts))) - budget
+        if overage <= 0:
+            break
+        allowed = max(MIN_COMPONENT, len(parts[index]) - overage)
+        if allowed < len(parts[index]):
+            parts[index] = shorten_component(parts[index], allowed)
+    fitted = Path(*parts)
+    if len(str(fitted)) > budget:
+        logger.warning(
+            "%s is still %d characters over the limit after shortening; it is too"
+            " deeply nested to fit",
+            relative,
+            len(str(fitted)) - budget,
+        )
+    return fitted
+
+
+def mirror_path_for(source, source_root, mirror_root, safe=False, budget=0):
     """Return the mirror path corresponding to a source file."""
-    return (mirror_root / source.relative_to(source_root)).with_suffix(MIRROR_SUFFIX)
+    relative = source.relative_to(source_root).with_suffix(MIRROR_SUFFIX)
+    if safe:
+        relative = Path(*(fat32_safe(part) for part in relative.parts))
+        if budget > 0 and len(str(relative)) > budget:
+            relative = fit_path(relative, budget)
+    return mirror_root / relative
 
 
 def is_current(source, mirror):
@@ -132,6 +254,30 @@ def is_current(source, mirror):
     return abs(source.stat().st_mtime - mirror.stat().st_mtime) <= MTIME_TOLERANCE_SECONDS
 
 
+@functools.lru_cache(maxsize=4096)
+def mirror_cover(source_directory, mirror_directory):
+    """Put a copy of the album's cover beside its tracks in the mirror.
+
+    Cached per directory: an album's tracks all ask for the same file, and the
+    answer cannot change within a pass.
+    """
+    cover = find_cover(source_directory)
+    if cover is None or cover.suffix.lower() not in (".jpg", ".jpeg"):
+        # Only JPEG is copied. Rockbox will read a BMP too, but converting a
+        # PNG is ffmpeg work for a file nothing else in the pass needs.
+        return None
+    destination = mirror_directory / MIRROR_COVER
+    try:
+        if destination.is_file() and destination.stat().st_size == cover.stat().st_size:
+            return destination
+        shutil.copy2(cover, destination)
+        make_group_readable(destination)
+    except OSError as error:
+        logger.warning("could not copy cover for %s: %s", source_directory, error)
+        return None
+    return destination
+
+
 def make_group_readable(path):
     """Add the group-read bit to a mirror file, leaving the rest of the mode alone."""
     mode = path.stat().st_mode
@@ -139,8 +285,13 @@ def make_group_readable(path):
         path.chmod(mode | GROUP_READ)
 
 
+@functools.lru_cache(maxsize=4096)
 def find_cover(directory):
-    """Return an external cover image for a directory, if one is present."""
+    """Return an external cover image for a directory, if one is present.
+
+    Cached because an album's tracks all ask the same question, and the answer
+    costs one stat per candidate name.
+    """
     for name in COVER_NAMES:
         candidate = directory / name
         if candidate.is_file():
@@ -217,7 +368,13 @@ def encode(source, mirror, quality_args, dry_run):
         return Result("encoded", mirror)
 
     mirror.parent.mkdir(parents=True, exist_ok=True)
-    cover = None if has_embedded_picture(source) else find_cover(source.parent)
+    mirror_cover(source.parent, mirror.parent)
+    # Probing costs an ffprobe process per file, so only ask when the answer
+    # can change the command. With no cover file beside the track, `-map
+    # 0:v:0?` carries embedded art if there is any and shrugs if there is not.
+    cover = find_cover(source.parent)
+    if cover is not None and has_embedded_picture(source):
+        cover = None
 
     # Read the source's mtime before encoding, not after. If the file is still
     # being written -- a Lidarr import landing mid-pass -- stamping the mirror
@@ -259,6 +416,7 @@ def copy(source, mirror, dry_run):
         return Result("copied", mirror)
 
     mirror.parent.mkdir(parents=True, exist_ok=True)
+    mirror_cover(source.parent, mirror.parent)
 
     # Through a temporary file and a rename, for the same reason encodes go
     # that way, and a sharper one: copy2 reproduces the source's mtime as well
@@ -283,8 +441,39 @@ def copy(source, mirror, dry_run):
     return Result("copied", mirror)
 
 
-def process(source, mirror, quality_args, dry_run):
+def adopt_existing(source, mirror, candidates, dry_run=False):
+    """Move an already-encoded file to its new name. Returns whether it moved.
+
+    Turning on FAT32-safe naming changes the path of every track whose name
+    held a reserved character. Without this the run would encode them all again
+    and then prune the originals -- hours of work to produce files that already
+    exist, byte for byte, under the old name.
+
+    Several candidates are tried because there is more than one previous
+    naming: the original, and the sanitised-but-not-yet-shortened form left by
+    an earlier version.
+    """
+    for previous in candidates:
+        if previous == mirror or not previous.is_file() or not is_current(source, previous):
+            continue
+        if dry_run:
+            logger.info("would rename %s -> %s", previous.name, mirror.name)
+            return True
+        mirror.parent.mkdir(parents=True, exist_ok=True)
+        os.replace(previous, mirror)
+        logger.info("renamed %s -> %s", previous.name, mirror.name)
+        return True
+    return False
+
+
+def process(source, mirror, quality_args, dry_run, previous=None):
     """Bring one source file's mirror entry up to date."""
+    # Counted separately from an encode, and reported in a dry run, because the
+    # difference between moving a file and re-encoding it is the difference
+    # between a minute and an afternoon.
+    if previous is not None and not mirror.exists():
+        if adopt_existing(source, mirror, previous, dry_run):
+            return Result("renamed", mirror)
     if is_current(source, mirror):
         # A mirror written before this bit was set has a correct mtime, so
         # nothing else in the pass would ever revisit it. Top it up here
@@ -308,7 +497,7 @@ def find_sources(root):
             yield path
 
 
-def plan(scan_root, source_root, mirror_root):
+def plan(scan_root, source_root, mirror_root, safe=False, budget=0):
     """Map each mirror path to the one source that should produce it.
 
     Two sources can want the same mirror path -- `01 Song.flac` alongside a
@@ -319,12 +508,20 @@ def plan(scan_root, source_root, mirror_root):
     the outcome stable and predictable instead.
     """
     chosen = {}
+    # Keyed case-insensitively when the target is FAT32, because two names
+    # differing only in case are two files here and one file there. Detecting
+    # that now beats discovering it as a silent overwrite during the copy.
+    seen = {}
     for source in find_sources(scan_root):
-        mirror = mirror_path_for(source, source_root, mirror_root)
+        mirror = mirror_path_for(source, source_root, mirror_root, safe, budget)
-        rival = chosen.get(mirror)
+        key = str(mirror).casefold() if safe else str(mirror)
+        rival_path = seen.get(key)
+        rival = chosen.get(rival_path) if rival_path else None
         if rival is None:
+            seen[key] = mirror
             chosen[mirror] = source
             continue
+        mirror = rival_path
         winner, loser = sorted((source, rival), key=source_rank)
         logger.warning("%s and %s both map to %s; using %s", rival, source, mirror, winner)
         chosen[mirror] = winner
@@ -358,6 +555,13 @@ def prune(mirror_root, expected, dry_run):
         mirror.unlink(missing_ok=True)
 
     if not dry_run:
+        # A cover copied for an album whose tracks have all gone is an orphan
+        # too, and while it sits there the directory never looks empty.
+        for cover in sorted(mirror_root.rglob(MIRROR_COVER)):
+            if not any(cover.parent.glob(f"*{MIRROR_SUFFIX}")):
+                logger.info("removing orphan %s", cover)
+                cover.unlink(missing_ok=True)
+
         # Deepest first, so a directory emptied by the loop above is caught.
         for directory in sorted(mirror_root.rglob("*"), reverse=True):
             if directory.is_dir() and not any(directory.iterdir()):
@@ -366,21 +570,46 @@ def prune(mirror_root, expected, dry_run):
     return removed
 
 
-def run_once(scan_root, source_root, mirror_root, quality_args, jobs, dry_run, do_prune):
+def run_once(
+    scan_root,
+    source_root,
+    mirror_root,
+    quality_args,
+    jobs,
+    dry_run,
+    do_prune,
+    safe=False,
+    budget=0,
+):
     """Run a single pass. Returns the number of failures.
 
     `scan_root` is what gets walked and `source_root` is what mirror paths are
     computed against; they differ only for a partial pass over one directory.
     """
     started = time.monotonic()
-    counts = {"encoded": 0, "copied": 0, "skipped": 0, "failed": 0}
+    logger.info("pass starting with %d concurrent encoders", jobs)
+    counts = {"encoded": 0, "copied": 0, "renamed": 0, "skipped": 0, "failed": 0}
     failures = []
 
-    work = plan(scan_root, source_root, mirror_root)
+    work = plan(scan_root, source_root, mirror_root, safe, budget)
 
     with concurrent.futures.ThreadPoolExecutor(max_workers=jobs) as pool:
         futures = [
-            pool.submit(process, source, mirror, quality_args, dry_run)
+            pool.submit(
+                process,
+                source,
+                mirror,
+                quality_args,
+                dry_run,
+                (
+                    [
+                        mirror_path_for(source, source_root, mirror_root),
+                        mirror_path_for(source, source_root, mirror_root, True),
+                    ]
+                    if safe
+                    else None
+                ),
+            )
             for mirror, source in work.items()
         ]
         for future in concurrent.futures.as_completed(futures):
@@ -389,16 +618,27 @@ def run_once(scan_root, source_root, mirror_root, quality_args, jobs, dry_run, d
             if result.action == "failed":
                 failures.append(result)
 
-    removed = prune(mirror_root, set(work), dry_run) if do_prune else 0
+    expected = set(work)
+    if safe and dry_run:
+        # Nothing was actually renamed, so the pre-sanitisation files are still
+        # on disk. They are not orphans -- they are the files a real run would
+        # move -- and reporting them for deletion would misrepresent the pass
+        # twice over.
+        for source in work.values():
+            expected.add(mirror_path_for(source, source_root, mirror_root))
+            expected.add(mirror_path_for(source, source_root, mirror_root, True))
+    removed = prune(mirror_root, expected, dry_run) if do_prune else 0
 
     for failure in failures:
         logger.error("failed: %s: %s", failure.path, failure.error)
 
     logger.info(
-        "pass complete in %.1fs: %d encoded, %d copied, %d up to date, %d removed, %d failed",
+        "pass complete in %.1fs: %d encoded, %d copied, %d renamed, %d up to date,"
+        " %d removed, %d failed",
         time.monotonic() - started,
         counts["encoded"],
         counts["copied"],
+        counts["renamed"],
         counts["skipped"],
         removed,
         counts["failed"],
@@ -418,6 +658,25 @@ def acquire_lock(mirror_root):
     return handle
 
 
+def default_jobs():
+    """Return the number of CPUs this process may actually use.
+
+    os.cpu_count() reports the host's total, which in a container with a `cpus:`
+    limit means starting several times more encoders than there is CPU to run
+    them. libmp3lame is single-threaded, so one process per available CPU is the
+    whole of the concurrency story.
+    """
+    try:
+        quota, period = Path("/sys/fs/cgroup/cpu.max").read_text().split()
+        if quota != "max":
+            return max(1, round(int(quota) / int(period)))
+    except (OSError, ValueError):
+        pass
+    if hasattr(os, "process_cpu_count"):  # 3.13+, respects CPU affinity
+        return os.process_cpu_count() or 4
+    return os.cpu_count() or 4
+
+
 def build_parser():
     """Return the argument parser. Every option also reads an env var, so the
     container can be configured without a command line."""
@@ -443,8 +702,8 @@ def build_parser():
     parser.add_argument(
         "--jobs",
         type=int,
-        default=int(os.getenv("MUSIC_MIRROR_JOBS", "0")) or (os.cpu_count() or 4),
+        default=int(os.getenv("MUSIC_MIRROR_JOBS", "0")) or default_jobs(),
-        help="concurrent encodes (env MUSIC_MIRROR_JOBS; default: CPU count)",
+        help="concurrent encodes (env MUSIC_MIRROR_JOBS; default: available CPUs)",
     )
     parser.add_argument(
         "--interval",
@@ -456,6 +715,26 @@ def build_parser():
         default=None,
         help="limit the pass to one directory below --source; skips pruning",
     )
+    parser.add_argument(
+        "--fat32-safe",
+        action="store_true",
+        default=os.getenv("MUSIC_MIRROR_FAT32_SAFE", "").lower() in ("1", "true", "yes"),
+        help="name mirror files so a FAT32 device will accept them"
+        " (env MUSIC_MIRROR_FAT32_SAFE)",
+    )
+    parser.add_argument(
+        "--max-path",
+        type=int,
+        default=int(os.getenv("MUSIC_MIRROR_MAX_PATH", str(MAX_PATH))),
+        help=f"longest path the device will take, counted from its root; Rockbox's"
+        f" MAX_PATH is {MAX_PATH} (env MUSIC_MIRROR_MAX_PATH)",
+    )
+    parser.add_argument(
+        "--device-prefix",
+        default=os.getenv("MUSIC_MIRROR_DEVICE_PREFIX", DEVICE_PREFIX),
+        help="directory the mirror is copied into on the device, whose length comes"
+        " out of the path budget (env MUSIC_MIRROR_DEVICE_PREFIX)",
+    )
     parser.add_argument(
         "--no-prune",
         action="store_true",
@@ -474,12 +753,14 @@ def main(argv=None):
     logging.basicConfig(format="%(asctime)s %(levelname)s %(message)s", level=logging.INFO)
     args = build_parser().parse_args(argv)
 
-    # Directories are created with 0o777 masked by the umask, so clear the group
+    # Directories are created with 0o777 masked by the umask, so clear the bits
-    # bits from it once here rather than chmod'ing every directory the walk
+    # that matter from it once here rather than chmod'ing every directory the
-    # creates. Files cannot be handled this way -- mkstemp and copy2 both set a
+    # walk creates. The `other` bits are left alone, since whether the mirror is
-    # mode outright -- so they get an explicit chmod instead.
+    # world-readable is a real policy question; owner and group access is not.
+    # Files cannot be handled this way -- mkstemp and copy2 both set a mode
+    # outright, ignoring the umask -- so they get an explicit chmod instead.
     inherited = os.umask(0o077)
-    os.umask(inherited & ~GROUP_ENTER)
+    os.umask(inherited & ~DIRECTORY_ACCESS)
 
     if not args.source or not args.mirror:
         logger.error("both --source and --mirror are required")
@@ -512,6 +793,17 @@ def main(argv=None):
         # A partial pass cannot tell an orphan from a file outside its scope.
         do_prune = False
 
+    # The device's limit covers the whole path it will see, so what the mirror
+    # may spend is that less the directory it gets copied into.
+    budget = max(0, args.max_path - len(device_prefix_length(args.device_prefix)))
+    if args.fat32_safe:
+        logger.info(
+            "paths are limited to %d characters, from --max-path %d less the %r prefix",
+            budget,
+            args.max_path,
+            args.device_prefix,
+        )
+
     lock = acquire_lock(mirror_root)
     if lock is None:
         logger.error("another pass is already running over %s", mirror_root)
@@ -537,6 +829,8 @@ def main(argv=None):
                 args.jobs,
                 args.dry_run,
                 do_prune,
+                args.fat32_safe,
+                budget,
             )
             if interval is None or stopping:
                 return 1 if failures else 0

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "music-mirror"
-version = "0.1.0"
+version = "0.4.0"
 description = "Maintain a lossy MP3 mirror of a lossless music library"
 readme = "README.md"
 requires-python = ">=3.11"

tests/conftest.py

@@ -27,6 +27,21 @@ def tight_umask():
     os.umask(previous)
 
 
+@pytest.fixture
+def owner_hostile_umask():
+    """Return a callable applying a umask that masks off the owner's read bit.
+
+    Unusual, but it is what produces a mirror tree of mode 0300 -- writable and
+    enterable, unreadable to the very process that built it. Applied on demand
+    rather than for the whole test, because the source library is built by
+    something else entirely and the same umask would make the test's own
+    fixtures unreadable before the run under test even started.
+    """
+    previous = os.umask(0o022)
+    yield lambda: os.umask(0o477)
+    os.umask(previous)
+
+
 @pytest.fixture
 def make_flac():
     """Return a factory writing a short tagged FLAC file."""
@@ -63,6 +78,23 @@ def make_flac():
     return factory
 
 
+@pytest.fixture
+def make_cover():
+    """Return a factory writing a small JPEG beside an album's tracks."""
+
+    def factory(path):
+        path.parent.mkdir(parents=True, exist_ok=True)
+        subprocess.run(
+            ["ffmpeg", "-nostdin", "-hide_banner", "-loglevel", "error", "-y",
+             "-f", "lavfi", "-i", "color=c=red:s=64x64:d=1", "-frames:v", "1", str(path)],
+            check=True,
+            capture_output=True,
+        )
+        return path
+
+    return factory
+
+
 @pytest.fixture
 def probe_tag():
     """Return a helper reading a single metadata tag from a file."""

tests/test_check_fat32.py

@@ -0,0 +1,64 @@
+import subprocess
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent / "tools"))
+
+import check_fat32  # noqa: E402
+
+
+def problems(name):
+    return check_fat32.problems_with(Path(name))
+
+
+def test_a_reserved_character_is_a_problem():
+    assert any("reserved" in p for p in problems("Album/Motherf**ker.mp3"))
+
+
+def test_a_trailing_dot_or_space_is_a_problem():
+    """FAT eats them silently, so the name round-trips as a different name."""
+    assert any("trailing" in p for p in problems("Trailing Dot./x.mp3"))
+    assert any("trailing" in p for p in problems("Space /x.mp3"))
+
+
+def test_an_ordinary_name_is_fine():
+    assert problems("Artist/Album/01 Fine Track.mp3") == []
+
+
+def test_accents_are_fine():
+    assert problems("Mötley Crüe/Album/Track.mp3") == []
+
+
+def test_an_over_long_path_is_a_problem():
+    deep = "/".join("d" * 40 for _ in range(10)) + "/track.mp3"
+    assert any("path of" in p for p in problems(deep))
+
+
+def test_case_collisions_are_reported(tmp_path):
+    album = tmp_path / "Album"
+    album.mkdir()
+    (album / "Song.mp3").write_bytes(b"x")
+    (album / "SONG.mp3").write_bytes(b"x")
+
+    completed = subprocess.run(
+        [sys.executable, str(Path(check_fat32.__file__)), str(tmp_path)],
+        capture_output=True,
+        text=True,
+    )
+
+    assert completed.returncode == 1
+    assert "collides case-insensitively" in completed.stdout
+
+
+def test_a_clean_tree_exits_zero(tmp_path):
+    (tmp_path / "Album").mkdir()
+    (tmp_path / "Album" / "Fine.mp3").write_bytes(b"x")
+
+    completed = subprocess.run(
+        [sys.executable, str(Path(check_fat32.__file__)), str(tmp_path)],
+        capture_output=True,
+        text=True,
+    )
+
+    assert completed.returncode == 0
+    assert "0 problems" in completed.stderr

tests/test_music_mirror.py

@@ -222,6 +222,27 @@ def test_mirror_directories_are_group_traversable(tmp_path, make_flac, tight_uma
         assert mode & stat.S_IXGRP, directory
 
 
+def test_mirror_directories_survive_an_owner_hostile_umask(
+    tmp_path, make_flac, owner_hostile_umask
+):
+    """A umask carrying 0400 otherwise builds a tree the run cannot read back."""
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "Artist" / "Album" / "a.flac")
+
+    # Applied only now: the library already exists, and the umask under test is
+    # the one the container starts this run with.
+    owner_hostile_umask()
+    run(source, mirror)
+
+    for directory in (mirror, mirror / "Artist", mirror / "Artist" / "Album"):
+        mode = directory.stat().st_mode
+        assert mode & stat.S_IRUSR, directory
+        assert mode & stat.S_IXUSR, directory
+        assert mode & stat.S_IRGRP, directory
+        assert mode & stat.S_IXGRP, directory
+
+
 def test_private_mirror_file_is_repaired_without_re_encoding(tmp_path, make_flac):
     """A mirror written by an older version has a correct mtime, so nothing
     else in the pass would revisit it."""
@@ -422,3 +443,346 @@ def test_mirror_inside_source_is_refused(tmp_path, make_flac):
 
 def test_missing_source_is_refused(tmp_path):
     assert run(tmp_path / "nope", tmp_path / "dst") == 2
+
+
+@pytest.mark.parametrize(
+    ("name", "expected"),
+    [
+        ("Kick Out the Epic Motherf**ker", "Kick Out the Epic Motherf__ker"),
+        ("Where Are You?", "Where Are You_"),
+        ("Song: Part 2", "Song_ Part 2"),
+        ('"Heroes"', "_Heroes_"),
+        ("trailing dot.", "trailing dot"),
+        ("trailing space ", "trailing space"),
+        ("...", "_"),
+        ("Mötley Crüe", "Mötley Crüe"),
+        ("perfectly ordinary", "perfectly ordinary"),
+    ],
+)
+def test_fat32_safe_names(name, expected):
+    """A name FAT32 will not take is a track that silently does not arrive."""
+    assert music_mirror.fat32_safe(name) == expected
+
+
+def test_a_reserved_character_is_replaced_in_the_mirror_path(tmp_path, make_flac):
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "Dada Life" / "Album" / "Kick Out the Epic Motherf**ker.flac")
+
+    run(source, mirror, "--fat32-safe")
+
+    assert (mirror / "Dada Life" / "Album" / "Kick Out the Epic Motherf__ker.mp3").is_file()
+
+
+def test_without_the_flag_the_name_is_left_alone(tmp_path, make_flac):
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "Album" / "Where Are You?.flac")
+
+    run(source, mirror)
+
+    assert (mirror / "Album" / "Where Are You?.mp3").is_file()
+
+
+def test_an_existing_mirror_is_renamed_not_re_encoded(tmp_path, make_flac):
+    """Turning the flag on changes the path of every track holding a reserved
+    character. Re-encoding those would be hours of work to produce files that
+    already exist byte for byte."""
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "Album" / "Where Are You?.flac")
+
+    run(source, mirror)
+    before = mirror / "Album" / "Where Are You?.mp3"
+    contents = before.read_bytes()
+    stamp = before.stat().st_mtime_ns
+
+    run(source, mirror, "--fat32-safe")
+
+    after = mirror / "Album" / "Where Are You_.mp3"
+    assert after.is_file()
+    assert not before.exists()
+    assert after.read_bytes() == contents, "it was re-encoded rather than moved"
+    assert after.stat().st_mtime_ns == stamp
+
+
+def test_names_colliding_only_by_case_are_caught(tmp_path, make_flac):
+    """Two files here, one file on FAT32. Better found now than as a silent
+    overwrite partway through the copy."""
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "Album" / "Song.flac")
+    make_flac(source / "Album" / "SONG.flac")
+
+    run(source, mirror, "--fat32-safe")
+
+    written = sorted(p.name for p in (mirror / "Album").glob("*.mp3"))
+    assert len(written) == 1, written
+
+
+def test_the_album_cover_is_copied_beside_the_tracks(tmp_path, make_flac, make_cover):
+    """Rockbox looks for art on the filesystem; its search never touches the
+    picture embedded in the tag."""
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "Album" / "a.flac")
+    make_cover(source / "Album" / "cover.jpg")
+
+    run(source, mirror)
+
+    assert (mirror / "Album" / "cover.jpg").is_file()
+
+
+def test_a_copied_cover_is_group_readable(tmp_path, make_flac, make_cover, tight_umask):
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "Album" / "a.flac")
+    make_cover(source / "Album" / "cover.jpg")
+
+    run(source, mirror)
+
+    assert (mirror / "Album" / "cover.jpg").stat().st_mode & stat.S_IRGRP
+
+
+def test_a_cover_left_without_tracks_is_pruned(tmp_path, make_flac, make_cover):
+    """Otherwise the directory never looks empty and never goes."""
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "Gone" / "a.flac")
+    make_cover(source / "Gone" / "cover.jpg")
+
+    run(source, mirror)
+    assert (mirror / "Gone" / "cover.jpg").is_file()
+
+    shutil.rmtree(source / "Gone")
+    run(source, mirror)
+
+    assert not (mirror / "Gone").exists()
+
+
+def test_a_dry_run_reports_a_rename_not_an_encode(tmp_path, make_flac, caplog):
+    """The difference between moving a file and re-encoding it is the
+    difference between a minute and an afternoon, so a dry run must not
+    describe the first as the second."""
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "Album" / "Where Are You?.flac")
+    run(source, mirror)
+
+    with caplog.at_level("INFO"):
+        run(source, mirror, "--fat32-safe", "--dry-run")
+
+    assert "would rename" in caplog.text
+    assert "would encode" not in caplog.text
+
+
+def test_a_dry_run_does_not_call_the_old_paths_orphans(tmp_path, make_flac, caplog):
+    """Nothing was renamed, so they are still there -- but they are the files a
+    real run would move, not files it would delete."""
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "Album" / "Where Are You?.flac")
+    run(source, mirror)
+
+    with caplog.at_level("INFO"):
+        run(source, mirror, "--fat32-safe", "--dry-run")
+
+    assert "would remove orphan" not in caplog.text
+
+
+def test_a_dry_run_moves_nothing(tmp_path, make_flac):
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "Album" / "Where Are You?.flac")
+    run(source, mirror)
+
+    run(source, mirror, "--fat32-safe", "--dry-run")
+
+    assert (mirror / "Album" / "Where Are You?.mp3").is_file()
+    assert not (mirror / "Album" / "Where Are You_.mp3").exists()
+
+
+def test_renames_are_counted_separately_from_encodes(tmp_path, make_flac, caplog):
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "Album" / "Where Are You?.flac")
+    run(source, mirror)
+
+    with caplog.at_level("INFO"):
+        run(source, mirror, "--fat32-safe")
+
+    assert "1 renamed" in caplog.text
+    assert "0 encoded" in caplog.text
+
+
+def test_a_long_path_is_shortened_from_the_deepest_component(tmp_path, make_flac):
+    """The track name carries the least navigational value and the artist the
+    most, so the filename is sacrificed before the album."""
+    artist = "A" * 60
+    album = "B" * 60
+    title = "C" * 150
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / artist / album / f"{title}.flac")
+
+    run(source, mirror, "--fat32-safe", "--max-path", "160", "--device-prefix", "/Music")
+
+    written = list(mirror.rglob("*.mp3"))
+    assert len(written) == 1
+    relative = written[0].relative_to(mirror)
+    assert relative.parts[0] == artist, "the artist directory should be untouched"
+    assert relative.parts[1] == album, "the album directory should be untouched"
+    assert len(str(relative)) <= 160 - len("Music") - 2
+
+
+def test_shortening_is_stable_across_passes(tmp_path, make_flac):
+    """An unstable name would rename every file on every pass, for ever."""
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / ("D" * 80) / ("E" * 80) / f"{'F' * 120}.flac")
+
+    run(source, mirror, "--fat32-safe", "--max-path", "180")
+    first = sorted(str(p.relative_to(mirror)) for p in mirror.rglob("*.mp3"))
+    stamp = next(mirror.rglob("*.mp3")).stat().st_mtime_ns
+
+    run(source, mirror, "--fat32-safe", "--max-path", "180")
+
+    assert sorted(str(p.relative_to(mirror)) for p in mirror.rglob("*.mp3")) == first
+    assert next(mirror.rglob("*.mp3")).stat().st_mtime_ns == stamp
+
+
+def test_two_long_names_do_not_collide_after_shortening(tmp_path, make_flac):
+    """They share a prefix and cut to the same string; the hash is what keeps
+    them apart."""
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    shared = "G" * 140
+    make_flac(source / "Album" / f"{shared}one.flac")
+    make_flac(source / "Album" / f"{shared}two.flac")
+
+    run(source, mirror, "--fat32-safe", "--max-path", "120")
+
+    assert len(list(mirror.rglob("*.mp3"))) == 2
+
+
+def test_a_sanitised_mirror_is_renamed_rather_than_re_encoded_when_shortening(
+    tmp_path, make_flac
+):
+    """The previous naming is sanitised-but-not-shortened, not the original."""
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "Album" / f"Where Are You? {'H' * 140}.flac")
+
+    run(source, mirror, "--fat32-safe", "--max-path", "400")
+    before = next(mirror.rglob("*.mp3"))
+    contents = before.read_bytes()
+
+    run(source, mirror, "--fat32-safe", "--max-path", "120")
+
+    after = next(mirror.rglob("*.mp3"))
+    assert after != before
+    assert after.read_bytes() == contents, "it was re-encoded rather than moved"
+
+
+def test_shortening_only_applies_when_over_budget(tmp_path, make_flac):
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "Artist" / "Album" / "Short Name.flac")
+
+    run(source, mirror, "--fat32-safe")
+
+    assert (mirror / "Artist" / "Album" / "Short Name.mp3").is_file()
+
+
+def test_a_path_that_cannot_be_made_to_fit_is_reported(tmp_path, make_flac, caplog):
+    """Too deeply nested to shorten without making every component unreadable."""
+    deep = Path(*["I" * 20 for _ in range(10)])
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / deep / "track.flac")
+
+    with caplog.at_level("WARNING"):
+        run(source, mirror, "--fat32-safe", "--max-path", "80")
+
+    assert "too" in caplog.text and "nested" in caplog.text
+
+
+# Lidarr writes "Artist - Album - NN - Title.mp3" inside a directory already
+# named for that artist and album, so a long album title appears three times in
+# one path. These are real paths from a real library.
+GIZZARD_ALBUM = (
+    "PetroDragonic Apocalypse; or, Dawn of Eternal Night - An Annihilation of"
+    " Planet Earth and the Beginning of Merciless Damnation"
+)
+GIZZARD_TRACKS = (
+    "01 - Motor Spirit",
+    "02 - Supercell",
+    "03 - Converge",
+    "04 - Witchcraft",
+    "05 - Gila Monster",
+    "06 - Dragon",
+    "07 - Flamethrower",
+)
+
+
+def gizzard_path(track):
+    return Path(
+        "King Gizzard & the Lizard Wizard",
+        f"{GIZZARD_ALBUM} (2023)",
+        f"King Gizzard & the Lizard Wizard - {GIZZARD_ALBUM} - {track}.mp3",
+    )
+
+
+def test_shortening_keeps_the_part_that_tells_tracks_apart():
+    """Cutting from the end discards the track number and title, which is all
+    that distinguishes one track on the record from another."""
+    for track in GIZZARD_TRACKS:
+        fitted = music_mirror.fit_path(gizzard_path(track), 253)
+        assert len(str(fitted)) <= 253
+        assert fitted.name.endswith(f"{track}.mp3"), fitted.name
+
+
+def test_every_track_on_a_long_album_keeps_a_distinct_name():
+    fitted = {music_mirror.fit_path(gizzard_path(t), 253).name for t in GIZZARD_TRACKS}
+
+    assert len(fitted) == len(GIZZARD_TRACKS)
+
+
+def test_a_long_title_keeps_both_ends():
+    """The Beatles' 'The Long One' is one track whose title is the long part."""
+    path = Path(
+        "The Beatles",
+        "Abbey Road (1969)",
+        "Digital Media 03",
+        "The Beatles - Abbey Road - 09 - The Long One - You Never Give Me Your Money"
+        " + Sun King + Mean Mr Mustard + Her Majesty + Polythene Pam + She Came In"
+        " Through the Bathroom Window+ Golden Slumbers + Carry That Weight + The End.mp3",
+    )
+
+    fitted = music_mirror.fit_path(path, 253)
+
+    assert len(str(fitted)) <= 253
+    assert fitted.name.startswith("The Beatles - Abbey Road - 09 - The Long One")
+    assert fitted.name.endswith("The End.mp3")
+
+
+@pytest.mark.parametrize(
+    ("prefix", "expected"),
+    [("/Music", 253), ("Music", 253), ("/Music/", 253), ("", 259), ("/", 259)],
+)
+def test_the_device_prefix_is_costed_exactly(prefix, expected):
+    """A mirror-relative path of 253 characters becomes 260 on the device once
+    /Music/ is in front of it, which is the whole of the limit. Approximating
+    the prefix loses a character at the root, where the longest paths are."""
+    assert 260 - len(music_mirror.device_prefix_length(prefix)) == expected
+
+
+def test_the_budget_is_reported_so_it_can_be_checked(tmp_path, make_flac, caplog):
+    source = tmp_path / "src"
+    mirror = tmp_path / "dst"
+    make_flac(source / "a.flac")
+
+    with caplog.at_level("INFO"):
+        run(source, mirror, "--fat32-safe")
+
+    assert "limited to 253 characters" in caplog.text

tests/test_rsync_progress.py

@@ -0,0 +1,180 @@
+import io
+import sys
+from pathlib import Path
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent / "tools"))
+
+import rsync_progress  # noqa: E402
+
+
+class NotATerminal(io.StringIO):
+    def isatty(self):
+        return False
+
+
+class Terminal(io.StringIO):
+    def isatty(self):
+        return True
+
+
+def run(lines, total=0, out=None, bytes_expected=0):
+    out = out or NotATerminal()
+    rsync_progress.main(
+        ["--total", str(total), "--bytes", str(bytes_expected)],
+        stream=io.StringIO(lines),
+        out=out,
+    )
+    return out.getvalue()
+
+
+def test_the_artist_and_album_are_pulled_from_the_path():
+    assert rsync_progress.album_of("Pendulum/Immersion/01 - Watercolour.mp3") == (
+        "Pendulum / Immersion"
+    )
+
+
+def test_a_shallower_path_degrades_rather_than_failing():
+    assert rsync_progress.album_of("Pendulum/loose.mp3") == "Pendulum"
+    assert rsync_progress.album_of("loose.mp3") == ""
+
+
+def test_directories_are_not_counted():
+    """rsync reports them too, and counting them puts the percentage past 100."""
+    output = run("Artist/\nArtist/Album/\nArtist/Album/track.mp3\n", total=1)
+
+    assert "1/1" in output
+    assert "100%" in output
+
+
+def test_the_percentage_tracks_the_total():
+    output = run("".join(f"A/B/{i}.mp3\n" for i in range(5)), total=10)
+
+    assert "5/10" in output
+    assert "50%" in output
+
+
+def test_without_a_total_it_counts_instead_of_guessing():
+    output = run("A/B/one.mp3\nA/B/two.mp3\n")
+
+    assert "2 files" in output
+    assert "%" not in output
+
+
+def test_a_final_line_is_always_printed():
+    """Otherwise the last state of a rewriting line is whatever it happened to
+    be when the interval last elapsed."""
+    output = run("A/B/one.mp3\n", total=1)
+
+    assert output.endswith("\n")
+    assert "1/1" in output
+
+
+def test_nothing_transferred_still_reports():
+    output = run("", total=0)
+
+    assert "0 files" in output
+
+
+def test_a_log_gets_no_carriage_returns():
+    """A non-terminal filling with \\r and escape codes is unreadable."""
+    output = run("".join(f"A/B/{i}.mp3\n" for i in range(50)), total=50)
+
+    assert "\r" not in output
+    assert "\033" not in output
+
+
+def test_a_terminal_rewrites_one_line():
+    output = run("".join(f"A/B/{i}.mp3\n" for i in range(50)), total=50, out=Terminal())
+
+    assert "\r\033[2K" in output
+
+
+@pytest.mark.parametrize(
+    ("text", "width", "expected"),
+    [
+        ("short", 20, "short"),
+        ("King Gizzard / PetroDragonic Apocalypse", 20, "…agonic Apocalypse"),
+    ],
+)
+def test_long_labels_are_trimmed_from_the_left(text, width, expected):
+    """The album is the informative end, so the artist is what gets cut."""
+    trimmed = rsync_progress.fit(text, width)
+
+    assert len(trimmed) <= width
+    if len(text) > width:
+        assert trimmed.startswith("…")
+        assert text.endswith(trimmed.lstrip("…"))
+
+
+def test_the_size_and_path_are_parsed():
+    assert rsync_progress.parse("5000 Artist/Album/Track.mp3\n") == (
+        5000,
+        "Artist/Album/Track.mp3",
+    )
+
+
+def test_a_filename_containing_spaces_survives():
+    """Splitting on every space would lose most of the library."""
+    assert rsync_progress.parse("1234 Artist/An Album/A Track With Spaces.mp3") == (
+        1234,
+        "Artist/An Album/A Track With Spaces.mp3",
+    )
+
+
+def test_a_bare_path_is_tolerated():
+    """In case this is fed --out-format='%n' by something older."""
+    assert rsync_progress.parse("Artist/Album/Track.mp3") == (0, "Artist/Album/Track.mp3")
+
+
+def test_directory_sizes_do_not_inflate_the_total():
+    """rsync reports directories with a 4096 inode size, which is several
+    megabytes of nothing across six thousand albums."""
+    output = run("4096 Artist/\n4096 Artist/Album/\n5000 Artist/Album/t.mp3\n", total=1)
+
+    assert "4.9 KiB" in output
+    assert "12" not in output.split("Artist")[0]
+
+
+def test_a_rate_is_not_reported_until_it_means_something():
+    """The first files arrive microseconds apart and would give a rate in the
+    gigabytes per second and an ETA of zero."""
+    rate = rsync_progress.Rate()
+    rate.add(100.0, 0)
+    rate.add(100.5, 5_000_000)
+
+    assert rate.per_second() == 0.0
+
+
+def test_a_rate_over_a_long_enough_window_is_reported():
+    rate = rsync_progress.Rate()
+    rate.add(100.0, 0)
+    rate.add(110.0, 10_000_000)
+
+    assert rate.per_second() == pytest.approx(1_000_000)
+
+
+def test_the_window_forgets_the_distant_past():
+    """So the estimate follows a device that slows down rather than averaging
+    the slowdown away."""
+    rate = rsync_progress.Rate(window=30.0)
+    for second in range(0, 100, 10):
+        rate.add(float(second), second * 1_000_000)
+    rate.add(200.0, 100_000_000)
+
+    assert rate.samples[0][0] >= 90.0
+
+
+@pytest.mark.parametrize(
+    ("seconds", "expected"),
+    [(0, "0s"), (45, "45s"), (60, "1m00s"), (1092, "18m12s"), (7500, "2h05m")],
+)
+def test_durations_read_without_arithmetic(seconds, expected):
+    assert rsync_progress.human_duration(seconds) == expected
+
+
+def test_a_summary_is_printed_at_the_end():
+    output = run("5000000 A/B/one.mp3\n", total=1, bytes_expected=5000000)
+
+    assert "copied 4.8 MiB in" in output

tests/test_submit_scrobbles.py

@@ -0,0 +1,563 @@
+import json
+import subprocess
+import sys
+from datetime import datetime, timezone
+from pathlib import Path
+from zoneinfo import ZoneInfo
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent / "tools"))
+
+import submit_scrobbles  # noqa: E402
+
+LOG = """#AUDIOSCROBBLER/1.1
+#TZ/UNKNOWN
+#CLIENT/Rockbox ipodvideo 4.0
+#ARTIST\t#ALBUM\t#TITLE\t#TRACKNUM\t#LENGTH\t#RATING\t#TIMESTAMP\t#MUSICBRAINZ_TRACKID
+Pendulum\tImmersion\tWatercolour\t3\t245\tL\t1700000300\t
+Green Day\tDookie\tBasket Case\t7\t180\tS\t1700000200\t
+Mötley Crüe\tDr. Feelgood\tKickstart My Heart\t2\t283\tL\t1700000100\tmb-1
+"""
+
+
+def test_only_listened_tracks_are_submitted():
+    """A skip is not a play."""
+    played, skipped, timeless = submit_scrobbles.parse_log(LOG)
+
+    assert [entry["track"] for entry in played] == ["Kickstart My Heart", "Watercolour"]
+    assert skipped == 1
+    assert timeless == 0
+
+
+def test_entries_come_back_oldest_first():
+    played, _, _ = submit_scrobbles.parse_log(LOG)
+
+    assert [entry["timestamp"] for entry in played] == ["1700000100", "1700000300"]
+
+
+def test_a_timeless_log_is_counted_and_not_submitted():
+    """Without a real-time clock Rockbox writes every timestamp as zero. Those
+    cannot be scrobbled without inventing when they happened."""
+    log = LOG + "Band\tAlbum\tTrack\t-1\t100\tL\t0\t\n"
+
+    played, _, timeless = submit_scrobbles.parse_log(log)
+
+    assert timeless == 1
+    assert all(int(entry["timestamp"]) > 0 for entry in played)
+
+
+def test_absent_optional_fields_are_dropped():
+    played, _, _ = submit_scrobbles.parse_log(LOG)
+    params = submit_scrobbles.batch_params(played)
+
+    assert "mbid[0]" in params  # Kickstart My Heart has one
+    assert "mbid[1]" not in params  # Watercolour does not
+    assert params["trackNumber[0]"] == "2"
+
+
+def test_a_track_number_of_minus_one_is_not_sent():
+    """Rockbox writes -1 when it does not know, which is not a track number."""
+    played, _, _ = submit_scrobbles.parse_log(
+        "Band\tAlbum\tTrack\t-1\t100\tL\t1700000000\t\n"
+    )
+
+    assert submit_scrobbles.batch_params(played).get("trackNumber[0]") is None
+
+
+def test_the_signature_sorts_names_by_ascii_not_by_number():
+    """Last.fm sorts parameter names as strings, so artist[10] precedes
+    artist[1]. Sorting numerically produces an invalid signature and nothing
+    else."""
+    params = {"artist[1]": "b", "artist[10]": "a", "api_key": "k"}
+
+    expected = submit_scrobbles.hashlib.md5(
+        ("api_keyk" + "artist[1]b" + "artist[10]a" + "s").encode()
+    ).hexdigest()
+    assert submit_scrobbles.sign(params, "s") != expected
+
+    correct = submit_scrobbles.hashlib.md5(
+        ("api_keyk" + "artist[10]a" + "artist[1]b" + "s").encode()
+    ).hexdigest()
+    assert submit_scrobbles.sign(params, "s") == correct
+
+
+def fake_transport(responses):
+    """Return a transport serving canned responses and recording requests."""
+    sent = []
+
+    def transport(request, timeout=None):
+        sent.append(dict(submit_scrobbles.urllib.parse.parse_qsl(request.data.decode())))
+        return json.dumps(responses[len(sent) - 1])
+
+    transport.sent = sent
+    return transport
+
+
+def test_scrobbles_are_sent_in_batches_of_fifty():
+    entries = [
+        {"artist": "A", "track": f"T{i}", "timestamp": str(1700000000 + i)}
+        for i in range(120)
+    ]
+    accepted = {"scrobbles": {"@attr": {"accepted": 50, "ignored": 0}}}
+    transport = fake_transport([accepted, accepted, accepted])
+
+    submit_scrobbles.submit(entries, "k", "s", "sk", transport, delay=0)
+
+    assert len(transport.sent) == 3
+    assert transport.sent[0]["method"] == "track.scrobble"
+    assert "artist[49]" in transport.sent[0]
+    assert "artist[50]" not in transport.sent[0]
+
+
+def test_every_request_carries_a_signature_and_session():
+    entries = [{"artist": "A", "track": "T", "timestamp": "1700000000"}]
+    transport = fake_transport([{"scrobbles": {"@attr": {"accepted": 1, "ignored": 0}}}])
+
+    submit_scrobbles.submit(entries, "k", "s", "session-key", transport, delay=0)
+
+    assert transport.sent[0]["sk"] == "session-key"
+    assert len(transport.sent[0]["api_sig"]) == 32
+
+
+def test_a_service_error_is_raised_not_swallowed():
+    entries = [{"artist": "A", "track": "T", "timestamp": "1700000000"}]
+    transport = fake_transport([{"error": 9, "message": "Invalid session key"}])
+
+    with pytest.raises(submit_scrobbles.LastfmError, match="error 9"):
+        submit_scrobbles.submit(entries, "k", "s", "sk", transport, delay=0)
+
+
+def test_the_log_is_set_aside_after_a_successful_submission(tmp_path, monkeypatch):
+    device = tmp_path / "IPOD"
+    device.mkdir()
+    (device / ".scrobbler.log").write_text(LOG, encoding="utf-8")
+    transport = fake_transport([{"scrobbles": {"@attr": {"accepted": 2, "ignored": 0}}}])
+    monkeypatch.setattr(submit_scrobbles, "load_session", lambda: "sk")
+
+    submit_scrobbles.main(
+        [str(device), "--api-key", "k", "--api-secret", "s"], transport=transport
+    )
+
+    assert not (device / ".scrobbler.log").exists()
+    # Renamed, not deleted: if Last.fm quietly dropped one, the evidence remains.
+    assert list(device.glob(".scrobbler.log.*.submitted"))
+
+
+def test_a_failed_submission_leaves_the_log_alone(tmp_path, monkeypatch):
+    device = tmp_path / "IPOD"
+    device.mkdir()
+    (device / ".scrobbler.log").write_text(LOG, encoding="utf-8")
+    transport = fake_transport([{"error": 29, "message": "Rate limit"}])
+    monkeypatch.setattr(submit_scrobbles, "load_session", lambda: "sk")
+
+    code = submit_scrobbles.main(
+        [str(device), "--api-key", "k", "--api-secret", "s"], transport=transport
+    )
+
+    assert code == 1
+    assert (device / ".scrobbler.log").is_file()
+
+
+def test_a_dry_run_submits_nothing(tmp_path):
+    device = tmp_path / "IPOD"
+    device.mkdir()
+    (device / ".scrobbler.log").write_text(LOG, encoding="utf-8")
+    transport = fake_transport([])
+
+    submit_scrobbles.main([str(device), "--dry-run"], transport=transport)
+
+    assert transport.sent == []
+    assert (device / ".scrobbler.log").is_file()
+
+
+def test_no_log_is_not_an_error(tmp_path):
+    device = tmp_path / "IPOD"
+    device.mkdir()
+
+    assert submit_scrobbles.main([str(device)], transport=fake_transport([])) == 0
+
+
+def test_write_credentials_are_required(tmp_path, capsys, monkeypatch):
+    """The read-only key used elsewhere is not enough for a write method."""
+    monkeypatch.delenv("LASTFM_API_KEY", raising=False)
+    monkeypatch.delenv("LASTFM_API_SECRET", raising=False)
+    device = tmp_path / "IPOD"
+    device.mkdir()
+    (device / ".scrobbler.log").write_text(LOG, encoding="utf-8")
+
+    code = submit_scrobbles.main([str(device)], transport=fake_transport([]))
+
+    assert code == 2
+    assert "LASTFM_API_SECRET" in capsys.readouterr().err
+
+
+PLAYBACK_LOG = """1700000300:180000:245000:/Music/Pendulum/Immersion/01.mp3
+1700000200:9000:180000:/Music/Green Day/Dookie/07.mp3
+0:180000:245000:/Music/No/Clock/track.mp3
+malformed line without colons
+"""
+
+
+def tags_of(artist="Pendulum", title="Watercolour", album="Immersion", track="1/11"):
+    def runner(path):
+        return json.dumps(
+            {"format": {"tags": {"ARTIST": artist, "TITLE": title,
+                                 "ALBUM": album, "track": track}}}
+        )
+
+    return runner
+
+
+def test_the_playback_log_format_is_four_fields():
+    """timestamp:elapsed_ms:length_ms:path, written by Rockbox core."""
+    plays = submit_scrobbles.parse_playback_log(PLAYBACK_LOG)
+
+    assert len(plays) == 3  # the malformed line is dropped
+    assert plays[0] == (1700000300, 180000, 245000, "/Music/Pendulum/Immersion/01.mp3")
+
+
+def test_a_device_path_maps_onto_the_mirror():
+    assert submit_scrobbles.device_to_local(
+        "/Music/Pendulum/Immersion/01.mp3", "/Music", "/mnt/mirror"
+    ) == Path("/mnt/mirror/Pendulum/Immersion/01.mp3")
+
+
+def test_a_path_outside_the_prefix_is_not_mapped():
+    """Something played from elsewhere on the card is not in the mirror."""
+    assert submit_scrobbles.device_to_local(
+        "/Podcasts/episode.mp3", "/Music", "/mnt/mirror"
+    ) is None
+
+
+def test_a_path_at_the_card_root_maps_straight_across():
+    assert submit_scrobbles.device_to_local(
+        "/Pendulum/Immersion/01.mp3", "/", "/mnt/mirror"
+    ) == Path("/mnt/mirror/Pendulum/Immersion/01.mp3")
+
+
+def test_a_short_play_is_a_skip_not_a_scrobble(tmp_path, monkeypatch):
+    """Nine seconds of a three minute track. The on-device plugin uses the same
+    fraction, so the two never disagree about what counted as a play."""
+    monkeypatch.setattr(Path, "is_file", lambda self: True)
+
+    result = submit_scrobbles.plays_from_playback_log(
+        PLAYBACK_LOG, "/Music", "/mnt/mirror", runner=tags_of()
+    )
+
+    assert result.skipped == 1
+    assert [entry["timestamp"] for entry in result.played] == ["1700000300"]
+
+
+def test_a_zero_timestamp_is_refused(tmp_path, monkeypatch):
+    """Without a real-time clock Rockbox logs ticks, not dates. Scrobbling
+    those would mean inventing when they happened."""
+    monkeypatch.setattr(Path, "is_file", lambda self: True)
+
+    result = submit_scrobbles.plays_from_playback_log(
+        PLAYBACK_LOG, "/Music", "/mnt/mirror", runner=tags_of()
+    )
+
+    assert result.timeless == 1
+
+
+def test_tags_come_from_the_mirror(tmp_path, monkeypatch):
+    """The log carries only a path -- which is precisely why the on-device
+    plugin exists. Off the mirror the tags are free."""
+    monkeypatch.setattr(Path, "is_file", lambda self: True)
+
+    played = submit_scrobbles.plays_from_playback_log(
+        PLAYBACK_LOG, "/Music", "/mnt/mirror", runner=tags_of()
+    ).played
+
+    assert played[0]["artist"] == "Pendulum"
+    assert played[0]["album"] == "Immersion"
+    assert played[0]["trackNumber"] == "1"       # "1/11" -> "1"
+    assert played[0]["duration"] == "245"        # milliseconds -> seconds
+
+
+def test_a_track_missing_from_the_mirror_is_counted_not_guessed(monkeypatch):
+    monkeypatch.setattr(Path, "is_file", lambda self: False)
+
+    result = submit_scrobbles.plays_from_playback_log(
+        PLAYBACK_LOG, "/Music", "/mnt/mirror", runner=tags_of()
+    )
+
+    assert result.played == []
+    # One: the skip and the clockless entry are filtered before the file is
+    # looked for, since neither would be submitted either way.
+    assert result.unresolved == 1
+    # And that one play is kept, so a later run can try it again.
+    assert len(result.retain) == 1
+
+
+LONDON = ZoneInfo("Europe/London")
+
+# Rockbox builds timestamps with mktime(get_time()), and its mktime applies no
+# zone at all, so the number is the RTC's local wall clock read as if it were
+# UTC. Under BST that puts every play an hour ahead of when it happened.
+SUMMER_LOGGED = 1787839200   # 2026-08-27 14:00 written by the device
+SUMMER_TRUE = 1787835600     # the instant that actually was, 13:00 UTC
+WINTER_LOGGED = 1796479200   # 2026-12-05 14:00, when London is already UTC
+WINTER_TRUE = 1796479200
+
+
+def test_a_summer_timestamp_is_pulled_back_to_real_utc():
+    """The device logged 14:00 local. That was 13:00 UTC, and UTC is what
+    Last.fm stores."""
+    assert submit_scrobbles.device_time_to_utc(SUMMER_LOGGED, LONDON) == SUMMER_TRUE
+
+
+def test_a_winter_timestamp_is_left_alone():
+    """London keeps UTC for half the year, so there is nothing to correct and
+    the correction must not invent an offset anyway."""
+    assert submit_scrobbles.device_time_to_utc(WINTER_LOGGED, LONDON) == WINTER_TRUE
+
+
+def test_the_converted_time_reads_back_as_the_clock_the_device_showed():
+    """The round trip, which is the property that actually matters: whatever
+    the player's screen said is what Last.fm should show in local time."""
+    shown = datetime.fromtimestamp(SUMMER_LOGGED, timezone.utc)
+    corrected = submit_scrobbles.device_time_to_utc(SUMMER_LOGGED, LONDON)
+
+    assert datetime.fromtimestamp(corrected, LONDON).strftime("%Y-%m-%d %H:%M") == (
+        shown.strftime("%Y-%m-%d %H:%M")
+    )
+
+
+def test_one_log_spanning_a_clock_change_converts_each_side_separately():
+    """A week's listening either side of the October change carries two
+    different offsets. Correcting the log by a single figure would put half of
+    it an hour out, which is why the offset is resolved per play."""
+    before, after = 1792888200, 1792899000  # 2026-10-25, 00:30 BST and 03:30 GMT
+
+    assert submit_scrobbles.device_time_to_utc(before, LONDON) == before - 3600
+    assert submit_scrobbles.device_time_to_utc(after, LONDON) == after
+
+
+def test_a_playback_log_is_corrected_before_submission(monkeypatch):
+    monkeypatch.setattr(Path, "is_file", lambda self: True)
+    log = f"{SUMMER_LOGGED}:180000:245000:/Music/Pendulum/Immersion/01.mp3\n"
+
+    played = submit_scrobbles.plays_from_playback_log(
+        log, "/Music", "/mnt/mirror", runner=tags_of(), zone=LONDON
+    ).played
+
+    assert played[0]["timestamp"] == str(SUMMER_TRUE)
+
+
+def test_a_scrobbler_log_claiming_utc_is_not_corrected_twice():
+    """The format's header exists for exactly this. A client that already did
+    the conversion says so, and correcting it again would break it."""
+    log = LOG.replace("#TZ/UNKNOWN", "#TZ/UTC")
+
+    played, _, _ = submit_scrobbles.parse_log(log, LONDON)
+
+    assert [entry["timestamp"] for entry in played] == ["1700000100", "1700000300"]
+
+
+def test_a_scrobbler_log_declaring_unknown_is_corrected():
+    """Rockbox writes UNKNOWN, meaning local wall clock, so the times are ours
+    to fix."""
+    played, _, _ = submit_scrobbles.parse_log(LOG, LONDON)
+
+    # 1700000100 and 1700000300 are November, when London is on UTC anyway;
+    # the point is that the header did not exempt them from being looked at.
+    assert submit_scrobbles.declares_utc(LOG) is False
+    assert [entry["timestamp"] for entry in played] == ["1700000100", "1700000300"]
+
+
+def test_the_system_zone_keeps_its_whole_name(monkeypatch, tmp_path):
+    """/etc/localtime points into the tzdata tree. Taking only the last
+    component gives "London", which no database holds -- and the fallback for
+    an unknown name is a fixed offset, which is wrong for half the year. The
+    region has to survive."""
+    zoneinfo_dir = tmp_path / "share" / "zoneinfo" / "Europe"
+    zoneinfo_dir.mkdir(parents=True)
+    (zoneinfo_dir / "London").write_bytes(b"TZif")
+    monkeypatch.setattr(
+        submit_scrobbles.Path,
+        "read_text",
+        lambda self, **kw: (_ for _ in ()).throw(OSError),
+    )
+    monkeypatch.setattr(
+        submit_scrobbles.Path,
+        "resolve",
+        lambda self: zoneinfo_dir / "London",
+    )
+
+    assert submit_scrobbles.system_zone_name() == "Europe/London"
+
+
+def test_a_named_zone_beats_the_machine_default():
+    """The player may not be in the same place as the laptop."""
+    assert submit_scrobbles.device_zone("Asia/Tokyo") == ZoneInfo("Asia/Tokyo")
+
+
+def test_plays_dated_after_now_are_reported():
+    """A clock never put forward, or the wrong zone, produces plays that have
+    not happened yet. Last.fm cannot tell those from real ones."""
+    now = 1787835600
+    played = [
+        {"timestamp": str(now - 60)},
+        {"timestamp": str(now + 3600)},
+    ]
+
+    ahead = submit_scrobbles.future_plays(played, now=now)
+
+    assert [entry["timestamp"] for entry in ahead] == [str(now + 3600)]
+
+
+def test_a_clock_a_minute_fast_is_not_reported():
+    """Devices drift. Only an offset large enough to be a zone error matters."""
+    now = 1787835600
+
+    assert submit_scrobbles.future_plays([{"timestamp": str(now + 60)}], now=now) == []
+
+
+def test_playback_logs_are_found_including_rotations(tmp_path):
+    """Rockbox rotates the log once it passes half a megabyte."""
+    rockbox = tmp_path / ".rockbox"
+    rockbox.mkdir()
+    for name in ("playback.log", "playback_0001.log", "playback_0002.log"):
+        (rockbox / name).write_text("1700000000:1:1:/Music/a.mp3\n")
+    (rockbox / "empty.log").write_text("")
+
+    found = submit_scrobbles.find_playback_logs(tmp_path)
+
+    assert [path.name for path in found] == [
+        "playback.log", "playback_0001.log", "playback_0002.log"
+    ]
+
+
+def test_without_a_mirror_it_says_what_is_needed(tmp_path, capsys):
+    device = tmp_path / "IPOD"
+    (device / ".rockbox").mkdir(parents=True)
+    (device / ".rockbox" / "playback.log").write_text("1700000000:1:1:/Music/a.mp3\n")
+
+    submit_scrobbles.main([str(device)], transport=fake_transport([]))
+
+    assert "pass --mirror" in capsys.readouterr().err
+
+
+MIRROR = "/mnt/mirror"
+
+
+def stub_ffprobe(monkeypatch, **tags):
+    """Answer for anything under the mirror without invoking ffprobe."""
+    monkeypatch.setattr(submit_scrobbles, "_ffprobe", tags_of(**tags))
+
+
+def only_mirror_files_exist(monkeypatch, resolvable=None):
+    """Make the mirror's files appear to exist, and nothing else.
+
+    Patching is_file wholesale makes the device directory look like a log file,
+    which sends main() down the .scrobbler.log path instead.
+    """
+    real = Path.is_file
+
+    def patched(self):
+        text = str(self)
+        if text.startswith(MIRROR):
+            return resolvable is None or text == resolvable
+        return real(self)
+
+    monkeypatch.setattr(Path, "is_file", patched)
+
+
+def playback_device(tmp_path, log=PLAYBACK_LOG):
+    device = tmp_path / "IPOD"
+    (device / ".rockbox").mkdir(parents=True)
+    (device / ".rockbox" / "playback.log").write_text(log)
+    return device
+
+
+def test_a_failed_submission_keeps_every_log(tmp_path, monkeypatch):
+    """Nothing got through, so nothing may be set aside."""
+    only_mirror_files_exist(monkeypatch)
+    stub_ffprobe(monkeypatch)
+    monkeypatch.setattr(submit_scrobbles, "load_session", lambda: "sk")
+    device = playback_device(tmp_path)
+    transport = fake_transport([{"error": 29, "message": "Rate limit"}])
+
+    code = submit_scrobbles.main(
+        [str(device), "--mirror", MIRROR, "--api-key", "k", "--api-secret", "s"],
+        transport=transport,
+    )
+
+    assert code == 1
+    assert (device / ".rockbox" / "playback.log").is_file()
+    assert not list((device / ".rockbox").glob("*.submitted"))
+
+
+def test_an_unmatched_play_is_written_back_not_lost(tmp_path, monkeypatch, capsys):
+    """A track the mirror does not yet hold is still a play that happened. It
+    is kept so a later run, after the file has been copied, can submit it."""
+    monkeypatch.setattr(submit_scrobbles, "load_session", lambda: "sk")
+    # Only the first track resolves; the rest are absent from the mirror.
+    only_mirror_files_exist(monkeypatch, f"{MIRROR}/Pendulum/Immersion/01.mp3")
+    stub_ffprobe(monkeypatch)
+    log = (
+        "1700000300:180000:245000:/Music/Pendulum/Immersion/01.mp3\n"
+        "1700000400:180000:245000:/Music/Missing/Album/09.mp3\n"
+    )
+    device = playback_device(tmp_path, log)
+    transport = fake_transport([{"scrobbles": {"@attr": {"accepted": 1, "ignored": 0}}}])
+
+    code = submit_scrobbles.main(
+        [str(device), "--mirror", MIRROR, "--api-key", "k", "--api-secret", "s"],
+        transport=transport,
+    )
+
+    assert code == 0
+    rockbox = device / ".rockbox"
+    # The original is preserved untouched...
+    assert list(rockbox.glob("playback.log.*.submitted"))
+    # ...and the unmatched play is back in a live log for the next attempt.
+    written = (rockbox / "playback.log").read_text()
+    assert "Missing/Album/09.mp3" in written
+    assert "Pendulum/Immersion/01.mp3" not in written
+
+
+def test_the_original_is_renamed_rather_than_deleted(tmp_path, monkeypatch):
+    """If Last.fm quietly dropped something, the evidence stays on the device."""
+    only_mirror_files_exist(monkeypatch)
+    stub_ffprobe(monkeypatch)
+    monkeypatch.setattr(submit_scrobbles, "load_session", lambda: "sk")
+    device = playback_device(tmp_path)
+    transport = fake_transport([{"scrobbles": {"@attr": {"accepted": 1, "ignored": 0}}}])
+
+    submit_scrobbles.main(
+        [str(device), "--mirror", MIRROR, "--api-key", "k", "--api-secret", "s"],
+        transport=transport,
+    )
+
+    aside = list((device / ".rockbox").glob("playback.log.*.submitted"))
+    assert len(aside) == 1
+    assert PLAYBACK_LOG.splitlines()[0] in aside[0].read_text()
+
+
+def test_keep_leaves_everything_alone(tmp_path, monkeypatch):
+    only_mirror_files_exist(monkeypatch)
+    stub_ffprobe(monkeypatch)
+    monkeypatch.setattr(submit_scrobbles, "load_session", lambda: "sk")
+    device = playback_device(tmp_path)
+    transport = fake_transport([{"scrobbles": {"@attr": {"accepted": 1, "ignored": 0}}}])
+
+    submit_scrobbles.main(
+        [str(device), "--mirror", MIRROR, "--keep",
+         "--api-key", "k", "--api-secret", "s"],
+        transport=transport,
+    )
+
+    assert (device / ".rockbox" / "playback.log").read_text() == PLAYBACK_LOG
+    assert not list((device / ".rockbox").glob("*.submitted"))
+
+
+def test_a_file_ffprobe_cannot_read_does_not_abandon_the_rest(monkeypatch):
+    """CalledProcessError is not an OSError, so one bad file used to take the
+    whole submission with it."""
+    def broken(path):
+        raise subprocess.CalledProcessError(1, "ffprobe")
+
+    assert submit_scrobbles.read_tags(Path("/mnt/mirror/x.mp3"), runner=broken) == {}

tests/test_sync_to_ipod.py

@@ -0,0 +1,348 @@
+"""The guards on sync-to-ipod.sh, which are the substance of the script.
+
+rsync --delete is being aimed at a whole filesystem, so every refusal here is
+protecting against emptying the wrong directory -- a mistake that does not
+announce itself.
+"""
+
+import shutil
+import subprocess
+from pathlib import Path
+
+import pytest
+
+SCRIPT = Path(__file__).resolve().parent.parent / "tools" / "sync-to-ipod.sh"
+
+# Skipped rather than failed where the tools are absent: this is a host-side
+# script, and a machine without rsync is not a machine that would run it.
+REQUIRED = ("bash", "rsync", "findmnt")
+pytestmark = pytest.mark.skipif(
+    not all(shutil.which(tool) for tool in REQUIRED),
+    reason=f"needs {', '.join(REQUIRED)} on PATH",
+)
+
+
+def run(*arguments):
+    return subprocess.run(
+        ["bash", str(SCRIPT), *arguments], capture_output=True, text=True
+    )
+
+
+@pytest.fixture
+def mirror(tmp_path):
+    source = tmp_path / "mirror"
+    (source / "Album").mkdir(parents=True)
+    (source / "Album" / "track.mp3").write_bytes(b"x")
+    return source
+
+
+def test_the_host_root_is_refused(mirror):
+    """Stripping the trailing slash from "/" leaves an empty string, and an
+    earlier version then reported it as "not a directory" instead."""
+    result = run(str(mirror), "/")
+
+    assert result.returncode == 1
+    assert "refusing to sync onto /" in result.stderr
+
+
+def test_an_empty_mirror_is_refused(tmp_path):
+    """Mirroring nothing onto the device would delete everything on it."""
+    empty = tmp_path / "empty"
+    empty.mkdir()
+    destination = tmp_path / "dest"
+    destination.mkdir()
+
+    result = run(str(empty), str(destination))
+
+    assert result.returncode == 1
+    assert "refusing to mirror nothing" in result.stderr
+
+
+def test_syncing_a_directory_onto_itself_is_refused(mirror):
+    result = run(str(mirror), str(mirror))
+
+    assert result.returncode == 1
+    assert "same directory" in result.stderr
+
+
+def test_a_non_fat_destination_is_refused(mirror, tmp_path):
+    """Which is also how an unmounted device is caught: /media/IPOD/Music then
+    resolves to the host's own root filesystem."""
+    destination = tmp_path / "dest"
+    destination.mkdir()
+
+    result = run(str(mirror), str(destination))
+
+    assert result.returncode == 1
+    assert "not FAT" in result.stderr
+    assert "Is the device mounted?" in result.stderr
+
+
+def test_a_missing_destination_is_refused(mirror, tmp_path):
+    result = run(str(mirror), str(tmp_path / "nowhere"))
+
+    assert result.returncode == 1
+    assert "not a directory" in result.stderr
+
+
+def test_a_subdirectory_of_the_device_is_a_valid_target(mirror, tmp_path):
+    """The better target, in fact: --delete is confined to it."""
+    destination = tmp_path / "dest" / "Music"
+    destination.mkdir(parents=True)
+
+    result = run("-f", "-n", str(mirror), str(destination))
+
+    assert result.returncode == 0, result.stderr
+    assert "dry run, nothing was written" in result.stderr
+
+
+def test_the_device_prefix_is_derived_from_the_destination(mirror, tmp_path):
+    """Derived rather than configured, so it cannot disagree with where the
+    files are actually going -- and the device's path limit applies to it."""
+    destination = tmp_path / "dest" / "Music"
+    destination.mkdir(parents=True)
+
+    result = run("-f", "-n", str(mirror), str(destination))
+
+    assert "the device will see this as /" in result.stderr
+
+
+def test_a_dry_run_writes_nothing(mirror, tmp_path):
+    destination = tmp_path / "dest"
+    destination.mkdir()
+
+    run("-f", "-n", str(mirror), str(destination))
+
+    assert list(destination.iterdir()) == []
+
+
+def test_rockbox_is_never_deleted(mirror, tmp_path):
+    """A sync to the card root would otherwise remove the Rockbox install,
+    since the mirror does not contain it."""
+    destination = tmp_path / "dest"
+    destination.mkdir()
+    (destination / ".rockbox").mkdir()
+    (destination / ".rockbox" / "rockbox.ipod").write_bytes(b"firmware")
+    (destination / ".scrobbler.log").write_bytes(b"#AUDIOSCROBBLER/1.1\n")
+    (destination / "Stale.mp3").write_bytes(b"old")
+
+    result = run("-f", "-S", "-U", str(mirror), str(destination))
+
+    assert result.returncode == 0, result.stderr
+    assert (destination / ".rockbox" / "rockbox.ipod").is_file()
+    assert (destination / ".scrobbler.log").is_file()
+    # But a track whose source has gone is still removed. That is the point.
+    assert not (destination / "Stale.mp3").exists()
+    assert (destination / "Album" / "track.mp3").is_file()
+
+
+def test_help_goes_to_stdout_and_exits_clean():
+    """Asking for help is not an error; getting the arguments wrong is."""
+    result = run("--help")
+
+    assert result.returncode == 0
+    assert result.stdout.startswith("usage:")
+    assert result.stderr == ""
+
+
+def test_short_help_behaves_the_same():
+    result = run("-h")
+
+    assert result.returncode == 0
+    assert result.stdout.startswith("usage:")
+
+
+def test_misuse_goes_to_stderr_and_does_not():
+    result = run("only-one-argument")
+
+    assert result.returncode == 2
+    assert result.stderr.startswith("usage:")
+    assert result.stdout == ""
+
+
+def test_the_help_explains_what_the_destination_should_be():
+    """The question this script actually gets asked."""
+    help_text = run("--help").stdout
+
+    assert "/media/IPOD/Music" in help_text
+    assert "artist folders" in help_text
+    assert ".rockbox" in help_text
+
+
+def test_the_help_says_how_to_reach_and_leave_disk_mode():
+    help_text = run("--help").stdout
+
+    assert "Menu+Select" in help_text
+    assert "holding Play" in help_text
+
+
+def test_counting_is_off_by_default(mirror, tmp_path):
+    """The counting pass walks and compares both trees exactly as the transfer
+    does. On a FAT card of fifty thousand files that costs more than moving the
+    data, so the percentage has to be asked for."""
+    destination = tmp_path / "dest"
+    destination.mkdir()
+
+    result = run("-f", "-S", "-U", str(mirror), str(destination))
+
+    assert result.returncode == 0, result.stderr
+    assert "files to copy" not in result.stderr
+    assert (destination / "Album" / "track.mp3").is_file()
+
+
+def test_the_delta_algorithm_is_disabled(mirror, tmp_path):
+    """It would read every destination file back over USB to checksum it, to
+    avoid resending an MP3 that has changed in its entirety anyway."""
+    script = SCRIPT.read_text()
+
+    assert "--whole-file" in script
+
+
+def test_directory_timestamps_are_not_set(mirror, tmp_path):
+    """One setattr round trip per directory, across six thousand albums, to set
+    timestamps nothing reads."""
+    script = SCRIPT.read_text()
+
+    assert "--omit-dir-times" in script
+
+
+def test_an_interrupt_is_trapped_so_the_filesystem_is_flushed():
+    """Ctrl-C during a transfer would otherwise skip the sync and the unmount,
+    leaving a journal-less FAT filesystem with dirty buffers -- which is the
+    corruption this script exists to prevent.
+
+    Structural rather than timed: reproducing a mid-transfer signal needs a
+    payload large enough to be slow, and a test that depends on losing a race
+    is a test that fails in CI for no reason.
+    """
+    script = SCRIPT.read_text()
+
+    assert "trap interrupted INT TERM" in script
+    assert "exit 130" in script
+
+
+def test_the_flush_and_unmount_happen_on_every_exit_path():
+    script = SCRIPT.read_text()
+
+    # Both the normal path and the interrupt path go through the same function,
+    # so one cannot drift from the other.
+    assert script.count("finish\n") >= 2
+    assert "--partial" not in script, "rsync must delete partial files, not keep them"
+
+
+def test_the_database_step_is_skipped_without_a_tool(mirror, tmp_path, monkeypatch):
+    """Opt-in, like the scrobbler: absent configuration is not an error."""
+    destination = tmp_path / "dest"
+    destination.mkdir()
+    monkeypatch.delenv("MUSIC_MIRROR_DATABASE_TOOL", raising=False)
+
+    result = run("-f", "-S", "-U", str(mirror), str(destination))
+
+    assert result.returncode == 0, result.stderr
+    assert "no database tool configured" in result.stderr
+
+
+def test_a_missing_database_tool_is_refused(mirror, tmp_path, monkeypatch):
+    destination = tmp_path / "dest"
+    destination.mkdir()
+    monkeypatch.setenv("MUSIC_MIRROR_DATABASE_TOOL", str(tmp_path / "nonexistent"))
+
+    result = run("-f", "-S", "-U", str(mirror), str(destination))
+
+    assert result.returncode == 1
+    assert "not executable" in result.stderr
+
+
+def test_the_database_step_can_be_skipped(mirror, tmp_path, monkeypatch):
+    destination = tmp_path / "dest"
+    destination.mkdir()
+    monkeypatch.setenv("MUSIC_MIRROR_DATABASE_TOOL", str(tmp_path / "nonexistent"))
+
+    result = run("-f", "-S", "-U", "-B", str(mirror), str(destination))
+
+    assert result.returncode == 0, result.stderr
+    assert "not executable" not in result.stderr
+
+
+def test_the_scan_reads_from_the_mirror_not_the_device():
+    """The whole point: tags come off the mirror, only the .tcd files go over
+    USB. Reading 49,600 files through an iPod's USB bridge is the slow path."""
+    script = SCRIPT.read_text()
+
+    assert 'ln -s "$mirror"' in script
+    assert 'cd "$scratch"' in script
+
+
+def can_bind_mount():
+    """User namespaces let an unprivileged process bind mount. Not everywhere,
+    notably not inside some containers, so the test that needs it skips."""
+    return (
+        subprocess.run(
+            ["unshare", "-Umr", "true"], capture_output=True, check=False
+        ).returncode
+        == 0
+    )
+
+
+@pytest.mark.skipif(not can_bind_mount(), reason="needs unprivileged user namespaces")
+def test_the_database_lands_at_the_device_root_not_the_music_folder(tmp_path):
+    """The two tools disagree about where the root is. rsync copies artist
+    folders into <device>/Music; the database tool must run one level up, where
+    .rockbox lives, and must record /Music/... paths while reading the bytes
+    from the mirror. device_prefix is what reconciles them.
+    """
+    mirror = tmp_path / "mirror" / "Pendulum" / "Immersion"
+    mirror.mkdir(parents=True)
+    (mirror / "01.mp3").write_bytes(b"not really an mp3")
+    card = tmp_path / "card"
+    (card / ".rockbox").mkdir(parents=True)
+    (card / "Music").mkdir()
+    device = tmp_path / "device"
+    device.mkdir()
+
+    tool = tmp_path / "fake-database"
+    # Records where it was run and what it could see, which is the whole
+    # question; producing a real database needs Rockbox's builder.
+    tool.write_text(
+        "#!/bin/sh\n"
+        "printf '%s\\n' \"$PWD\" > .rockbox/where.txt\n"
+        "ls Music/ > .rockbox/saw.txt\n"
+        "echo db > .rockbox/database_0.tcd\n"
+    )
+    tool.chmod(0o755)
+
+    script = (
+        f"mount --bind {card} {device} && "
+        f"XDG_CACHE_HOME={tmp_path / 'cache'} MUSIC_MIRROR_DATABASE_TOOL={tool} "
+        f"bash {SCRIPT} -f -S -U {tmp_path / 'mirror'} {device / 'Music'}"
+    )
+    result = subprocess.run(
+        ["unshare", "-Umr", "sh", "-c", script], capture_output=True, text=True
+    )
+
+    assert result.returncode == 0, result.stderr
+    # The database lands beside the device root, not inside Music.
+    assert (card / ".rockbox" / "database_0.tcd").is_file()
+    # Only *.tcd is copied across, so the markers stay in the scratch root --
+    # which is itself the point: nothing else is written to the device.
+    scratch = tmp_path / "cache" / "music-mirror" / "database" / ".rockbox"
+    assert not (card / ".rockbox" / "where.txt").exists()
+
+    # It ran in the scratch root, not on the card.
+    where = (scratch / "where.txt").read_text().strip()
+    assert where.endswith("music-mirror/database"), where
+    # ...and could walk into the mirror through a symlink named for the device
+    # prefix, which is how the paths come out as /Music/... while the bytes are
+    # read from somewhere else entirely.
+    assert "Pendulum" in (scratch / "saw.txt").read_text()
+
+
+def test_counting_can_be_asked_for(mirror, tmp_path):
+    """When the destination is cheap to traverse, the percentage is worth it."""
+    destination = tmp_path / "dest"
+    destination.mkdir()
+
+    result = run("-f", "-S", "-U", "-P", str(mirror), str(destination))
+
+    assert result.returncode == 0, result.stderr
+    assert "files to copy" in result.stderr

tools/check_fat32.py

@@ -0,0 +1,121 @@
+#!/usr/bin/env python3
+"""Report paths a FAT32 device will not accept, before copying rather than during.
+
+Run this against the mirror before an rsync to a Rockbox iPod. rsync will
+report the failures too, but scattered through a run of fifty thousand files,
+where they are easy to lose.
+
+Checks the four ways a name fails on FAT32: reserved characters, trailing dots
+or spaces that FAT silently eats, components longer than 255 characters, and
+names that differ only in case -- two files here, one file there, and the
+second silently overwrites the first.
+"""
+
+import argparse
+import os
+import re
+import sys
+import unicodedata
+from collections import defaultdict
+from pathlib import Path
+
+RESERVED = re.compile(r'[<>:"\\|?*\x00-\x1f]')
+COMPONENT_LIMIT = 255
+# Rockbox's MAX_PATH, from firmware/include/fs_defines.h. It bounds the path as
+# the device sees it, so the directory the mirror is copied into comes out of
+# the same budget.
+PATH_LIMIT = 260
+DEVICE_PREFIX = "/Music"
+
+
+def device_prefix_length(prefix):
+    """Return the on-device prefix as it will actually appear, with slashes.
+
+    "/Music" costs seven characters -- the leading slash, the name, and the
+    separator before the mirror's own path -- while an empty prefix costs one.
+    Approximating that loses a character at the root, which is precisely where
+    the longest paths are.
+    """
+    cleaned = prefix.strip("/")
+    return f"/{cleaned}/" if cleaned else "/"
+
+
+def problems_with(relative, budget=PATH_LIMIT):
+    """Return every reason this relative path is unfit for FAT32."""
+    found = []
+    for part in relative.parts:
+        if RESERVED.search(part):
+            found.append(f"reserved character in {part!r}")
+        if part != part.rstrip(". "):
+            found.append(f"trailing dot or space in {part!r}")
+        if len(part) > COMPONENT_LIMIT:
+            found.append(f"component of {len(part)} characters")
+    if len(str(relative)) > budget:
+        found.append(f"path of {len(str(relative))} characters, over a budget of {budget}")
+    return found
+
+
+def walk(root):
+    """Yield every file below a root, as a path relative to it."""
+    for base, _, names in os.walk(root):
+        for name in names:
+            yield Path(os.path.join(base, name)).relative_to(root)
+
+
+def main(argv=None):
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("root", help="directory to check, e.g. the mirror")
+    parser.add_argument("--limit", type=int, default=0, help="show at most this many")
+    parser.add_argument(
+        "--max-path",
+        type=int,
+        default=PATH_LIMIT,
+        help=f"longest path the device will take, from its root (default {PATH_LIMIT},"
+        " Rockbox's MAX_PATH)",
+    )
+    parser.add_argument(
+        "--device-prefix",
+        default=DEVICE_PREFIX,
+        help="directory the mirror is copied into on the device; its length comes out"
+        f" of the budget (default {DEVICE_PREFIX})",
+    )
+    args = parser.parse_args(argv)
+    budget = max(0, args.max_path - len(device_prefix_length(args.device_prefix)))
+
+    root = Path(args.root)
+    if not root.is_dir():
+        print(f"{root} is not a directory", file=sys.stderr)
+        return 2
+
+    faults = []
+    by_case = defaultdict(list)
+    total = 0
+    for relative in walk(root):
+        total += 1
+        # NFC first: the same name written by two systems is otherwise two
+        # different strings, and the collision check would miss it.
+        key = unicodedata.normalize("NFC", str(relative)).casefold()
+        by_case[key].append(relative)
+        for problem in problems_with(relative, budget):
+            faults.append((relative, problem))
+
+    for relative, group in sorted(by_case.items()):
+        if len(group) > 1:
+            names = ", ".join(str(path) for path in sorted(group))
+            faults.append((group[0], f"collides case-insensitively with: {names}"))
+
+    for relative, problem in faults[: args.limit or None]:
+        print(f"{relative}\t{problem}")
+
+    print(f"\n{len(faults)} problems across {total} files", file=sys.stderr)
+    if faults:
+        print(
+            "Run music-mirror with --fat32-safe to have the mirror named acceptably"
+            " in the first place; it shortens over-long paths as well.",
+            file=sys.stderr,
+        )
+    return 1 if faults else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())

tools/rsync_progress.py

@@ -0,0 +1,190 @@
+#!/usr/bin/env python3
+"""Render rsync's per-file output as a single updating status line.
+
+Fed the size and path rsync reports with --out-format='%l %n', one per line.
+The size is what makes an estimate possible: rsync's own rate is not exposed
+per file, but bytes completed over time elapsed is the same arithmetic and
+needs nothing rsync does not already print. Prints one
+line that rewrites itself, showing how far through the transfer is and which
+album is currently going across, rather than either scrolling fifty thousand
+filenames past or -- as rsync does while it builds its file list -- saying
+nothing at all for several minutes.
+
+Falls back to periodic plain lines when stderr is not a terminal, so a log does
+not fill up with carriage returns.
+"""
+
+import argparse
+import collections
+import os
+import shutil
+import sys
+import time
+
+# The estimate is taken over a trailing window rather than the whole run, so it
+# follows a device that slows down instead of averaging the slowdown away.
+RATE_WINDOW_SECONDS = 30.0
+
+# Below this the window is too narrow to divide by: the first few files arrive
+# in microseconds and produce a rate in the gigabytes per second, and an ETA of
+# nothing at all. Better to show neither until the figure means something.
+RATE_MINIMUM_SPAN_SECONDS = 2.0
+
+
+def parse(line):
+    """Return (bytes, path) for one line of rsync output.
+
+    Tolerates a bare path, in case someone runs this against --out-format='%n'.
+    """
+    line = line.rstrip("\n")
+    size, separator, path = line.partition(" ")
+    if separator and size.isdigit():
+        return int(size), path
+    return 0, line
+
+
+def human_bytes(count):
+    size = float(count)
+    for unit in ("B", "KiB", "MiB", "GiB", "TiB"):
+        if size < 1024 or unit == "TiB":
+            return f"{size:.1f} {unit}"
+        size /= 1024
+
+
+def human_duration(seconds):
+    """Return a duration nobody has to do arithmetic on."""
+    seconds = int(seconds)
+    if seconds < 60:
+        return f"{seconds}s"
+    if seconds < 3600:
+        return f"{seconds // 60}m{seconds % 60:02d}s"
+    return f"{seconds // 3600}h{(seconds % 3600) // 60:02d}m"
+
+
+class Rate:
+    """Bytes per second over a trailing window."""
+
+    def __init__(self, window=RATE_WINDOW_SECONDS):
+        self.window = window
+        self.samples = collections.deque()
+
+    def add(self, when, total_bytes):
+        self.samples.append((when, total_bytes))
+        while len(self.samples) > 2 and when - self.samples[0][0] > self.window:
+            self.samples.popleft()
+
+    def per_second(self):
+        if len(self.samples) < 2:
+            return 0.0
+        (first_time, first_bytes), (last_time, last_bytes) = (
+            self.samples[0],
+            self.samples[-1],
+        )
+        elapsed = last_time - first_time
+        if elapsed < RATE_MINIMUM_SPAN_SECONDS:
+            return 0.0
+        return (last_bytes - first_bytes) / elapsed
+
+
+def album_of(path):
+    """Return "Artist / Album" for a mirror-relative path."""
+    parts = [part for part in path.strip("/").split("/") if part]
+    if len(parts) >= 3:
+        return f"{parts[0]} / {parts[1]}"
+    if len(parts) == 2:
+        return parts[0]
+    return ""
+
+
+def fit(text, width):
+    """Trim to the terminal, from the left: the album matters more than the artist."""
+    if width <= 1 or len(text) <= width:
+        return text
+    return "…" + text[-(width - 1) :]
+
+
+def render(done, total, copied, expected, rate, label, width):
+    """Build the status line, giving whatever room is left to the album."""
+    if total > 0:
+        share = min(100, done * 100 // total)
+        head = f"[{share:>3}%] {done:,}/{total:,}"
+    else:
+        head = f"[{done:,} files]"
+
+    if expected > 0:
+        head += f"  {human_bytes(copied)}/{human_bytes(expected)}"
+    if rate > 0:
+        head += f"  {human_bytes(rate)}/s"
+        remaining = expected - copied
+        if remaining > 0:
+            head += f"  ETA {human_duration(remaining / rate)}"
+    head += "  "
+    return head + fit(label, max(0, width - len(head)))
+
+
+def main(argv=None, stream=None, out=None):
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--total", type=int, default=0, help="files expected")
+    parser.add_argument("--bytes", type=int, default=0, help="bytes expected")
+    parser.add_argument("--interval", type=float, default=0.1, help="seconds between redraws")
+    args = parser.parse_args(argv)
+
+    stream = stream or sys.stdin
+    out = out or sys.stderr
+    interactive = out.isatty()
+    width = shutil.get_terminal_size((100, 24)).columns - 1
+
+    done = 0
+    copied = 0
+    rate = Rate()
+    started = time.monotonic()
+    rate.add(started, 0)
+    last_drawn = 0.0
+    label = ""
+
+    for line in stream:
+        size, path = parse(line)
+        # rsync reports directories too, with a trailing slash and an inode
+        # size. Counting them puts the percentage past a hundred and the byte
+        # total well over what will actually be transferred.
+        if not path or path.endswith("/"):
+            continue
+        done += 1
+        copied += size
+        label = album_of(path) or os.path.basename(path)
+
+        now = time.monotonic()
+        rate.add(now, copied)
+        if interactive:
+            if now - last_drawn >= args.interval:
+                out.write(
+                    "\r\033[2K"
+                    + render(done, args.total, copied, args.bytes, rate.per_second(),
+                             label, width)
+                )
+                out.flush()
+                last_drawn = now
+        elif now - last_drawn >= 30:
+            out.write(
+                render(done, args.total, copied, args.bytes, rate.per_second(), label, width)
+                + "\n"
+            )
+            out.flush()
+            last_drawn = now
+
+    elapsed = max(1e-9, time.monotonic() - started)
+    if interactive:
+        out.write("\r\033[2K")
+    summary = render(done, args.total, copied, args.bytes, 0, label, width).rstrip()
+    out.write(f"{summary}\n")
+    if copied:
+        out.write(
+            f"copied {human_bytes(copied)} in {human_duration(elapsed)}"
+            f" at {human_bytes(copied / elapsed)}/s\n"
+        )
+    out.flush()
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())

tools/submit_scrobbles.py

@@ -0,0 +1,643 @@
+#!/usr/bin/env python3
+"""Submit a Rockbox scrobbler log to Last.fm, then set it aside.
+
+Rockbox writes /.scrobbler.log on the device in AUDIOSCROBBLER 1.1 format: one
+tab-separated line per track, rated `L` for listened or `S` for skipped. Only
+the listened ones are submitted; a skip is not a play.
+
+Scrobbling is a write method, so unlike everything else here it needs the API
+secret and a session key, obtained once through the browser. Read-only calls
+elsewhere in these projects need neither.
+"""
+
+import argparse
+import hashlib
+import json
+import os
+import subprocess
+import sys
+import time
+import urllib.error
+import urllib.parse
+import urllib.request
+from dataclasses import dataclass
+from datetime import datetime, timezone
+from pathlib import Path
+from zoneinfo import ZoneInfo, ZoneInfoNotFoundError
+
+API_ROOT = "https://ws.audioscrobbler.com/2.0/"
+
+# Last.fm's documented ceiling for one track.scrobble call.
+BATCH = 50
+
+# Rockbox names the log for whether the target has a real-time clock. Without
+# one every timestamp it writes is zero, which is not a time anything can
+# scrobble.
+LOG_NAMES = (".scrobbler.log", ".scrobbler-timeless.log")
+
+# Rockbox core writes this whenever "play log" is on, with no plugin running:
+#     timestamp:elapsed_ms:length_ms:/Music/Artist/Album/Track.mp3
+# It is rotated once it grows past half a megabyte. Converting it needs tags,
+# which is why the on-device plugin exists -- reading them back off the player
+# is slow. Off the mirror it is free, so the plugin can be skipped entirely.
+PLAYBACK_LOG_NAMES = ("playback.log", "playback_*.log")
+
+# The plugin counts a track as listened at savepct of its length, defaulting to
+# fifty. Same rule here, or the two disagree about what a play is.
+LISTENED_FRACTION = 0.5
+
+# Below this a timestamp is not a wall-clock time. Without a real-time clock
+# Rockbox logs ticks in milliseconds instead, which is not a date.
+EARLIEST_PLAUSIBLE = 1_000_000_000
+
+# How far ahead of now a converted play may sit before it is reported. Some
+# slack absorbs a device clock drifting by a minute or two; an hour out means
+# the zone is wrong or the clock was never put forward.
+FUTURE_TOLERANCE_SECONDS = 300
+
+SESSION_FILE = Path(
+    os.getenv("XDG_CONFIG_HOME", Path.home() / ".config")
+) / "music-mirror" / "lastfm.json"
+
+
+class LastfmError(Exception):
+    """A Last.fm request that failed."""
+
+
+def system_zone_name():
+    """Return this machine's IANA zone name, or "" if nothing states it.
+
+    The name has to come out whole. /etc/localtime is a symlink into the
+    tzdata tree, so the part after `zoneinfo/` is the name -- taking only the
+    last component yields "London", which no database has, and falls back to a
+    fixed offset that would then be wrong for half the year.
+    """
+    try:
+        named = Path("/etc/timezone").read_text(encoding="utf-8").strip()
+        if named:
+            return named
+    except OSError:
+        pass
+    try:
+        parts = Path("/etc/localtime").resolve().parts
+    except OSError:
+        return ""
+    if "zoneinfo" in parts:
+        return "/".join(parts[len(parts) - parts[::-1].index("zoneinfo"):])
+    return ""
+
+
+def device_zone(name=None):
+    """Return the zone the device's clock is keeping.
+
+    Rockbox has no concept of a timezone, so its clock is set to local wall
+    time and the zone has to be supplied from outside. Defaulting to this
+    machine's zone is right whenever the player and the laptop are in the same
+    place, which for a device synced by cable they are.
+    """
+    if name:
+        return ZoneInfo(name)
+    for candidate in (os.getenv("TZ"), system_zone_name()):
+        if not candidate:
+            continue
+        try:
+            return ZoneInfo(candidate)
+        except (ZoneInfoNotFoundError, ValueError):
+            continue
+    # Nothing named the zone, so the offset cannot be resolved per play: this
+    # is today's offset applied to every timestamp, which is wrong either side
+    # of a daylight saving change. Still better than pretending it logged UTC.
+    print(
+        "warning: no IANA timezone found for this machine; using its current"
+        " offset for every play. Pass --device-timezone to fix older plays"
+        " across a daylight saving change.",
+        file=sys.stderr,
+    )
+    return datetime.now().astimezone().tzinfo
+
+
+def device_time_to_utc(stamp, zone):
+    """Return the true UTC epoch of a timestamp Rockbox wrote.
+
+    Rockbox builds its timestamps with `mktime(get_time())`, and its mktime
+    (firmware/libc/mktime.c) is plain calendar arithmetic with no zone applied.
+    Fed the RTC's local fields it yields local-wall-clock-as-if-UTC, so the
+    number is ahead of real UTC by whatever the offset was. Its own scrobbler
+    plugin admits this by writing `#TZ/UNKNOWN`, leaving the correction here.
+
+    Decoding the number back into those fields and reinterpreting them in the
+    device's zone recovers the instant, and does so per play, so a log
+    straddling a daylight-saving change converts each side by its own offset.
+    """
+    fields = datetime.fromtimestamp(stamp, timezone.utc).replace(tzinfo=zone)
+    return int(fields.timestamp())
+
+
+def future_plays(played, now=None):
+    """Return the plays timestamped later than now, which cannot have happened.
+
+    A device clock left on the wrong offset, or never adjusted across a
+    daylight-saving change, shows up here. Last.fm has no way to tell such a
+    scrobble from a real one, so it is worth saying out loud.
+    """
+    now = time.time() if now is None else now
+    return [
+        entry for entry in played
+        if int(entry["timestamp"]) > now + FUTURE_TOLERANCE_SECONDS
+    ]
+
+
+def declares_utc(text):
+    """Whether an AUDIOSCROBBLER log says its timestamps are already UTC.
+
+    The format's header carries `#TZ/UTC` or `#TZ/UNKNOWN`, and its spec is
+    explicit that a device may only claim UTC if it converted. Rockbox writes
+    UNKNOWN, meaning the times are local wall clock and want correcting; a log
+    from anything that claims UTC must be left alone.
+    """
+    for line in text.splitlines():
+        if not line.startswith("#"):
+            break
+        if line.strip().upper().startswith("#TZ/"):
+            return line.strip().upper() == "#TZ/UTC"
+    return False
+
+
+def parse_log(text, zone=None):
+    """Return the listened tracks in an AUDIOSCROBBLER log, oldest first.
+
+    Fields are artist, album, title, track number, length, rating, timestamp
+    and MusicBrainz id. Rockbox converts any tab inside a field to a space
+    before writing, so splitting on tabs is safe.
+
+    Timestamps are corrected from the device's local wall clock to UTC unless
+    the log's own header claims it did that already.
+    """
+    played, skipped, timeless = [], 0, 0
+    convert = zone is not None and not declares_utc(text)
+    for line in text.splitlines():
+        if not line or line.startswith("#"):
+            continue
+        fields = line.split("\t")
+        if len(fields) < 7:
+            continue
+        artist, album, title, number, length, rating, timestamp = fields[:7]
+        mbid = fields[7] if len(fields) > 7 else ""
+        if rating.strip().upper() != "L":
+            skipped += 1
+            continue
+        try:
+            when = int(timestamp)
+        except ValueError:
+            continue
+        if when <= 0:
+            timeless += 1
+            continue
+        if not artist or not title:
+            continue
+        if convert:
+            when = device_time_to_utc(when, zone)
+        played.append(
+            {
+                "artist": artist,
+                "album": album,
+                "track": title,
+                "trackNumber": number if number not in ("", "-1") else "",
+                "duration": length if length.isdigit() and int(length) > 0 else "",
+                "timestamp": str(when),
+                "mbid": mbid,
+            }
+        )
+    played.sort(key=lambda entry: int(entry["timestamp"]))
+    return played, skipped, timeless
+
+
+def parse_playback_log(text):
+    """Return (timestamp, elapsed_ms, length_ms, path) for each logged play."""
+    plays = []
+    for line in text.splitlines():
+        fields = line.strip().split(":", 3)
+        if len(fields) != 4:
+            continue
+        stamp, elapsed, length, path = fields
+        try:
+            plays.append((int(stamp), int(elapsed), int(length), path))
+        except ValueError:
+            continue
+    return plays
+
+
+def device_to_local(path, device_prefix, mirror):
+    """Map a path as the player sees it onto the mirror it was copied from."""
+    prefix = "/" + device_prefix.strip("/")
+    if prefix != "/":
+        if not path.startswith(prefix + "/"):
+            return None
+        path = path[len(prefix) :]
+    return Path(mirror) / path.lstrip("/")
+
+
+def read_tags(path, runner=None):
+    """Return the tags of a local file, via ffprobe. Empty if it cannot be read.
+
+    A file ffprobe chokes on is one play left unidentified, not a reason to
+    abandon the rest -- and CalledProcessError is not an OSError, so catching
+    the obvious things is not enough.
+    """
+    runner = runner or _ffprobe
+    try:
+        payload = json.loads(runner(path))
+    except (OSError, ValueError, subprocess.SubprocessError):
+        return {}
+    return {
+        key.lower(): value
+        for key, value in (payload.get("format", {}).get("tags") or {}).items()
+    }
+
+
+def _ffprobe(path):
+    return subprocess.run(
+        ["ffprobe", "-v", "error", "-show_entries", "format_tags",
+         "-of", "json", str(path)],
+        capture_output=True, text=True, check=True,
+    ).stdout
+
+
+@dataclass
+class Conversion:
+    """What a playback log turned into, and what must not be thrown away.
+
+    `retain` holds the raw lines of plays that were real but could not be
+    submitted -- a track absent from the mirror, usually because the sync had
+    not copied it yet. Those are written back so a later run can try again.
+    Skips and clockless entries are not retained: neither can ever be
+    submitted, and the untouched original is set aside regardless.
+    """
+
+    played: list
+    skipped: int = 0
+    unresolved: int = 0
+    timeless: int = 0
+    retain: list = None
+
+    def __post_init__(self):
+        if self.retain is None:
+            self.retain = []
+
+
+def plays_from_playback_log(text, device_prefix, mirror, runner=None, zone=None):
+    """Return the conversion of a playback log.
+
+    Skips are decided by the same fraction the on-device plugin uses, so the
+    two never disagree about what counted as a play. Timestamps are corrected
+    from the device's wall clock to UTC; the core log has no header to say so,
+    but it is written the same way the plugin's UNKNOWN times are.
+    """
+    result = Conversion(played=[])
+    for line in text.splitlines():
+        parsed = parse_playback_log(line)
+        if not parsed:
+            continue
+        stamp, elapsed, length, device_path = parsed[0]
+        if stamp < EARLIEST_PLAUSIBLE:
+            result.timeless += 1
+            continue
+        if length > 0 and elapsed < length * LISTENED_FRACTION:
+            result.skipped += 1
+            continue
+        local = device_to_local(device_path, device_prefix, mirror)
+        tags = read_tags(local, runner) if local and local.is_file() else {}
+        artist = tags.get("artist") or tags.get("album_artist") or ""
+        title = tags.get("title") or ""
+        if not artist or not title:
+            # A real play of a track this run could not identify. Kept, so a
+            # later run -- after the file has been copied, or the tags fixed --
+            # can submit it rather than the play being lost.
+            result.unresolved += 1
+            result.retain.append(line)
+            continue
+        result.played.append(
+            {
+                "artist": artist,
+                "track": title,
+                "album": tags.get("album", ""),
+                "trackNumber": (tags.get("track") or "").split("/")[0],
+                "duration": str(length // 1000) if length > 0 else "",
+                "timestamp": str(
+                    device_time_to_utc(stamp, zone) if zone is not None else stamp
+                ),
+                "mbid": tags.get("musicbrainz_trackid", ""),
+                "line": line,
+            }
+        )
+    result.played.sort(key=lambda entry: int(entry["timestamp"]))
+    return result
+
+
+def find_playback_logs(device):
+    """Return every playback log on a device, oldest first."""
+    found = []
+    for pattern in PLAYBACK_LOG_NAMES:
+        found.extend(sorted(Path(device).glob(f".rockbox/{pattern}")))
+    return [path for path in found if path.is_file() and path.stat().st_size]
+
+
+def sign(params, secret):
+    """Return Last.fm's method signature for a set of parameters.
+
+    Names are sorted by the ASCII table rather than numerically, which is why
+    `artist[10]` comes before `artist[1]`. Getting that wrong produces an
+    invalid signature and nothing else.
+    """
+    joined = "".join(f"{name}{params[name]}" for name in sorted(params))
+    return hashlib.md5((joined + secret).encode("utf-8")).hexdigest()  # noqa: S324
+
+
+def post(params, transport):
+    """Sign, post, and return the decoded response."""
+    body = urllib.parse.urlencode(params).encode("utf-8")
+    request = urllib.request.Request(API_ROOT, data=body)
+    try:
+        payload = json.loads(transport(request))
+    except urllib.error.HTTPError as error:
+        detail = error.read().decode("utf-8", "replace")[:300]
+        raise LastfmError(f"HTTP {error.code}: {detail}") from error
+    except (urllib.error.URLError, TimeoutError, json.JSONDecodeError) as error:
+        raise LastfmError(str(error)) from error
+    if payload.get("error"):
+        raise LastfmError(f"error {payload['error']}: {payload.get('message', '')}")
+    return payload
+
+
+def call(method, params, key, secret, session, transport):
+    """Make one signed, authenticated call."""
+    full = {**params, "method": method, "api_key": key}
+    if session:
+        full["sk"] = session
+    full["api_sig"] = sign(full, secret)
+    full["format"] = "json"
+    return post(full, transport)
+
+
+def authorise(key, secret, transport, opener=print):
+    """Walk the one-time browser authorisation and return a session key."""
+    token = call("auth.getToken", {}, key, secret, None, transport)["token"]
+    url = f"https://www.last.fm/api/auth/?api_key={key}&token={token}"
+    opener(f"Open this, approve the application, then press Enter:\n\n  {url}\n")
+    input()
+    session = call("auth.getSession", {"token": token}, key, secret, None, transport)
+    return session["session"]["key"]
+
+
+def load_session():
+    if SESSION_FILE.is_file():
+        return json.loads(SESSION_FILE.read_text(encoding="utf-8")).get("session")
+    return None
+
+
+def save_session(session):
+    SESSION_FILE.parent.mkdir(parents=True, exist_ok=True)
+    SESSION_FILE.write_text(json.dumps({"session": session}), encoding="utf-8")
+    SESSION_FILE.chmod(0o600)
+
+
+def batch_params(entries):
+    """Return the indexed parameters for one track.scrobble call."""
+    params = {}
+    for index, entry in enumerate(entries):
+        for name in ("artist", "track", "timestamp", "album", "trackNumber", "duration", "mbid"):
+            if entry.get(name):
+                params[f"{name}[{index}]"] = entry[name]
+    return params
+
+
+def submit(entries, key, secret, session, transport, delay=1.0, on_sent=None):
+    """Submit every entry. Returns how many the service accepted.
+
+    Batches are counted as they succeed rather than at the end, so a failure
+    partway through leaves an honest number and the caller can keep the rest of
+    the log instead of losing it.
+    """
+    accepted = 0
+    for start in range(0, len(entries), BATCH):
+        chunk = entries[start : start + BATCH]
+        payload = call(
+            "track.scrobble", batch_params(chunk), key, secret, session, transport
+        )
+        block = payload.get("scrobbles", {})
+        summary = block.get("@attr", block)
+        accepted += int(summary.get("accepted", len(chunk)))
+        if on_sent is not None:
+            on_sent(chunk)
+        ignored = int(summary.get("ignored", 0))
+        if ignored:
+            print(f"  {ignored} of {len(chunk)} ignored by Last.fm", file=sys.stderr)
+        if start + BATCH < len(entries):
+            time.sleep(delay)
+    return accepted
+
+
+def http_post(request, timeout=30):
+    with urllib.request.urlopen(request, timeout=timeout) as response:  # noqa: S310
+        return response.read().decode("utf-8")
+
+
+def find_log(device):
+    """Return the scrobbler log on a mounted device, or None."""
+    for name in LOG_NAMES:
+        candidate = Path(device) / name
+        if candidate.is_file() and candidate.stat().st_size:
+            return candidate
+    return None
+
+
+def main(argv=None, transport=http_post):
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("device", help="the mounted device, or a scrobbler log file")
+    parser.add_argument(
+        "--mirror",
+        help="the mirror the device was copied from. Given this, Rockbox's own"
+        " playback.log is converted here rather than needing the on-device"
+        " plugin run first",
+    )
+    parser.add_argument(
+        "--device-prefix",
+        default="/Music",
+        help="where the music sits on the device, stripped when mapping a logged"
+        " path back onto the mirror",
+    )
+    parser.add_argument(
+        "--device-timezone",
+        default=os.getenv("ROCKBOX_TIMEZONE"),
+        help="the zone the player's clock is set to, as an IANA name such as"
+        " Europe/London. Rockbox keeps local wall time and cannot record an"
+        " offset, so its timestamps need this to become the UTC Last.fm wants."
+        " Defaults to this machine's zone",
+    )
+    parser.add_argument("--api-key", default=os.getenv("LASTFM_API_KEY"))
+    parser.add_argument("--api-secret", default=os.getenv("LASTFM_API_SECRET"))
+    parser.add_argument("--dry-run", action="store_true", help="parse and report only")
+    parser.add_argument(
+        "--keep", action="store_true", help="do not set the log aside afterwards"
+    )
+    args = parser.parse_args(argv)
+
+    try:
+        zone = device_zone(args.device_timezone)
+    except (ZoneInfoNotFoundError, ValueError) as error:
+        print(f"unknown timezone {args.device_timezone!r}: {error}", file=sys.stderr)
+        return 2
+
+    target = Path(args.device)
+    log = target if target.is_file() else find_log(target)
+    logs = []
+
+    conversion = None
+    if log is not None:
+        played, skipped, timeless = parse_log(
+            log.read_text(encoding="utf-8", errors="replace"), zone
+        )
+        unresolved = 0
+        logs = [log]
+        print(f"{log}: {len(played)} listened, {skipped} skipped", file=sys.stderr)
+    elif args.mirror:
+        # No plugin has been run, but the core log is there. Tags come off the
+        # mirror, which is the only reason the plugin was needed at all.
+        logs = find_playback_logs(target)
+        if not logs:
+            print("no scrobbler log to submit", file=sys.stderr)
+            return 0
+        text = "\n".join(
+            path.read_text(encoding="utf-8", errors="replace") for path in logs
+        )
+        conversion = plays_from_playback_log(
+            text, args.device_prefix, args.mirror, zone=zone
+        )
+        played = conversion.played
+        skipped, unresolved, timeless = (
+            conversion.skipped,
+            conversion.unresolved,
+            conversion.timeless,
+        )
+        print(
+            f"{len(logs)} playback log(s): {len(played)} listened, {skipped} skipped",
+            file=sys.stderr,
+        )
+        if unresolved:
+            print(
+                f"  {unresolved} could not be matched to a file in the mirror."
+                " Those plays are kept for a later run rather than discarded.",
+                file=sys.stderr,
+            )
+    else:
+        print(
+            "no scrobbler log to submit. Rockbox's own playback.log can be used"
+            " instead -- pass --mirror so tags can be read from it.",
+            file=sys.stderr,
+        )
+        return 0
+
+    if timeless:
+        print(
+            f"  {timeless} entries have no timestamp, so this target has no clock."
+            " They cannot be scrobbled without inventing when they happened.",
+            file=sys.stderr,
+        )
+    if not played:
+        return 0
+
+    ahead = future_plays(played)
+    if ahead:
+        newest = int(ahead[-1]["timestamp"]) - int(time.time())
+        print(
+            f"  {len(ahead)} plays are timestamped up to {newest // 60} minutes in"
+            f" the future, converting from {zone}. Either the player's clock is"
+            " wrong or that is not the zone it is set to.",
+            file=sys.stderr,
+        )
+
+    if args.dry_run:
+        for entry in played[:20]:
+            when = datetime.fromtimestamp(int(entry["timestamp"]), zone)
+            print(
+                f"{entry['timestamp']}\t{when:%Y-%m-%d %H:%M %Z}"
+                f"\t{entry['artist']}\t{entry['track']}"
+            )
+        return 0
+
+    if not args.api_key or not args.api_secret:
+        print(
+            "scrobbling is a write method: it needs LASTFM_API_KEY and"
+            " LASTFM_API_SECRET, not just the read-only key",
+            file=sys.stderr,
+        )
+        return 2
+
+    session = load_session()
+    if not session:
+        session = authorise(args.api_key, args.api_secret, transport)
+        save_session(session)
+
+    # Recorded as each batch is accepted, so a failure partway through knows
+    # exactly what got through and what did not.
+    sent = []
+    try:
+        accepted = submit(
+            played, args.api_key, args.api_secret, session, transport,
+            on_sent=sent.extend,
+        )
+    except LastfmError as error:
+        print(f"submission failed after {len(sent)} scrobbles: {error}", file=sys.stderr)
+        keep_history(logs, played, sent, conversion, args.keep)
+        return 1
+
+    print(f"{accepted} scrobbles accepted", file=sys.stderr)
+    keep_history(logs, played, sent, conversion, args.keep)
+    return 0
+
+
+def keep_history(logs, played, sent, conversion, keep):
+    """Set the logs aside, writing back anything still owed a submission.
+
+    Two separate obligations. The original is preserved untouched, renamed
+    rather than deleted, so a play is never lost to a mistake here. And any
+    play that was not submitted -- unmatched, or in a batch that failed -- is
+    written back into a live log, so the next run tries it again instead of it
+    quietly vanishing with the rest.
+    """
+    if keep or not logs:
+        if keep:
+            print("logs left in place", file=sys.stderr)
+        return
+
+    submitted = {id(entry) for entry in sent}
+    # A .scrobbler.log was converted by the on-device plugin and carries no
+    # per-line record, so there is nothing to write back for it -- only the
+    # rename below, which loses nothing.
+    pending = list(conversion.retain) if conversion is not None else []
+    pending += [
+        entry["line"] for entry in played
+        if "line" in entry and id(entry) not in submitted
+    ]
+
+    if not sent:
+        print("nothing was accepted; logs left untouched", file=sys.stderr)
+        return
+
+    stamp = played[-1]["timestamp"] if played else "0"
+    for path in logs:
+        path.rename(path.with_name(f"{path.name}.{stamp}.submitted"))
+
+    if pending:
+        live = logs[0].with_name("playback.log")
+        live.write_text("\n".join(pending) + "\n", encoding="utf-8")
+        print(
+            f"{len(pending)} plays not submitted were written back to"
+            f" {live.name} for the next run",
+            file=sys.stderr,
+        )
+    print(f"{len(logs)} log(s) set aside as .submitted", file=sys.stderr)
+
+
+if __name__ == "__main__":
+    sys.exit(main())

tools/sync-to-ipod.sh

@@ -0,0 +1,323 @@
+#!/usr/bin/env bash
+# Copy the mirror onto a Rockbox device, then unmount it cleanly.
+#
+# The device is FAT32 with no journal, reached through the Apple firmware's
+# disk mode because Rockbox's own mass storage is unreliable on an iFlash. An
+# interrupted write is corruption that needs fsck.vfat from another machine, so
+# this syncs and unmounts rather than leaving that to whoever pulls the cable.
+#
+# rsync --delete is pointed at a whole filesystem, so the checks below are the
+# point of the script rather than decoration.
+
+set -euo pipefail
+
+usage() {
+    # Help goes to stdout and exits clean; misuse goes to stderr and does not.
+    local stream=2 code=2
+    if [ "${1:-}" = "help" ]; then
+        stream=1
+        code=0
+    fi
+    cat >&"$stream" <<'USAGE'
+usage: sync-to-ipod.sh [options] <mirror> <destination>
+
+  -n  dry run; show what would change and touch nothing
+  -P  count what needs copying first, so progress can show a percentage and
+      an estimate. Costs a second full traversal of both trees, which on a
+      FAT card of fifty thousand files is slower than the transfer itself
+  -f  copy even if the FAT32 check finds unacceptable paths
+  -S  skip submitting the Rockbox scrobbler log to Last.fm
+  -B  skip rebuilding the Rockbox database
+  -U  leave the destination mounted afterwards
+
+Rebuilding the database needs MUSIC_MIRROR_DATABASE_TOOL pointing at Rockbox's
+host-side builder (tools/database, built with ./tools/configure --type=d). It
+is skipped with a note when unset. The scan reads tags from the mirror rather
+than from the device, so it costs seconds rather than the hours an on-device
+commit takes -- and on a large library the on-device commit may not finish at
+all.
+
+Submitting scrobbles needs LASTFM_API_KEY and LASTFM_API_SECRET; it is skipped
+with a note when they are unset. Scrobbling is a write method and needs the
+secret, unlike the read-only calls elsewhere in these projects.
+
+Rockbox has no notion of a timezone: its clock holds local wall time and its
+logs record that, not UTC. Set ROCKBOX_TIMEZONE to the zone the player's clock
+is keeping (an IANA name, such as Europe/London) if it differs from this
+machine's, which is otherwise assumed. Getting it wrong shifts every scrobble
+by the difference.
+
+The mirror is the directory holding the artist folders. The destination is
+where those folders should end up on the device -- not the card root, unless
+that is genuinely where you want them:
+
+  sync-to-ipod.sh /mnt/tank/media/music-mp3 /media/IPOD/Music
+
+A subdirectory is the better target: --delete is confined to it, and the path
+budget is derived from it, since the device's 260-character limit counts the
+whole path as the device sees it. /.rockbox and the scrobbler logs are never
+deleted wherever you point this.
+
+The destination must be on a mounted FAT filesystem. That check is also what
+catches an unmounted device: /media/IPOD/Music then resolves to the host's own
+root filesystem, and this refuses to empty that.
+
+Progress is one line that rewrites itself, showing the album currently going
+across, how far through the transfer is, the rate, and an estimate of what is
+left. Working the totals out first means a second pass over the tree, which is
+the price of figures that mean something; rsync's own percentage is computed
+against a file list it is still building.
+
+Reach the device with the Apple firmware's disk mode: Menu+Select to reboot,
+then immediately Select+Play. Power off afterwards by holding Play.
+USAGE
+    exit "$code"
+}
+
+dry_run=false
+counting=false
+force=false
+unmount=true
+scrobble=true
+database=true
+for argument in "$@"; do
+    [ "$argument" = "--help" ] && usage help
+done
+while getopts ":nPfSBUh" option; do
+    case "$option" in
+        n) dry_run=true ;;
+        P) counting=true ;;
+        f) force=true ;;
+        S) scrobble=false ;;
+        B) database=false ;;
+        U) unmount=false ;;
+        h) usage help ;;
+        *) usage ;;
+    esac
+done
+shift $((OPTIND - 1))
+[ $# -eq 2 ] || usage
+
+# Trailing slashes are stripped for tidiness, but stripping one from "/" leaves
+# an empty string, and the guard below would then never see the root it is
+# there to refuse.
+mirror=${1%/}
+mirror=${mirror:-/}
+destination=${2%/}
+destination=${destination:-/}
+here=$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)
+
+die() {
+    # Every argument, not just the first: the second half of a message is
+    # usually the half that says what to do about it.
+    printf 'sync-to-ipod: %s\n' "$*" >&2
+    exit 1
+}
+
+[ -d "$mirror" ] || die "mirror $mirror is not a directory"
+[ -n "$(ls -A "$mirror")" ] || die "mirror $mirror is empty; refusing to mirror nothing"
+[ -d "$destination" ] || die "destination $destination is not a directory"
+
+# --delete makes every one of these load-bearing. Emptying the wrong directory
+# is not a mistake that announces itself.
+case "$destination" in
+    "" | "/" | "$HOME") die "refusing to sync onto $destination" ;;
+esac
+[ "$(readlink -f "$mirror")" != "$(readlink -f "$destination")" ] ||
+    die "mirror and destination are the same directory"
+
+# The filesystem the destination sits on, which is the check that matters: a
+# subdirectory of the card is a perfectly good target, and is the better one,
+# because --delete is then confined to it. Being FAT is also what proves the
+# card is mounted at all -- an unmounted /media/IPOD/Music resolves to the
+# host's own root filesystem, and this refuses to empty that.
+filesystem=$(findmnt -no FSTYPE --target "$destination")
+mounted_on=$(findmnt -no TARGET --target "$destination")
+case "$filesystem" in
+    vfat | exfat) ;;
+    *)
+        $force ||
+            die "$destination is on a $filesystem filesystem, not FAT." \
+                "Is the device mounted? Pass -f if this is deliberate."
+        printf 'sync-to-ipod: destination is %s, not FAT\n' "$filesystem" >&2
+        ;;
+esac
+
+# What the device will call this directory, which is what its path limit
+# applies to. Derived rather than configured, so it cannot disagree with where
+# the files are actually going.
+device_prefix=${destination#"$mounted_on"}
+device_prefix="/${device_prefix#/}"
+printf 'sync-to-ipod: the device will see this as %s\n' "$device_prefix" >&2
+
+if $force; then
+    printf 'sync-to-ipod: skipping the FAT32 check\n' >&2
+elif ! python3 "$here/check_fat32.py" --device-prefix "$device_prefix" "$mirror"; then
+    die "the mirror holds paths FAT32 will not take; run music-mirror with --fat32-safe"
+fi
+
+# Before the copy, not after: the plays already happened, and if the transfer
+# then fails there is no reason to have lost them too.
+if $scrobble; then
+    if [ -z "${LASTFM_API_KEY:-}" ] || [ -z "${LASTFM_API_SECRET:-}" ]; then
+        printf 'sync-to-ipod: no Last.fm credentials, skipping the scrobbler log\n' >&2
+    else
+        scrobble_options=()
+        $dry_run && scrobble_options+=(--dry-run)
+        # Rockbox keeps local wall time with no notion of a zone, so its
+        # timestamps are not the UTC Last.fm expects. Naming the zone the
+        # player's clock is set to lets them be corrected.
+        [ -n "${ROCKBOX_TIMEZONE:-}" ] &&
+            scrobble_options+=(--device-timezone "$ROCKBOX_TIMEZONE")
+        # --mirror lets it convert Rockbox's own playback.log, so the on-device
+        # scrobbler plugin never has to be run. The device root, not the music
+        # directory: the logs live in .rockbox.
+        python3 "$here/submit_scrobbles.py" "${scrobble_options[@]}" \
+            --mirror "$mirror" --device-prefix "$device_prefix" "$mounted_on" ||
+            die "submitting scrobbles failed; nothing has been copied"
+    fi
+fi
+
+# -rt rather than -a: owners, groups and permissions mean nothing on FAT, and
+# asking for them produces a screenful of errors and a non-zero exit.
+# --modify-window=2 because FAT stores mtimes to two-second resolution, without
+# which every file looks changed and the whole library is copied every time.
+# --whole-file is already the default when both ends are local paths, and an
+# SMB or FAT mount counts as one, but stating it documents that the delta
+# algorithm is deliberately not wanted: it would read every destination file
+# back over USB to compute a checksum, to save sending an MP3 that has changed
+# entirely anyway.
+#
+# --omit-dir-times drops a setattr round trip per directory. Across six
+# thousand album folders on a FAT card that is six thousand operations to set
+# timestamps nothing reads.
+options=(--recursive --times --delete --modify-window=2 --whole-file --omit-dir-times)
+# --delete removes tracks whose source has gone, which is the point. It would
+# also remove everything on the device that the mirror does not contain -- and
+# if the destination is the card root that means /.rockbox, the Rockbox install
+# itself. Excluded paths are not deleted unless --delete-excluded is given,
+# which it never is here.
+for owned in "/.rockbox" "/.scrobbler.log" "/.scrobbler.log.*" "/.playlist_control" \
+    "/System Volume Information" "/.Spotlight-V100" "/.Trashes" "/.fseventsd"; do
+    options+=(--exclude "$owned")
+done
+
+printf 'sync-to-ipod: %s -> %s\n' "$mirror" "$destination" >&2
+
+if $dry_run; then
+    rsync "${options[@]}" --dry-run --verbose "$mirror/" "$destination/"
+    printf 'sync-to-ipod: dry run, nothing was written\n' >&2
+    exit 0
+fi
+
+# rsync says nothing at all while it builds its file list, which on fifty
+# thousand files over USB is minutes of apparent hang. Counting first costs a
+# second pass over the tree but means the transfer can show a real percentage
+# rather than a number that grows as rsync discovers more work.
+# Counting is opt-in because it is not cheap. It walks and compares both trees
+# in full, exactly as the transfer does, and on a FAT card holding fifty
+# thousand files that traversal costs more than moving the data. Without it the
+# progress line still shows the running count, the rate and the album in
+# flight; only the percentage and the estimate are lost, and those were the
+# least useful part of it.
+total=0
+total_bytes=0
+if $counting; then
+    printf 'sync-to-ipod: working out what needs copying...\n' >&2
+    # %l is the file's size, which is what makes an estimate possible.
+    # Directories are dropped: rsync reports those too, with an inode size that
+    # would inflate the total by several megabytes of nothing.
+    counted=$(rsync "${options[@]}" --dry-run --out-format='%l %n' "$mirror/" "$destination/" |
+        awk '!/\/$/ { files++; bytes += $1 } END { print files + 0, bytes + 0 }')
+    total=${counted% *}
+    total_bytes=${counted#* }
+    printf 'sync-to-ipod: %s files to copy\n' "$total" >&2
+fi
+
+# Flushing and unmounting is the whole reason this is a script, so it has to
+# happen on the way out whichever way that is. Ctrl-C during a transfer would
+# otherwise leave a FAT filesystem with dirty buffers and no journal, which is
+# the corruption this exists to avoid.
+finish() {
+    sync
+    if $unmount; then
+        device=$(findmnt -no SOURCE --target "$destination" 2>/dev/null || true)
+        if [ -n "$device" ]; then
+            printf 'sync-to-ipod: unmounting %s\n' "$device" >&2
+            if command -v udisksctl >/dev/null 2>&1; then
+                udisksctl unmount -b "$device" || umount -- "$destination" || true
+            else
+                umount -- "$destination" || true
+            fi
+            printf 'sync-to-ipod: safe to disconnect\n' >&2
+        fi
+    else
+        printf 'sync-to-ipod: still mounted; unmount before disconnecting\n' >&2
+    fi
+}
+
+interrupted() {
+    trap - INT TERM
+    printf '\nsync-to-ipod: interrupted -- rsync leaves no partial files, but the\n' >&2
+    printf 'sync-to-ipod: filesystem still needs flushing before you pull anything\n' >&2
+    finish
+    exit 130
+}
+
+trap interrupted INT TERM
+
+rsync "${options[@]}" --out-format='%l %n' "$mirror/" "$destination/" |
+    python3 "$here/rsync_progress.py" --total "$total" --bytes "$total_bytes"
+status=${PIPESTATUS[0]}
+[ "$status" -eq 0 ] || die "rsync exited $status"
+
+# Rockbox reads its database from .tcd files in .rockbox. Building them here
+# rather than on the device is not just faster: the on-device commit sorts the
+# whole index in whatever memory it can scrape together, and on a large library
+# it runs for hours or dies outright.
+#
+# The scan reads tags through a scratch root -- a real .rockbox beside a symlink
+# standing in for where the music lands on the device -- so the paths recorded
+# match what Rockbox will look up, while the bytes are read from the mirror
+# instead of over USB. The scratch is kept between runs because the builder is
+# incremental: a second pass over unchanged files does no work at all.
+rebuild_database() {
+    local tool=${MUSIC_MIRROR_DATABASE_TOOL:-}
+    if [ -z "$tool" ]; then
+        printf 'sync-to-ipod: no database tool configured, skipping the database\n' >&2
+        return 0
+    fi
+    [ -x "$tool" ] || die "$tool is not executable"
+
+    local device_rockbox="$mounted_on/.rockbox"
+    if [ ! -d "$device_rockbox" ]; then
+        printf 'sync-to-ipod: no .rockbox on the device, skipping the database\n' >&2
+        return 0
+    fi
+
+    local scratch="${XDG_CACHE_HOME:-$HOME/.cache}/music-mirror/database"
+    mkdir -p "$scratch/.rockbox"
+
+    # Rebuild the symlink layout each time; the mirror path or the device
+    # prefix may have changed since the last run.
+    find "$scratch" -maxdepth 1 -type l -delete
+    if [ "$device_prefix" = "/" ]; then
+        ln -s "$mirror"/* "$scratch/" 2>/dev/null || true
+    else
+        local under=${device_prefix#/}
+        rm -rf "${scratch:?}/${under%%/*}"
+        mkdir -p "$scratch/$(dirname "$under")"
+        ln -s "$mirror" "$scratch/$under"
+    fi
+
+    printf 'sync-to-ipod: building the database from the mirror...\n' >&2
+    ( cd "$scratch" && "$tool" ) >/dev/null || die "the database build failed"
+
+    cp -- "$scratch"/.rockbox/*.tcd "$device_rockbox/" ||
+        die "could not copy the database onto the device"
+    printf 'sync-to-ipod: database copied to %s\n' "$device_rockbox" >&2
+}
+
+$database && rebuild_database
+
+finish