Phases 5b and 5c of the metadata writer.
5b — single-file audio (MP3/FLAC/M4A/M4B): the container's tags are
rewritten with ffmpeg -c copy (no re-encode; -map 0 keeps cover art,
-map_chapters 0 keeps chapters). The key set is exactly what the
scanner's applyAudioTagFallbacks reads back — title+album, subtitle,
artist/album_artist, composer (narrators, Audible's convention),
grouping + iTunes series-part, date, genre — so a write + rescan
round-trips. Output is ffprobe-VALIDATED (duration within 1s/0.5% of
the source) before the atomic rename; a lossy container rewrite never
replaces a good book. Fields the format can't carry per our own read
chain are deliberately not written, and fields_written reports only
what really round-trips (subtitle rides MP3/FLAC — this image's mp4
muxer drops subtitle keys, discovered empirically and pinned by test).
5c — CBZ: ComicInfo.xml injected (or replaced) with the same atomic zip
rewrite as the EPUB path, extended with an append-if-missing mode
since a comic may not carry one yet. Field map mirrors the scanner's
comic block: Title/Series/Number/Count/Volume, Writer (authors),
Summary, Publisher, Year/Month/Day, LanguageISO, Web, Genre+Tags;
fractional series numbers are omitted rather than truncated to a lie.
CBR/CB7 stay on the sidecar path (RAR/7z can't be rewritten in place).
Integration tests generate media at test runtime with ffmpeg (m4b) and
in-memory zips (cbz) — no binary fixtures — pinning the tag round-trip
through ffprobe, post-write rescan titles, the ComicInfo injection and
the validation-gate behavior.
Phase 5a of the metadata-writer feature: the book editor gains a
"Write to file" action (admin) that persists the item's CURRENT
database metadata into the artifact on disk, so the library stays
portable to Calibre and other apps — the thing Save alone can't do.
Dispatch (services/metadata_writer.go): EPUB/KEPUB gets its embedded
OPF spliced in place (fresh dc:* block generated inside the ORIGINAL
metadata open tag; manifest/spine/guide untouched; every other zip
entry copied byte-identical in original order so mimetype stays first
and STORED; original dc:identifiers preserved — the package's
unique-identifier must keep resolving). Everything else takes a
Calibre-style sidecar: metadata.opf when the book is ALONE in its
directory (Calibre's own convention — round-trips through Calibre
too), <basename>.opf when siblings share the folder, so one book's
sidecar can never poison dir-mates' scans (the scanner now reads both,
basename first).
Safety contract: temp file + atomic rename — the original is never
truncated on failure; the generated OPF is unit-tested to ROUND-TRIP
through the scanner's own parser (write side and read side can never
disagree); metadata_overrides are deliberately kept (file and DB agree
after the write; overrides still guard against stale sidecars). The
endpoint re-extracts inline (RescanMediaItem) and reports rescan
failure separately — a failed rescan doesn't undo the write. SHA-256
changes are inherent and documented for users (Kobo/KOReader re-key).
UI: the metadata editor modal carries the file path as a data
attribute; the button confirms before mutating, labels the target
(embedded vs sidecar) and toasts the outcome. Integration tests pin
the 403, missing-file error, EPUB embed + post-write rescan title, the
lone-book metadata.opf case, and the shared-dir <basename>.opf case
with no dir-scoped litter.
John O'Keefe