minstrel/internal/coverart/embedded.go

package coverart

import (
	"errors"
	"fmt"
	"os"

	"github.com/dhowden/tag"
)

// ErrNoEmbeddedArt indicates the audio file has no embedded picture.
// Distinct from "tag read failed" so the enricher can quietly fall
// through to the provider chain without logging at warn/error level.
var ErrNoEmbeddedArt = errors.New("coverart: no embedded album art")

// ExtractEmbeddedAlbumArt reads the embedded picture from an audio
// file's tag block: ID3v2 APIC frames (MP3), FLAC
// METADATA_BLOCK_PICTURE blocks, MP4 covr atoms (M4A/M4B/AAC), and OGG
// METADATA_BLOCK_PICTURE base64 in vorbis comments. Whichever format
// dhowden/tag recognises.
//
// Returns ErrNoEmbeddedArt when the tag block parses but contains no
// usable picture; returns a wrapped error for I/O or tag-parse
// failures so callers can distinguish "no art" from "couldn't read".
//
// Picard-tagged libraries typically carry the canonical
// art in this block already — extracting it beats every external API
// guess for accuracy and avoids a network round-trip.
func ExtractEmbeddedAlbumArt(path string) ([]byte, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, fmt.Errorf("open: %w", err)
	}
	defer func() { _ = f.Close() }()

	meta, err := tag.ReadFrom(f)
	if err != nil {
		return nil, fmt.Errorf("tag read: %w", err)
	}
	pic := meta.Picture()
	if pic == nil || len(pic.Data) == 0 {
		return nil, ErrNoEmbeddedArt
	}
	return pic.Data, nil
}