feat(server/m7-379): extract MusicBrainz IDs from audio tags during scan

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/library/mbids.go

@@ -0,0 +1,59 @@
+package library
+
+// MusicBrainz tag keys vary by audio format. dhowden/tag's Raw() map exposes
+// the format-specific entries as-is — so we look up multiple candidates and
+// take the first non-empty match.
+//
+// References:
+//   - ID3v2 (MP3):       TXXX frames keyed by description.
+//   - Vorbis (FLAC/OGG): all-caps comment names.
+//   - MP4 (M4A):         iTunes freeform atoms ("----:com.apple.iTunes:...").
+
+var albumMBIDKeys = []string{
+	"TXXX:MusicBrainz Album Id",
+	"MUSICBRAINZ_ALBUMID",
+	"----:com.apple.iTunes:MusicBrainz Album Id",
+}
+
+var artistMBIDKeys = []string{
+	"TXXX:MusicBrainz Artist Id",
+	"MUSICBRAINZ_ARTISTID",
+	"----:com.apple.iTunes:MusicBrainz Artist Id",
+}
+
+// extractMBIDs reads MusicBrainz album and artist IDs from a tag.Metadata
+// Raw() map. Returns ("", "") when the keys are absent. Multi-value keys
+// (some formats list multiple artist IDs for collaborations) take only the
+// first value — Minstrel uses the primary release artist for MBCAA lookup.
+func extractMBIDs(raw map[string]interface{}) (albumMBID, artistMBID string) {
+	albumMBID = firstStringValue(raw, albumMBIDKeys)
+	artistMBID = firstStringValue(raw, artistMBIDKeys)
+	return
+}
+
+func firstStringValue(raw map[string]interface{}, keys []string) string {
+	for _, k := range keys {
+		v, ok := raw[k]
+		if !ok || v == nil {
+			continue
+		}
+		switch x := v.(type) {
+		case string:
+			if x != "" {
+				return x
+			}
+		case []string:
+			if len(x) > 0 && x[0] != "" {
+				return x[0]
+			}
+		case []interface{}:
+			if len(x) == 0 {
+				continue
+			}
+			if s, ok := x[0].(string); ok && s != "" {
+				return s
+			}
+		}
+	}
+	return ""
+}

internal/library/mbids_test.go

@@ -0,0 +1,69 @@
+package library
+
+import "testing"
+
+func TestExtractMBIDs_ID3v2(t *testing.T) {
+	raw := map[string]interface{}{
+		"TXXX:MusicBrainz Album Id":  "abc-123",
+		"TXXX:MusicBrainz Artist Id": "def-456",
+	}
+	a, ar := extractMBIDs(raw)
+	if a != "abc-123" || ar != "def-456" {
+		t.Errorf("got (%q, %q), want (\"abc-123\", \"def-456\")", a, ar)
+	}
+}
+
+func TestExtractMBIDs_Vorbis(t *testing.T) {
+	raw := map[string]interface{}{
+		"MUSICBRAINZ_ALBUMID":  "abc",
+		"MUSICBRAINZ_ARTISTID": "def",
+	}
+	a, ar := extractMBIDs(raw)
+	if a != "abc" || ar != "def" {
+		t.Errorf("got (%q, %q)", a, ar)
+	}
+}
+
+func TestExtractMBIDs_MP4(t *testing.T) {
+	raw := map[string]interface{}{
+		"----:com.apple.iTunes:MusicBrainz Album Id":  "abc",
+		"----:com.apple.iTunes:MusicBrainz Artist Id": "def",
+	}
+	a, ar := extractMBIDs(raw)
+	if a != "abc" || ar != "def" {
+		t.Errorf("got (%q, %q)", a, ar)
+	}
+}
+
+func TestExtractMBIDs_MissingKeysReturnEmpty(t *testing.T) {
+	a, ar := extractMBIDs(map[string]interface{}{})
+	if a != "" || ar != "" {
+		t.Errorf("got (%q, %q), want empty", a, ar)
+	}
+}
+
+func TestExtractMBIDs_MultiValueArtistTakesFirst(t *testing.T) {
+	// Some FLAC tags list collaborators as multi-value MUSICBRAINZ_ARTISTID.
+	// Minstrel uses the primary release artist; first wins.
+	raw := map[string]interface{}{
+		"MUSICBRAINZ_ARTISTID": []string{"primary-mbid", "collab-mbid"},
+	}
+	_, ar := extractMBIDs(raw)
+	if ar != "primary-mbid" {
+		t.Errorf("got %q, want %q", ar, "primary-mbid")
+	}
+}
+
+func TestExtractMBIDs_NilEntryIgnored(t *testing.T) {
+	raw := map[string]interface{}{
+		"TXXX:MusicBrainz Album Id":  nil,
+		"TXXX:MusicBrainz Artist Id": "real-id",
+	}
+	a, ar := extractMBIDs(raw)
+	if a != "" {
+		t.Errorf("nil album entry should be ignored, got %q", a)
+	}
+	if ar != "real-id" {
+		t.Errorf("artist = %q", ar)
+	}
+}

internal/library/scanner.go

@@ -131,6 +131,7 @@ func (s *Scanner) scanFile(ctx context.Context, q *dbq.Queries, path string, sta
 	if err != nil {
 		return fmt.Errorf("tag read: %w", err)
 	}
+	albumMBID, artistMBID := extractMBIDs(meta.Raw())
 
 	artistName := meta.Artist()
 	if artistName == "" {
@@ -145,11 +146,11 @@ func (s *Scanner) scanFile(ctx context.Context, q *dbq.Queries, path string, sta
 		trackTitle = strings.TrimSuffix(filepath.Base(path), filepath.Ext(path))
 	}
 
-	artist, err := s.resolveArtist(ctx, q, artistName)
+	artist, err := s.resolveArtist(ctx, q, artistName, artistMBID)
 	if err != nil {
 		return fmt.Errorf("artist: %w", err)
 	}
-	album, err := s.resolveAlbum(ctx, q, artist.ID, albumTitle, meta.Year())
+	album, err := s.resolveAlbum(ctx, q, artist.ID, albumTitle, meta.Year(), albumMBID)
 	if err != nil {
 		return fmt.Errorf("album: %w", err)
 	}
@@ -197,7 +198,7 @@ func (s *Scanner) scanFile(ctx context.Context, q *dbq.Queries, path string, sta
 	return nil
 }
 
-func (s *Scanner) resolveArtist(ctx context.Context, q *dbq.Queries, name string) (dbq.Artist, error) {
+func (s *Scanner) resolveArtist(ctx context.Context, q *dbq.Queries, name, mbid string) (dbq.Artist, error) {
 	existing, err := q.GetArtistByName(ctx, name)
 	if err == nil {
 		return existing, nil
@@ -205,13 +206,18 @@ func (s *Scanner) resolveArtist(ctx context.Context, q *dbq.Queries, name string
 	if !errors.Is(err, pgx.ErrNoRows) {
 		return dbq.Artist{}, err
 	}
-	return q.UpsertArtist(ctx, dbq.UpsertArtistParams{
+	params := dbq.UpsertArtistParams{
 		Name:     name,
 		SortName: sortKey(name),
-	})
+	}
+	if mbid != "" {
+		m := mbid
+		params.Mbid = &m
+	}
+	return q.UpsertArtist(ctx, params)
 }
 
-func (s *Scanner) resolveAlbum(ctx context.Context, q *dbq.Queries, artistID pgtype.UUID, title string, year int) (dbq.Album, error) {
+func (s *Scanner) resolveAlbum(ctx context.Context, q *dbq.Queries, artistID pgtype.UUID, title string, year int, mbid string) (dbq.Album, error) {
 	existing, err := q.GetAlbumByArtistAndTitle(ctx, dbq.GetAlbumByArtistAndTitleParams{ArtistID: artistID, Title: title})
 	if err == nil {
 		return existing, nil
@@ -233,6 +239,10 @@ func (s *Scanner) resolveAlbum(ctx context.Context, q *dbq.Queries, artistID pgt
 		s.logger.Warn("library scan: dropping invalid release year",
 			"year", year, "album", title)
 	}
+	if mbid != "" {
+		m := mbid
+		params.Mbid = &m
+	}
 	return q.UpsertAlbum(ctx, params)
 }