Better handling of album comments (#1013)

* Change album comment behaviour

* Don't check first item

* Fix previously imported album comments.

* Remove song comments if album comment is present

db/migration/20210418232815_fix_album_comments.go

@@ -0,0 +1,66 @@
+package migrations
+
+import (
+	"database/sql"
+	"strings"
+
+	"github.com/navidrome/navidrome/log"
+	"github.com/pressly/goose"
+)
+
+func init() {
+	goose.AddMigration(upFixAlbumComments, downFixAlbumComments)
+}
+
+func upFixAlbumComments(tx *sql.Tx) error {
+	const zwsp = string('\u200b')
+	rows, err := tx.Query(`
+	SELECT album.id, group_concat(media_file.comment, '` + zwsp + `') FROM album, media_file WHERE media_file.album_id = album.id GROUP BY album.id;
+	   `)
+	if err != nil {
+		return err
+	}
+	defer rows.Close()
+
+	stmt, err := tx.Prepare("UPDATE album SET comment = ? WHERE id = ?")
+	if err != nil {
+		return err
+	}
+	var id string
+	var comments sql.NullString
+
+	for rows.Next() {
+		err = rows.Scan(&id, &comments)
+		if err != nil {
+			return err
+		}
+		if !comments.Valid {
+			continue
+		}
+		comment := getComment(comments.String, zwsp)
+		_, err = stmt.Exec(comment, id)
+
+		if err != nil {
+			log.Error("Error setting album's comments", "albumId", id, err)
+		}
+	}
+	return rows.Err()
+}
+
+func downFixAlbumComments(tx *sql.Tx) error {
+	return nil
+}
+
+func getComment(comments string, separator string) string {
+	cs := strings.Split(comments, separator)
+	if len(cs) == 0 {
+		return ""
+	}
+	first := cs[0]
+	for _, c := range cs[1:] {
+		if first != c {
+			return ""
+		}
+	}
+	return first
+}