next

    pub fn next(self: *Self) !?Attribute {
        // Pax extended header consists of one or more attributes, each constructed as follows:
        // "%d %s=%s\n", <length>, <keyword>, <value>
        while (self.size > 0) {
            const length_buf = try self.reader.takeSentinel(' ');
            const length = try std.fmt.parseInt(usize, length_buf, 10); // record length in bytes

            const keyword = try self.reader.takeSentinel('=');
            if (hasNull(keyword)) return error.PaxNullInKeyword;

            // calculate value_len
            const value_start = length_buf.len + keyword.len + 2; // 2 separators
            if (length < value_start + 1 or self.size < length) return error.UnexpectedEndOfStream;
            const value_len = length - value_start - 1; // \n separator at end
            self.size -= length;

            const kind: PaxAttributeKind = if (eql(keyword, "path"))
                .path
            else if (eql(keyword, "linkpath"))
                .linkpath
            else if (eql(keyword, "size"))
                .size
            else {
                try self.reader.discardAll(value_len);
                try validateAttributeEnding(self.reader);
                continue;
            };
            if (kind == .size and value_len > pax_max_size_attr_len) {
                return error.PaxSizeAttrOverflow;
            }
            return .{
                .kind = kind,
                .len = value_len,
                .reader = self.reader,
            };
        }

        return null;
    }