undo_no_t undo_no; 
/** Undo number */
 typedef ib_id_t undo_no_t;/** The generic InnoDB system object identifier data type */
typedef ib_uint64_t ib_id_t;

事务对象
struct trx_t{
...
undo_no_t undo_no;
...
}
 next undo log record number to assign; since the undo log is private for a transaction, this is a simple ascending sequence with no gaps; thus it represents the number of modified/inserted rows in a transaction
下一个要分配的undo log record number。因为undo log是一个事务私有，所以这是一个递增的序列，中间没有间隔。表示一个事务中modified/inserted行的number。