mvcc-可见性源代码

class MVCC;
/** Read view lists the trx ids of those transactions for which a consistent
read should not see the modifications to the database. */
...
class ReadView {
    ...
    private:
        // Prevent copying
        ids_t(const ids_t&);
        ids_t& operator=(const ids_t&);
    private:
        /** Memory for the array */
        value_type* m_ptr;
        /** Number of active elements in the array */
        ulint       m_size;
        /** Size of m_ptr in elements */
        ulint       m_reserved;
        friend class ReadView;
    };
public:
    ReadView();
    ~ReadView();
    /** Check whether transaction id is valid.
    @param[in]  id      transaction id to check
    @param[in]  name        table name */
    static void check_trx_id_sanity(
        trx_id_t        id,
        const table_name_t& name);
// 判断一个修改是否可见
    /** Check whether the changes by id are visible.
    @param[in]  id  transaction id to check against the view
    @param[in]  name    table name
    @return whether the view sees the modifications of id. */
    bool changes_visible(
        trx_id_t        id,
        const table_name_t& name) const
        MY_ATTRIBUTE((warn_unused_result))
    {
        ut_ad(id > 0);
        if (id < m_up_limit_id || id == m_creator_trx_id) {
            return(true);
        }
        check_trx_id_sanity(id, name);
        if (id >= m_low_limit_id) {
            return(false);
        } else if (m_ids.empty()) {
            return(true);
        }
        const ids_t::value_type*    p = m_ids.data();
        return(!std::binary_search(p, p + m_ids.size(), id));
    }

private:
    // Disable copying
    ReadView(const ReadView&);
    ReadView& operator=(const ReadView&);
private:
   // 活动事务中的id的最大
    /** The read should not see any transaction with trx id >= this
    value. In other words, this is the "high water mark". */
    trx_id_t    m_low_limit_id;
    // 活动事务id的最小值
    /** The read should see all trx ids which are strictly
    smaller (<) than this value.  In other words, this is the
    low water mark". */
    //
    trx_id_t    m_up_limit_id;
    /** trx id of creating transaction, set to TRX_ID_MAX for free
    views. */
    trx_id_t    m_creator_trx_id;
    /** Set of RW transactions that was active when this snapshot
    was taken */
    ids_t       m_ids;
    /** The view does not need to see the undo logs for transactions
    whose transaction number is strictly smaller (<) than this value:
    they can be removed in purge if not needed by other views */
    trx_id_t    m_low_limit_no;
    /** AC-NL-RO transaction view that has been "closed". */
    bool        m_closed;
    typedef UT_LIST_NODE_T(ReadView) node_t;
    /** List of read views in trx_sys */
    byte        pad1[64 - sizeof(node_t)];
    node_t      m_view_list;
};