I have made a separate struct XA_data to store online_alter_cache:

I was thinking on whether to change the declaration in handlerton - this would require may changes. Besides that:

int
innobase_commit_by_xid(
/*===================*/
        handlerton*    hton,
        XID*          xid)   /*!< in: X/Open XA transaction identification */

I also decided not to leave the comment for the commit/rollback callbacks, since there's no use for this extension to the other engines apart from online alter, in the form it is done now.