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The documentation of ha_rollback_trans() suggests that
handlerton::rollback() should never return error:

      if ((err= ht->rollback(ht, thd, all)))
      {
        // cannot happen

Further, any error returned from ha_rollback_trans() is discarded in
trans_rollback_stmt(), and the code path in mysql_admin_table() after
calling trans_rollback_stmt() calls my_eof() which asserts clear
dastatus. So we clear the dastatus in spider_rollback() on error.