./sql/rpl_slave.cc

bool show_slave_status_send_data(THD *thd, Master_info *mi,
                                 char* io_gtid_set_buffer,
                                 char* sql_gtid_set_buffer)
......
if ((mi->get_master_log_pos() == mi->rli->get_group_master_log_pos()) 
        (!strcmp(mi->get_master_log_name(), mi->rli->get_group_master_log_name())))
    {
      if (mi->slave_running == MYSQL_SLAVE_RUN_CONNECT)
        protocol->store(0LL);
      else
        protocol->store_null();
    }
    else
    {
      long time_diff= ((long)(time(0) - mi->rli->last_master_timestamp)
                       - mi->clock_diff_with_master);

      protocol->store((longlong)(mi->rli->last_master_timestamp ?
                                   max(0L, time_diff) : 0));
    }
......

  /*
     The pseudo code to compute Seconds_Behind_Master:
     if (SQL thread is running)
     {
       if (SQL thread processed all the available relay log)
       {
         if (IO thread is running)
            print 0;
         else
            print NULL;
       }
        else
          compute Seconds_Behind_Master;
      }
      else
       print NULL;
  */

./sql/rpl_slave.cc

......
  if (ev)
  {
    enum enum_slave_apply_event_and_update_pos_retval exec_res;

    ptr_ev= ev;
    /*
      Even if we don't execute this event, we keep the master timestamp,
      so that seconds behind master shows correct delta (there are events
      that are not replayed, so we keep falling behind).

      If it is an artificial event, or a relay log event (IO thread generated
      event) or ev->when is set to 0, or a FD from master, or a heartbeat
      event with server_id '0' then  we don't update the last_master_timestamp.

      In case of parallel execution last_master_timestamp is only updated when
      a job is taken out of GAQ. Thus when last_master_timestamp is 0 (which
      indicates that GAQ is empty, all slave workers are waiting for events from
      the Coordinator), we need to initialize it with a timestamp from the first
      event to be executed in parallel.
    */
    if ((!rli->is_parallel_exec() || rli->last_master_timestamp == 0) 
         !(ev->is_artificial_event() || ev->is_relay_log_event() ||
          (ev->common_header->when.tv_sec == 0) ||
          ev->get_type_code() == binary_log::FORMAT_DESCRIPTION_EVENT ||
          ev->server_id == 0))
    {
      rli->last_master_timestamp= ev->common_header->when.tv_sec +
                                  (time_t) ev->exec_time;
      DBUG_ASSERT(rli->last_master_timestamp >= 0);
    }
......

./sql/rpl\_slave.cc

......
  /*
    We need to ensure that this is never called at this point when
    cnt is zero. This value means that the checkpoint information
    will be completely reset.
  */

  /*
    Update the rli->last_master_timestamp for reporting correct Seconds_behind_master.

    If GAQ is empty, set it to zero.
    Else, update it with the timestamp of the first job of the Slave_job_queue
    which was assigned in the Log_event::get_slave_worker() function.
  */
  ts= rli->gaq->empty()
    ? 0
    : reinterpret_castSlave_job_group*>(rli->gaq->head_queue())->ts;
  rli->reset_notified_checkpoint(cnt, ts, need_data_lock, true);
  /* end-of "Coordinator::"commit_positions" */

......

  /*
    Currently, the checkpoint routine is being called by the SQL Thread.
    For that reason, this function is called call from appropriate points
    in the SQL Thread's execution path and the elapsed time is calculated
    here to check if it is time to execute it.
  */
  set_timespec_nsec(curr_clock, 0);
  ulonglong diff= diff_timespec(curr_clock, rli->last_clock);
  if (!force  diff  period)
  {
    /*
      We do not need to execute the checkpoint now because
      the time elapsed is not enough.
    */
    DBUG_RETURN(FALSE);
  }