How to flush job output in vim8?

Question

Trying to do something with job output in exit_cb or close_cb, but it's all buffered, how to flush it. e.g.:

function! s:exit_cb(job, status)
  echom getbufline(s:bufnr, '$')
endfunction

function! s:close_cb(channel)
  echom getbufline(s:bufnr, '$')
endfunction

let s:bufnr = term_start('seq 1000',
      \ {'exit_cb': function("s:exit_cb"), 'close_cb':function("s:close_cb") })

None of them echo 1000, how to flush it ?

One work around is to call :h :sleep, but i don't think that's the right way.

update

As B Layer pointed out, result of close_cb depends on terminal window height. e.g., if it's height is 30, it prints 971, as 971 = 1000 - 30 + 1.

I reread :h close_cb , it says:

Vim will invoke callbacks that handle data before invoking close_cb, thus when this function is called no more data will be passed to the callbacks.

I guess the callback vim invoked to handle data haven't finished by the time close_cb is called. Tried to call term_wait() in close_cb, but it cause hang.

update

A similar issue on github.