bug#25122: 24.5; function describe-variable hangs on large variables

[npostavs]

Thierry Volpiatto <thierry.volpiatto@gmail.com> writes:

> npostavs@users.sourceforge.net writes:
>
>> and suspend it after printing X lines.
>
> Instead just print something like "Computing foo value ..." and let finish
> the thread, letting user reading and moving cursor in docstring while it
> finishes, once done save-excursion and send message "Computing foo value
> done".

Threads aren't truly parallel.  The user can't do anything while the
thread is running.

>
>> Then hitting RET on "..."  would just print another X lines.
>
> I think like Drew that this would be annoying.

I wonder if we could just hook this into scrolling somehow?  So the
lines would only be printed when you scroll to look at them.

>
> That said, what's the reason of choosing the slower approach to compute
> value (in a thread or not) instead of using the approach described in
> the advice I sent here which takes 1s to compute load-history instead of
> 3mn ? (I use this advice since one year now without any problems).

As mentioned in https://debbugs.gnu.org/cgi/bugreport.cgi?bug=21717#8,
it breaks circularity.  Try describing this variable:

    (defvar circular-list
      (let ((l (number-sequence 1 100)))
        (setcdr (last l) l)
        l)
      "A circular list that has problems with (mapc 'pp val).")

We could probably achieve something similar without breaking circular
printing by not calling indent-sexp on the full list, but 1s is longer
than "instant" anyway (which is about 50ms or less) which is why I'm
exploring other options.