bug#41531: 27.0.91; Better handle asynchronous eldoc backends

[Stefan Monnier <monnier@iro.umontreal.ca>]

> It's not complete, is it?

Don't know.  I have the impression that it's complete enough to give you
the same "power" as the callback argument.

I.e. instead of (funcall BACKEND CALLBACK)
you (eldoc-future-set-callback (funcall BACKEND) CALLBACK) and instead
of (funcall CALLBACK VALUE) you (eldoc-future-set-value FUT VALUE).

> And how to I use it to solve the
> eldoc-documentation-compose problem?

AFAIK this is orthogonal to the technique we use for the backend to run
eldoc's callback code.

> I suppose it's possible, anything
> is.  But do we really want to hand-roll futures in eldoc.el when we got
> this nice https://github.com/skeeto/emacs-aio/commits/master that we
> could be looking into?

I'm not familiar with that package, so I can't judge.  It might be an
even better option, indeed.

> Also the latest patch I attach solves the eldoc-documentation-compose
> problem decently (should actually be less LOC than previous versions).
> I suggest we go with this tried-and-tested well-understood solution and
> then adjust as more sophisticated solutions come along and we evaluate
> their merits.

The use of futures has been discussed forever in the context of
several packages.  That's why at this stage, I think either we decide to
drop the idea or to start using it.
I'm OK with either option.


        Stefan