bug#41531: 27.0.91; Better handle asynchronous eldoc backends

[João Távora]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> Here's a modified approach that doesn't use a global var and should make it
>> easier to transition to using futures.
> I think at this stage, we should either start using futures or we should
> forget about ever using them.

<DRAMATIC MUSIC PLAYS>

But really: now we have deadlock too?  I just want to solve this
problem: please let's commit something, and move on to the next bug.  It
can be functions that return functions that receive functions with the
airplane.

João