bug#45117: 28.0.50; process-send-string mysteriously exiting non-locally when called from timer

[João Távora]

On Thu, Dec 10, 2020 at 5:51 PM Stefan Monnier <monnier@iro.umontreal.ca> wrote:
>
> > Aha! I found the culprit.  It is eldoc.el.  It seems to be a
> > longstanding policy to call Eldoc backends with `with-no-input`.  This
> > is obviously badly problematic for aynchronous backends.
>
> Maybe "obviously" so from a pragmatic point of view, but in
> a theoretical sense, I don't see why:

I'm not sure.  Every such async system eventually boils down
to a point that sends something to the wire (step A) and a process
filter (step C) that runs later on and finds some suitable "continuation",
(a callback).  To find that suitable continuation it has to be added
to the continuation registry (step B). I'd say step A and B have to
be atomic, whatever the order.  If you interrupt between
A and B you get either an unexpected reply. If you interrupt between
B and A you get a stale leftover continuation.  Both are inconsistent
state, in my opinion.

There are workarounds for all of this, but I think this while-no-input
isn't conceptually sound there, for this reason.

Actually, thinking more about it. I don't think it's sound to have a
while-no-input at all under library control.  A programmer using
that library should be given a predictable evaluation model. At
any rate, this is a regression from 26.3, where things didn't work
like this.

João