Hi Guilers,

This is a (buggy) patch to guile-fibers that adds
a ‘wait-until-port-readable-operation’ and ‘wait-until-port-writable’
that can be used to wait until a port (backed by a fd) becomes readable
or writable respectively.

Now I seem to have hit a problem: the first few tests (more to be written
later) pass just fine, but the following test never completes:

  ;; XXX why does this hang?  Shouldn't there
  ;; be a time-out?
  (assert-run-fibers-returns (#f)
			     (writable/timeout A))

(writable/timeout is a combination of wait-until-port-writable and
sleep-operation, that also retries a few times to allow for spurious
wakeups.)

Oddly, the same test but with readable/timeout completes just fine.

Any thoughts?  (Warning: I don't know what I'm doing; I'm not
familiar with relevant literature on operations, I'm just trying
to complete wingo's proposal).

Wild guess: maybe schedule-task-when-fd-writable prevents the fiber
from being resumed by other causes?

Greetings,
Maxime.