Hello,

As of Guile 2.2.3, the Scheme-level ‘select’ can return non-empty sets
when the C-level ‘select’ returns EINTR or EAGAIN.  The program below
illustrates this:

--8<---------------cut here---------------start------------->8---
(use-modules (ice-9 match))

(sigaction SIGINT (lambda args
                    (pk 'signal! args)))

(let ((target (getpid)))
  (match (primitive-fork)
    ((? zero?)
     (sleep 4)
     (kill target SIGINT)
     (primitive-exit 0))
    (_ #t)))

(match (select (list (current-input-port)) '() '())
  (((port) () ())
   (pk 'reading-from port)
   (read-char port))
  (lst
   (pk 'done lst)))
--8<---------------cut here---------------end--------------->8---

On 2.2.3, it prints:

--8<---------------cut here---------------start------------->8---
$ guile select.scm


;;; (signal! (2))
;;; (reading-from #<input: file /dev/pts/9>)
--8<---------------cut here---------------end--------------->8---

From there on it’s stuck in a read(0, …) call.

The attached patch fixes it by clearing the returned FD sets on
EINTR/EAGAIN.

(Besides it seems that select(2) never returns EAGAIN.)

I’m not sure how to write a test for this; the one above is
timing-sensitive, which wouldn’t be great.

Thoughts?

Ludo’.