Subject: bug#30368: ‘select’ returns non-empty sets upon EINTR or EAGAIN

From: [email-masked] (Ludovic Courtès)
Date: Tue, 06 Feb 2018 11:39:30 +0100

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’.

diff --git a/libguile/filesys.c b/libguile/filesys.c
index e39dc4a0d..05dd2bd16 100644
--- a/libguile/filesys.c
+++ b/libguile/filesys.c
@@ -1,5 +1,5 @@
 /* Copyright (C) 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2004, 2006,
- *   2009, 2010, 2011, 2012, 2013, 2014, 2016, 2017 Free Software Foundation, Inc.
+ *   2009, 2010, 2011, 2012, 2013, 2014, 2016, 2017, 2018 Free Software Foundation, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Lesser General Public License
@@ -900,10 +900,20 @@ SCM_DEFINE (scm_select, "select", 3, 2, 0,
     int rv = scm_std_select (max_fd + 1,
                              &read_set, &write_set, &except_set,
                              time_ptr);
-    /* Let EINTR / EAGAIN cause a return to the user and let them loop
-       to run any asyncs that might be pending.  */
-    if (rv < 0 && errno != EINTR && errno != EAGAIN)
-      SCM_SYSERROR;
+    if (rv < 0)
+      {
+        /* Let EINTR / EAGAIN cause a return to the user and let them
+           loop to run any asyncs that might be pending.  */
+        if (errno != EINTR && errno != EAGAIN)
+          SCM_SYSERROR;
+        else
+          {
+            /* Return empty sets.  */
+            FD_ZERO (&read_set);
+            FD_ZERO (&write_set);
+            FD_ZERO (&except_set);
+          }
+      }
   }
 
   return scm_list_3 (retrieve_select_type (&read_set, read_ports_ready, reads),