kern-6525.txt

	From: 	  [email protected]
	Subject: 	NAB - Re: kern/76525: select() hangs on EOF from named pipe (FIFO)
	Date: 	April 9, 2005 1:49:10 PM PDT
	To: 	  [email protected], [email protected]

We believe that the behavior of select() on fifo
in the absence of a writer is the desired behavior of FreeBSD5.3
and is therefore not a bug.

We propose the following fix to the bug author's source code fifo.c

--- fifo_orig.c Sun Mar 13 08:54:14 2005
+++ fifo.c      Sun Mar 13 09:45:30 2005
@@ -4,26 +4,21 @@
 #include 
 #include 
 #include 
+#include 

 const char      FIFO[] = "/tmp/fifo";

 void
 look(int fd)
 {
-        fd_set  readfds, excepfds;
+        struct pollfd poll_list[1];

-        FD_ZERO(&readfds);
-        FD_ZERO(&excepfds);
-        FD_SET(fd, &readfds);
-        FD_SET(fd, &excepfds);
-        if (select(fd+1, &readfds, NULL, &excepfds, NULL) < 0) {
-                perror("select");
+        poll_list[0].fd = fd;
+        poll_list[0].events = POLLIN|POLLINIGNEOF;
+        if (poll(poll_list , 1, -1) < 0) {
+                perror("poll");
                 exit(1);
         }
-        if (FD_ISSET(fd, &excepfds))
-                puts("Exception on FIFO");
-        if (FD_ISSET(fd, &readfds))
-                puts("FIFO available for read");
 }

In FreeBSD4.x, select() on fifo always returned when there was no
writer.  In FreeBSD5.x, select() on fifo was changed to block
in the absence of a writer.

Historically, some users have desired select() to block waiting for a
writer to appear.  They want fifo to behave as a data source without
worrying about connections coming and going, but there are also
users who want select() to return in the absence of a writer.

This disagreement has played out in CVS, specifically
/usr/src/sys/fs/fifofs/fifo_vnops.c has experienced
a series of changes, alternating between blocking
and non-blocking behavior for select().

The following CVS revisions are relevant:
In 1.40, select() on fifo with no writer did not block.
In 1.56 it was restored to blocking.  In revision 1.62,
the fixes of 1.40 were restored but select() on fifo was
made to block waiting for a writer to appear.

Also, a new event bitmask, POLLINIGNEOF for poll() has been
implemented.  If the users want non-blocking behavior
when there is no writer, they can call poll() instead,
setting the event bitmask POLLINIGNEOF.  To illustrate,
we have implemented this change in the bug author's test program.

We also believe that the current 5.x behavior is consistent
with the POSIX.1 standard as well as the overall intent
of select(), but we are aware that this interpretation
is not universally shared.

If the current behavior of select() on fifo,
is not desirable, the following patch can be applied to
filo_poll() in fifo_vnops.c, which reverses the select()
behavior for fifo by reverting it to non-blocking.Users
need to set the event bitmask POLLINIGNEOF to get blocking
behavior.

While offering the change, we would like to reiterate
that we believe the change is inappropriate (inconsistent
with POSIX.1) and should not be captured into CVS.

--- fifo_vnops_orig.c   Wed Mar 16 16:13:24 2005
+++ fifo_vnops.c        Wed Mar 16 16:25:12 2005
@@ -531,7 +531,7 @@
                 * set POLLINIGNEOF to get non-blocking behavior.
                 */
                if (events & (POLLIN | POLLRDNORM) &&
-                   !(events & POLLINIGNEOF)) {
+                   (events & POLLINIGNEOF)) {
                        events &= ~(POLLIN | POLLRDNORM);
                        events |= POLLINIGNEOF;
                }
@@ -544,7 +544,7 @@

                /* Reverse the above conversion. */
                if ( (revents & POLLINIGNEOF) &&
-                   !(ap->a_events & POLLINIGNEOF)) {
+                   (ap->a_events & POLLINIGNEOF)) {
                        revents |= (ap->a_events & (POLLIN | POLLRDNORM));
                        revents &= ~POLLINIGNEOF;
                }



Shikha Shrivastava, engineer
Dorr H. Clark, advisor
COEN 284 - Operating Systems Case Study
Santa Clara University,
Santa Clara CA.