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Date: Wed, 03 Aug 2011 17:09:25 -0600
From: Eric Blake <eblake AT redhat DOT com>
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To: cygwin <cygwin AT cygwin DOT com>
Subject: odd socketpair() failure
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A while ago, I tested pipe() for EMFILE failures [1].  Well, I repeated 
those tests for socketpair() [2], and cygwin is once again the odd man out.

[1] http://cygwin.com/ml/cygwin/2011-06/msg00328.html
[2] http://austingroupbugs.net/view.php?id=483

$ cat foo.c
#define _POSIX_C_SOURCE 200811L
#define __EXTENSIONS__ 1
#include <sys/types.h>
#include <sys/time.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/socket.h>
int
main (int argc, char **argv)
{
   int last;
   int fd[2] = {-2,-3};
   int err;
   /* Get to an EMFILE condition.  */
   while (1) {
     int fd = open("/dev/null", O_RDONLY);
     if (fd < 0) {
       printf ("after fd %d, open failed with errno %d %s\n",
	      last, errno, strerror(errno));
       break;
     }
     last = fd;
   }
   /* Probe behavior */
   err = socketpair(AF_UNIX, SOCK_STREAM, 0, fd);
   if (err)
     printf ("try 1, socketpair returned %d errno %d %s, fds %d %d\n",
	    err, errno, strerror(errno), fd[0], fd[1]);
   else
     printf ("try 1, socketpair succeeded, fds %d %d\n", fd[0], fd[1]);
   if (close(last))
     return 1;
   err = socketpair(AF_UNIX, SOCK_STREAM, 0, fd);
   if (err)
     printf ("try 2, socketpair returned %d errno %d %s, fds %d %d\n",
	    err, errno, strerror(errno), fd[0], fd[1]);
   else
     printf ("try 2, socketpair succeeded, fds %d %d\n", fd[0], fd[1]);
   if (close(0))
     return 1;
   err = socketpair(AF_UNIX, SOCK_STREAM, 0, fd);
   if (err)
     printf ("try 3, socketpair returned %d errno %d %s, fds %d %d\n",
	    err, errno, strerror(errno), fd[0], fd[1]);
   else
     printf ("try 3, socketpair succeeded, fds %d %d\n", fd[0], fd[1]);
   return 0;
}
$ ./foo
after fd 3199, open failed with errno 24 Too many open files
try 1, socketpair returned -1 errno 24 Too many open files, fds -2 -3
try 2, socketpair returned -1 errno 24 Too many open files, fds -2 -3
try 3, socketpair returned -1 errno 24 Too many open files, fds -2 -3

But on Linux, try 3 succeeds.  Something in cygwin is not quite right on 
try 3 - the program explicitly freed two fd slots (0 and 3199), so it 
should have plenty of room to create the socketpair without hitting EMFILE.

Disclaimer: I tested on 1.7.9 rather than the latest snapshot; maybe the 
pipe() fix in the meantime also fixed socketpair()?

-- 
Eric Blake   eblake AT redhat DOT com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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