http://bugs.gentoo.org/250342
http://sources.redhat.com/bugzilla/show_bug.cgi?id=9685

we cant assume sock_cloexec and pipe2 are bound together as the former defines
are found in glibc only while the latter are a combo of kernel headers and
glibc.  so if we do a runtime detection of SOCK_CLOEXEC, but pipe2() is a stub
inside of glibc, we hit a problem.  for example:

#include <grp.h>
#include <stdio.h>
main()
{
	getgrnam("portage");
	if (!popen("ls", "r"))
		perror("popen()");
}

getgrnam() will detect that the kernel supports SOCK_CLOEXEC and then set both
__have_sock_cloexec and __have_pipe2 to true.  but if glibc was built against
older kernel headers where __NR_pipe2 does not exist, glibc will have a ENOSYS
stub for it.  so popen() will always fail as glibc assumes pipe2() works.

---
 include/unistd.h           |    5 +----
 socket/have_sock_cloexec.c |    5 +++++
 2 files changed, 6 insertions(+), 4 deletions(-)

--- a/include/unistd.h
+++ b/include/unistd.h
@@ -167,9 +167,6 @@
 extern int __pause_nocancel (void) attribute_hidden;
 
 extern int __have_sock_cloexec;
-/* At lot of other functionality became available at the same time as
-   SOCK_CLOEXEC.  Avoid defining separate variables for all of them
-   unless it is really necessary.  */
-#define __have_pipe2 __have_sock_cloexec
+extern int __have_pipe2;
 
 #endif
--- a/socket/have_sock_cloexec.c
+++ b/socket/have_sock_cloexec.c
@@ -16,9 +16,14 @@
    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
    02111-1307 USA.  */
 
+#include <fcntl.h>
 #include <sys/socket.h>
 #include <kernel-features.h>
 
 #if defined SOCK_CLOEXEC && !defined __ASSUME_SOCK_CLOEXEC
 int __have_sock_cloexec;
 #endif
+
+#if defined O_CLOEXEC && !defined __ASSUME_PIPE2
+int __have_pipe2;
+#endif