[PATCH][BZ #16996] get_nprocs: Only return explictly set cache values

[Meador Inge <meadori at codesourcery dot com>]

The implementation of __get_nprocs uses a stactic variable to cache
the value of the current number of processors.  The caching breaks when
'time (NULL) == 0':

  $ cat nproc.c
  #include <stdio.h>
  #include <time.h>
  #include <sys/time.h>

  int main(int argc, char *argv[])
  {
    time_t t;
    struct timeval tv = {0, 0};
    printf("settimeofday({0, 0}, NULL) = %d\n", settimeofday(&tv, NULL));
    t = time(NULL);
    printf("Time: %d, CPUs: %d\n", (unsigned int)t, get_nprocs());
    return 0;
  }
  $ gcc -O3 nproc.c
  $ ./a.out
  settimeofday({0, 0}, NULL) = -1
  Time: 1401311578, CPUs: 4
  $ sudo ./a.out
  settimeofday({0, 0}, NULL) = 0
  Time: 0, CPUs: 0

The problem is with the condition used to check whether a cached
value should be returned or not:

  static int cached_result;
  static time_t timestamp;

  time_t now = time (NULL);
  time_t prev = timestamp;
  atomic_read_barrier ();
  if (now == prev)
    return cached_result;

When 'timestamp == 0', as in the above test case, an unitialized
cache value is returned.

This patch fixes the problem by ensuring that 'cached_result' has
been set at least once before returning it.

I checked for regressions with 'make check' and verified that the
described test case works, but couldn't find a good way to automate
the test in the GLIBC testsuite.

OK?

2014-06-12  Meador Inge  <meadori@codesourcery.com>

	[BZ #16996]
	sysdeps/unix/sysv/linux/getsysstats.c (__get_nprocs): Ensure that
	the cached result has been set before returning it.

---
 sysdeps/unix/sysv/linux/getsysstats.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/getsysstats.c b/sysdeps/unix/sysv/linux/getsysstats.c
index b6a6fe3..755c259 100644
--- a/sysdeps/unix/sysv/linux/getsysstats.c
+++ b/sysdeps/unix/sysv/linux/getsysstats.c
@@ -126,13 +126,13 @@ next_line (int fd, char *const buffer, char **cp, char **re,
 int
 __get_nprocs (void)
 {
-  static int cached_result;
+  static int cached_result = -1;
   static time_t timestamp;
 
   time_t now = time (NULL);
   time_t prev = timestamp;
   atomic_read_barrier ();
-  if (now == prev)
+  if (now == prev && cached_result > -1)
     return cached_result;
 
   /* XXX Here will come a test for the new system call.  */
-- 
1.7.9.5