get_nprocs: Only return explictly set cache values (BZ #16996)

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;

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

1 files changed, 2 insertions, 2 deletions

diff --git a/sysdeps/unix/sysv/linux/getsysstats.c b/sysdeps/unix/sysv/linux/getsysstats.c
index b6a6fe3e2f..755c259862 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.  */