[Bug libc/16996] get_nprocs caching breaks when 'time (NULL) == 0'

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https://sourceware.org/bugzilla/show_bug.cgi?id=16996

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=995a46bbfba9964e328e3947130919d8bd3cd62a

commit 995a46bbfba9964e328e3947130919d8bd3cd62a
Author: Meador Inge <meadori@codesourcery.com>
Date:   Fri Jun 13 14:02:04 2014 +0530

    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.

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog                             |    6 ++++++
 NEWS                                  |    2 +-
 sysdeps/unix/sysv/linux/getsysstats.c |    4 ++--
 3 files changed, 9 insertions(+), 3 deletions(-)

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