view tests/nap.h @ 17427:afd09cd62311

tests/nap.h: use an adaptive delay to avoid ctime update issues The recent change in nap.h (5191133e) decreased the probability of lost races to about a third, however such problems could still be observed in virtual machines and openSUSE's OBS. Instead of calulating the nap() time once and using it (together with a small correction multiplier), avoid the race alltogether by verifying on a reference file whether a timestamp difference has happened. Before, nap() detected the needed time once empirically and then used that delay (together with a small correction multiplier) in further calls. This problem has been reported and discussed several times, including guesses about possible kernel issues: Now, nap() avoids the race alltogether by verifying on a reference file whether a timestamp difference has happened. * tests/nap.h (nap_fd): Define file descriptor variable for the witness file. (nap_works): Change return value to bool. Change passing the old file's status by value instead of by reference as this function does no longer update that timestamp; rename the function argument from st to old_st. Remove the local variables cdiff and mdiff because that function now returns true/false instead of the precise delay. (guess_delay): Remove function. (clear_tmp_file): Add new function to close and unlink the witness file. (nap): Instead of re-using the delay which has been calculated during the first call, avoid the race by actually verifying that a timestamp difference can be observed on the current file system. Use an adaptive approach for the delay to minimize execution time. Assert that the maximum delay is <= ~2 seconds, more precisely sum(2^n) from 0 to 30 = 2^31 - 1 = 2.1s. Use atexit to call clear_tmp_file when the process terminates.
author Bernhard Voelker <>
date Wed, 05 Jun 2013 09:20:15 +0200
parents 3c592b4deb04
line wrap: on
line source

/* Assist in file system timestamp tests.
   Copyright (C) 2009-2013 Free Software Foundation, Inc.

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <>.  */

/* Written by Eric Blake <>, 2009.  */

#ifndef GLTEST_NAP_H
# define GLTEST_NAP_H

# include <limits.h>
# include <stdbool.h>

/* File descriptor used for the witness file.  */
static int nap_fd = -1;

/* Return A - B, in ns.
   Return 0 if the true result would be negative.
   Return INT_MAX if the true result would be greater than INT_MAX.  */
static int
diff_timespec (struct timespec a, struct timespec b)
  time_t as = a.tv_sec;
  time_t bs = b.tv_sec;
  int ans = a.tv_nsec;
  int bns = b.tv_nsec;

  if (! (bs < as || (bs == as && bns < ans)))
    return 0;
  if (as - bs <= INT_MAX / 1000000000)
      int sdiff = (as - bs) * 1000000000;
      int usdiff = ans - bns;
      if (usdiff < INT_MAX - sdiff)
        return sdiff + usdiff;
  return INT_MAX;

static void
get_stat (int fd, struct stat *st, int do_write)
  if (do_write)
    ASSERT (write (fd, "\n", 1) == 1);
  ASSERT (fstat (fd, st) == 0);

/* Given a file whose descriptor is FD, see whether delaying by DELAY
   nanoseconds causes a change in a file's ctime and mtime.
   OLD_ST is the file's status, recently gotten.  */
static bool
nap_works (int fd, int delay, struct stat old_st)
  struct stat st;
  struct timespec delay_spec;
  delay_spec.tv_sec = delay / 1000000000;
  delay_spec.tv_nsec = delay % 1000000000;
  ASSERT (nanosleep (&delay_spec, 0) == 0);
  get_stat (fd, &st, 1);

  if (   diff_timespec (get_stat_ctime (&st), get_stat_ctime (&old_st))
      && diff_timespec (get_stat_mtime (&st), get_stat_mtime (&old_st)))
    return true;

  return false;

#define TEMPFILE BASE "nap.tmp"

static void
clear_temp_file (void)
  if (0 <= nap_fd)
      ASSERT (close (nap_fd) != -1);
      ASSERT (unlink (TEMPFILE) != -1);

/* Sleep long enough to notice a timestamp difference on the file
   system in the current directory.  Use an adaptive approach, trying
   to find the smallest delay which works on the current file system
   to make the timestamp difference appear.  Assert a maximum delay of
   ~2 seconds, more precisely sum(2^n) from 0 to 30 = 2^31 - 1 = 2.1s.
   Assumes that BASE is defined, and requires that the test module
   depends on nanosleep.  */
static void
nap (void)
  struct stat old_st;
  static int delay = 1;

  if (-1 == nap_fd)
      atexit (clear_temp_file);
      ASSERT ((nap_fd = creat (TEMPFILE, 0600)) != -1);
      get_stat (nap_fd, &old_st, 0);
      ASSERT (0 <= nap_fd);
      get_stat (nap_fd, &old_st, 1);

  if (1 < delay)
    delay = delay / 2;  /* Try half of the previous delay.  */
  ASSERT (0 < delay);

  for ( ; delay <= 2147483647; delay = delay * 2)
    if (nap_works (nap_fd, delay, old_st))

  /* Bummer: even the highest nap delay didn't work. */
  ASSERT (0);

#endif /* GLTEST_NAP_H */