text: /* utimecmp.c -- compare file time stamps
  
     Copyright (C) 2004-2007, 2009-2011 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
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
     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 <http://www.gnu.org/licenses/>.  */
  
  /* Written by Paul Eggert.  */
  
  #include <config.h>
  
  #include "utimecmp.h"
  
  #include <limits.h>
  #include <stdbool.h>
  #include <stdint.h>
  #include <stdlib.h>
  #include <time.h>
  #include <unistd.h>
  
  #include "hash.h"
  #include "intprops.h"
  #include "stat-time.h"
  #include "utimens.h"
  #include "verify.h"
  #include "xalloc.h"
  
  #ifndef MAX
  # define MAX(a, b) ((a) > (b) ? (a) : (b))
  #endif
  
  #define BILLION (1000 * 1000 * 1000)
  
  /* Best possible resolution that utimens can set and stat can return,
     due to system-call limitations.  It must be a power of 10 that is
     no greater than 1 billion.  */
  #if HAVE_UTIMENSAT
  enum { SYSCALL_RESOLUTION = 1 };
  #elif ((HAVE_FUTIMESAT || HAVE_WORKING_UTIMES)                  \
         && (defined HAVE_STRUCT_STAT_ST_ATIM_TV_NSEC             \
             || defined HAVE_STRUCT_STAT_ST_ATIMESPEC_TV_NSEC     \
             || defined HAVE_STRUCT_STAT_ST_ATIMENSEC             \
             || defined HAVE_STRUCT_STAT_ST_ATIM_ST__TIM_TV_NSEC  \
             || defined HAVE_STRUCT_STAT_ST_SPARE1))
  enum { SYSCALL_RESOLUTION = 1000 };
  #else
  enum { SYSCALL_RESOLUTION = BILLION };
  #endif
  
  /* Describe a file system and its time stamp resolution in nanoseconds.  */
  struct fs_res
  {
    /* Device number of file system.  */
    dev_t dev;
  
    /* An upper bound on the time stamp resolution of this file system,
       ignoring any resolution that cannot be set via utimens.  It is
       represented by an integer count of nanoseconds.  It must be
       either 2 billion, or a power of 10 that is no greater than a
       billion and is no less than SYSCALL_RESOLUTION.  */
    int resolution;
  
    /* True if RESOLUTION is known to be exact, and is not merely an
       upper bound on the true resolution.  */
    bool exact;
  };
  
  /* Hash some device info.  */
  static size_t
  dev_info_hash (void const *x, size_t table_size)
  {
    struct fs_res const *p = x;
  
    /* Beware signed arithmetic gotchas.  */
    if (TYPE_SIGNED (dev_t) && SIZE_MAX < MAX (INT_MAX, TYPE_MAXIMUM (dev_t)))
      {
        uintmax_t dev = p->dev;
        return dev % table_size;
      }
  
    return p->dev % table_size;
  }
  
  /* Compare two dev_info structs.  */
  static bool
  dev_info_compare (void const *x, void const *y)
  {
    struct fs_res const *a = x;
    struct fs_res const *b = y;
    return a->dev == b->dev;
  }
  
  /* Return -1, 0, 1 based on whether the destination file (with name
     DST_NAME and status DST_STAT) is older than SRC_STAT, the same age
     as SRC_STAT, or newer than SRC_STAT, respectively.
  
     DST_NAME may be NULL if OPTIONS is 0.
  
     If OPTIONS & UTIMECMP_TRUNCATE_SOURCE, do the comparison after SRC is
     converted to the destination's timestamp resolution as filtered through
     utimens.  In this case, return -2 if the exact answer cannot be
     determined; this can happen only if the time stamps are very close and
     there is some trouble accessing the file system (e.g., the user does not
     have permission to futz with the destination's time stamps).  */
  
  int
  utimecmp (char const *dst_name,
            struct stat const *dst_stat,
            struct stat const *src_stat,
            int options)
  {
    /* Things to watch out for:
  
       The code uses a static hash table internally and is not safe in the
       presence of signals, multiple threads, etc.
  
       int and long int might be 32 bits.  Many of the calculations store
       numbers up to 2 billion, and multiply by 10; they have to avoid
       multiplying 2 billion by 10, as this exceeds 32-bit capabilities.
  
       time_t might be unsigned.  */
  
    verify (TYPE_IS_INTEGER (time_t));
    verify (TYPE_TWOS_COMPLEMENT (int));
  
    /* Destination and source time stamps.  */
    time_t dst_s = dst_stat->st_mtime;
    time_t src_s = src_stat->st_mtime;
    int dst_ns = get_stat_mtime_ns (dst_stat);
    int src_ns = get_stat_mtime_ns (src_stat);
  
    if (options & UTIMECMP_TRUNCATE_SOURCE)
      {
        /* Look up the time stamp resolution for the destination device.  */
  
        /* Hash table for devices.  */
        static Hash_table *ht;
  
        /* Information about the destination file system.  */
        static struct fs_res *new_dst_res;
        struct fs_res *dst_res;
  
        /* Time stamp resolution in nanoseconds.  */
        int res;
  
        /* Quick exit, if possible.  Since the worst resolution is 2
           seconds, anything that differs by more than that does not
           needs source truncation.  */
        if (dst_s == src_s && dst_ns == src_ns)
          return 0;
        if (dst_s <= src_s - 2)
          return -1;
        if (src_s <= dst_s - 2)
          return 1;
  
        if (! ht)
          ht = hash_initialize (16, NULL, dev_info_hash, dev_info_compare, free);
        if (! new_dst_res)
          {
            new_dst_res = xmalloc (sizeof *new_dst_res);
            new_dst_res->resolution = 2 * BILLION;
            new_dst_res->exact = false;
          }
        new_dst_res->dev = dst_stat->st_dev;
        dst_res = hash_insert (ht, new_dst_res);
        if (! dst_res)
          xalloc_die ();
  
        if (dst_res == new_dst_res)
          {
            /* NEW_DST_RES is now in use in the hash table, so allocate a
               new entry next time.  */
            new_dst_res = NULL;
          }
  
        res = dst_res->resolution;
  
  #ifdef _PC_TIMESTAMP_RESOLUTION
        /* If the system will tell us the resolution, we're set!  */
        if (! dst_res->exact)
          {
            res = pathconf (dst_name, _PC_TIMESTAMP_RESOLUTION);
            if (0 < res)
              {
                dst_res->resolution = res;
                dst_res->exact = true;
              }
          }
  #endif
  
        if (! dst_res->exact)
          {
            /* This file system's resolution is not known exactly.
               Deduce it, and store the result in the hash table.  */
  
            time_t dst_a_s = dst_stat->st_atime;
            time_t dst_c_s = dst_stat->st_ctime;
            time_t dst_m_s = dst_s;
            int dst_a_ns = get_stat_atime_ns (dst_stat);
            int dst_c_ns = get_stat_ctime_ns (dst_stat);
            int dst_m_ns = dst_ns;
  
            /* Set RES to an upper bound on the file system resolution
               (after truncation due to SYSCALL_RESOLUTION) by inspecting
               the atime, ctime and mtime of the existing destination.
               We don't know of any file system that stores atime or
               ctime with a higher precision than mtime, so it's valid to
               look at them too.  */
            {
              bool odd_second = (dst_a_s | dst_c_s | dst_m_s) & 1;
  
              if (SYSCALL_RESOLUTION == BILLION)
                {
                  if (odd_second | dst_a_ns | dst_c_ns | dst_m_ns)
                    res = BILLION;
                }
              else
                {
                  int a = dst_a_ns;
                  int c = dst_c_ns;
                  int m = dst_m_ns;
  
                  /* Write it this way to avoid mistaken GCC warning
                     about integer overflow in constant expression.  */
                  int SR10 = SYSCALL_RESOLUTION;  SR10 *= 10;
  
                  if ((a % SR10 | c % SR10 | m % SR10) != 0)
                    res = SYSCALL_RESOLUTION;
                  else
                    for (res = SR10, a /= SR10, c /= SR10, m /= SR10;
                         (res < dst_res->resolution
                          && (a % 10 | c % 10 | m % 10) == 0);
                         res *= 10, a /= 10, c /= 10, m /= 10)
                      if (res == BILLION)
                        {
                          if (! odd_second)
                            res *= 2;
                          break;
                        }
                }
  
              dst_res->resolution = res;
            }
  
            if (SYSCALL_RESOLUTION < res)
              {
                struct timespec timespec[2];
                struct stat dst_status;
  
                /* Ignore source time stamp information that must necessarily
                   be lost when filtered through utimens.  */
                src_ns -= src_ns % SYSCALL_RESOLUTION;
  
                /* If the time stamps disagree widely enough, there's no need
                   to interrogate the file system to deduce the exact time
                   stamp resolution; return the answer directly.  */
                {
                  time_t s = src_s & ~ (res == 2 * BILLION);
                  if (src_s < dst_s || (src_s == dst_s && src_ns <= dst_ns))
                    return 1;
                  if (dst_s < s
                      || (dst_s == s && dst_ns < src_ns - src_ns % res))
                    return -1;
                }
  
                /* Determine the actual time stamp resolution for the
                   destination file system (after truncation due to
                   SYSCALL_RESOLUTION) by setting the access time stamp of the
                   destination to the existing access time, except with
                   trailing nonzero digits.  */
  
                timespec[0].tv_sec = dst_a_s;
                timespec[0].tv_nsec = dst_a_ns;
                timespec[1].tv_sec = dst_m_s | (res == 2 * BILLION);
                timespec[1].tv_nsec = dst_m_ns + res / 9;
  
                /* Set the modification time.  But don't try to set the
                   modification time of symbolic links; on many hosts this sets
                   the time of the pointed-to file.  */
                if ((S_ISLNK (dst_stat->st_mode)
                     ? lutimens (dst_name, timespec)
                     : utimens (dst_name, timespec)) != 0)
                  return -2;
  
                /* Read the modification time that was set.  */
                {
                  int stat_result = (S_ISLNK (dst_stat->st_mode)
                                     ? lstat (dst_name, &dst_status)
                                     : stat (dst_name, &dst_status));
  
                  if (stat_result
                      | (dst_status.st_mtime ^ dst_m_s)
                      | (get_stat_mtime_ns (&dst_status) ^ dst_m_ns))
                    {
                      /* The modification time changed, or we can't tell whether
                         it changed.  Change it back as best we can.  */
                      timespec[1].tv_sec = dst_m_s;
                      timespec[1].tv_nsec = dst_m_ns;
                      if (S_ISLNK (dst_stat->st_mode))
                        lutimens (dst_name, timespec);
                      else
                        utimens (dst_name, timespec);
                    }
  
                  if (stat_result != 0)
                    return -2;
                }
  
                /* Determine the exact resolution from the modification time
                   that was read back.  */
                {
                  int old_res = res;
                  int a = (BILLION * (dst_status.st_mtime & 1)
                           + get_stat_mtime_ns (&dst_status));
  
                  res = SYSCALL_RESOLUTION;
  
                  for (a /= res; a % 10 == 0; a /= 10)
                    {
                      if (res == BILLION)
                        {
                          res *= 2;
                          break;
                        }
                      res *= 10;
                      if (res == old_res)
                        break;
                    }
                }
              }
  
            dst_res->resolution = res;
            dst_res->exact = true;
          }
  
        /* Truncate the source's time stamp according to the resolution.  */
        src_s &= ~ (res == 2 * BILLION);
        src_ns -= src_ns % res;
      }
  
    /* Compare the time stamps and return -1, 0, 1 accordingly.  */
    return (dst_s < src_s ? -1
            : dst_s > src_s ? 1
            : dst_ns < src_ns ? -1
            : dst_ns > src_ns);
  }
  