413 lines
14 KiB
C
413 lines
14 KiB
C
/* utimecmp.c -- compare file timestamps
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Copyright (C) 2004-2007, 2009-2022 Free Software Foundation, Inc.
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This program is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>. */
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/* Written by Paul Eggert. */
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#include <config.h>
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#include "utimecmp.h"
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#include <fcntl.h>
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#include <limits.h>
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#include <stdbool.h>
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#include <stdint.h>
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#include <stdlib.h>
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#include <sys/stat.h>
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#include <time.h>
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#include <unistd.h>
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#include "dirname.h"
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#include "hash.h"
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#include "intprops.h"
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#include "stat-time.h"
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#include "verify.h"
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#ifndef MAX
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# define MAX(a, b) ((a) > (b) ? (a) : (b))
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#endif
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#define BILLION (1000 * 1000 * 1000)
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/* Best possible resolution that utimens can set and stat can return,
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due to system-call limitations. It must be a power of 10 that is
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no greater than 1 billion. */
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#if HAVE_UTIMENSAT
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enum { SYSCALL_RESOLUTION = 1 };
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#elif defined _WIN32 && ! defined __CYGWIN__
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/* On native Windows, file times have 100 ns resolution. See
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<https://docs.microsoft.com/en-us/windows/desktop/api/minwinbase/ns-minwinbase-filetime> */
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enum { SYSCALL_RESOLUTION = 100 };
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#elif ((HAVE_FUTIMESAT || HAVE_WORKING_UTIMES) \
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&& (defined HAVE_STRUCT_STAT_ST_ATIM_TV_NSEC \
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|| defined HAVE_STRUCT_STAT_ST_ATIMESPEC_TV_NSEC \
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|| defined HAVE_STRUCT_STAT_ST_ATIMENSEC \
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|| defined HAVE_STRUCT_STAT_ST_ATIM_ST__TIM_TV_NSEC \
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|| defined HAVE_STRUCT_STAT_ST_SPARE1))
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enum { SYSCALL_RESOLUTION = 1000 };
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#else
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enum { SYSCALL_RESOLUTION = BILLION };
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#endif
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/* Describe a file system and its timestamp resolution in nanoseconds. */
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struct fs_res
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{
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/* Device number of file system. */
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dev_t dev;
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/* An upper bound on the timestamp resolution of this file system,
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ignoring any resolution that cannot be set via utimens. It is
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represented by an integer count of nanoseconds. It must be
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either 2 billion, or a power of 10 that is no greater than a
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billion and is no less than SYSCALL_RESOLUTION. */
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int resolution;
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/* True if RESOLUTION is known to be exact, and is not merely an
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upper bound on the true resolution. */
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bool exact;
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};
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/* Hash some device info. */
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static size_t
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dev_info_hash (void const *x, size_t table_size)
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{
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struct fs_res const *p = x;
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/* Beware signed arithmetic gotchas. */
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if (TYPE_SIGNED (dev_t) && SIZE_MAX < MAX (INT_MAX, TYPE_MAXIMUM (dev_t)))
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{
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uintmax_t dev = p->dev;
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return dev % table_size;
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}
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return p->dev % table_size;
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}
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/* Compare two dev_info structs. */
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static bool
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dev_info_compare (void const *x, void const *y)
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{
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struct fs_res const *a = x;
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struct fs_res const *b = y;
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return a->dev == b->dev;
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}
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/* Return -1, 0, 1 based on whether the destination file (relative
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to openat-like directory file descriptor DFD with name
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DST_NAME and status DST_STAT) is older than SRC_STAT, the same age
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as SRC_STAT, or newer than SRC_STAT, respectively.
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DST_NAME may be NULL if OPTIONS is 0.
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If OPTIONS & UTIMECMP_TRUNCATE_SOURCE, do the comparison after SRC is
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converted to the destination's timestamp resolution as filtered through
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utimens. In this case, return -2 if the exact answer cannot be
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determined; this can happen only if the timestamps are very close and
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there is some trouble accessing the file system (e.g., the user does not
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have permission to futz with the destination's timestamps). */
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int
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utimecmp (char const *dst_name,
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struct stat const *dst_stat,
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struct stat const *src_stat,
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int options)
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{
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return utimecmpat (AT_FDCWD, dst_name, dst_stat, src_stat, options);
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}
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int
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utimecmpat (int dfd, char const *dst_name,
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struct stat const *dst_stat,
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struct stat const *src_stat,
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int options)
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{
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/* Things to watch out for:
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The code uses a static hash table internally and is not safe in the
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presence of signals, multiple threads, etc. However, memory pressure
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that prevents use of the hash table is not fatal - we just fall back
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to redoing the computations on every call in that case.
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int and long int might be 32 bits. Many of the calculations store
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numbers up to 2 billion, and multiply by 10; they have to avoid
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multiplying 2 billion by 10, as this exceeds 32-bit capabilities.
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time_t might be unsigned. */
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verify (TYPE_IS_INTEGER (time_t));
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/* Destination and source timestamps. */
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time_t dst_s = dst_stat->st_mtime;
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time_t src_s = src_stat->st_mtime;
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int dst_ns = get_stat_mtime_ns (dst_stat);
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int src_ns = get_stat_mtime_ns (src_stat);
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if (options & UTIMECMP_TRUNCATE_SOURCE)
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{
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#if defined _AIX
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/* On AIX 7.2, on a jfs2 file system, the times may differ by up to
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0.01 seconds in either direction. But it does not seem to come
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from clock ticks of 0.01 seconds each. */
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long long difference =
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((long long) dst_s - (long long) src_s) * BILLION
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+ ((long long) dst_ns - (long long) src_ns);
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if (difference < 10000000 && difference > -10000000)
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return 0;
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#endif
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/* Look up the timestamp resolution for the destination device. */
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/* Hash table for caching information learned about devices. */
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static Hash_table *ht;
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/* Information about the destination file system. */
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static struct fs_res *new_dst_res;
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struct fs_res *dst_res = NULL;
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struct fs_res tmp_dst_res;
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/* timestamp resolution in nanoseconds. */
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int res;
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/* Quick exit, if possible. Since the worst resolution is 2
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seconds, anything that differs by more than that does not
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needs source truncation. */
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if (dst_s == src_s && dst_ns == src_ns)
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return 0;
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if (dst_s <= src_s - 2)
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return -1;
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if (src_s <= dst_s - 2)
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return 1;
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/* Try to do a hash lookup, but fall back to stack variables and
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recomputation on low memory situations. */
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if (! ht)
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ht = hash_initialize (16, NULL, dev_info_hash, dev_info_compare, free);
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if (ht)
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{
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if (! new_dst_res)
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{
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new_dst_res = malloc (sizeof *new_dst_res);
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if (!new_dst_res)
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goto low_memory;
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new_dst_res->resolution = 2 * BILLION;
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new_dst_res->exact = false;
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}
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new_dst_res->dev = dst_stat->st_dev;
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dst_res = hash_insert (ht, new_dst_res);
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if (! dst_res)
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goto low_memory;
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if (dst_res == new_dst_res)
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{
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/* NEW_DST_RES is now in use in the hash table, so allocate a
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new entry next time. */
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new_dst_res = NULL;
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}
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}
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else
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{
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low_memory:
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if (ht)
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{
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tmp_dst_res.dev = dst_stat->st_dev;
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dst_res = hash_lookup (ht, &tmp_dst_res);
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}
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if (!dst_res)
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{
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dst_res = &tmp_dst_res;
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dst_res->resolution = 2 * BILLION;
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dst_res->exact = false;
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}
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}
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res = dst_res->resolution;
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#ifdef _PC_TIMESTAMP_RESOLUTION
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/* If the system will tell us the resolution, we're set! */
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if (! dst_res->exact)
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{
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res = -1;
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if (dfd == AT_FDCWD)
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res = pathconf (dst_name, _PC_TIMESTAMP_RESOLUTION);
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else
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{
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char *dstdir = mdir_name (dst_name);
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if (dstdir)
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{
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int destdirfd = openat (dfd, dstdir,
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O_SEARCH | O_CLOEXEC | O_DIRECTORY);
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if (0 <= destdirfd)
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{
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res = fpathconf (destdirfd, _PC_TIMESTAMP_RESOLUTION);
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close (destdirfd);
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}
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free (dstdir);
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}
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}
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if (0 < res)
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{
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dst_res->resolution = res;
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dst_res->exact = true;
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}
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}
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#endif
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if (! dst_res->exact)
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{
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/* This file system's resolution is not known exactly.
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Deduce it, and store the result in the hash table. */
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time_t dst_a_s = dst_stat->st_atime;
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time_t dst_c_s = dst_stat->st_ctime;
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time_t dst_m_s = dst_s;
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int dst_a_ns = get_stat_atime_ns (dst_stat);
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int dst_c_ns = get_stat_ctime_ns (dst_stat);
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int dst_m_ns = dst_ns;
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/* Set RES to an upper bound on the file system resolution
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(after truncation due to SYSCALL_RESOLUTION) by inspecting
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the atime, ctime and mtime of the existing destination.
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We don't know of any file system that stores atime or
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ctime with a higher precision than mtime, so it's valid to
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look at them too. */
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{
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bool odd_second = (dst_a_s | dst_c_s | dst_m_s) & 1;
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if (SYSCALL_RESOLUTION == BILLION)
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{
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if (odd_second | dst_a_ns | dst_c_ns | dst_m_ns)
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res = BILLION;
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}
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else
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{
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int a = dst_a_ns;
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int c = dst_c_ns;
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int m = dst_m_ns;
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/* Write it this way to avoid mistaken GCC warning
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about integer overflow in constant expression. */
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int SR10 = SYSCALL_RESOLUTION; SR10 *= 10;
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if ((a % SR10 | c % SR10 | m % SR10) != 0)
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res = SYSCALL_RESOLUTION;
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else
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for (res = SR10, a /= SR10, c /= SR10, m /= SR10;
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(res < dst_res->resolution
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&& (a % 10 | c % 10 | m % 10) == 0);
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res *= 10, a /= 10, c /= 10, m /= 10)
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if (res == BILLION)
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{
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if (! odd_second)
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res *= 2;
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break;
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}
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}
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dst_res->resolution = res;
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}
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if (SYSCALL_RESOLUTION < res)
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{
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struct timespec timespec[2];
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struct stat dst_status;
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/* Ignore source timestamp information that must necessarily
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be lost when filtered through utimens. */
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src_ns -= src_ns % SYSCALL_RESOLUTION;
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/* If the timestamps disagree widely enough, there's no need
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to interrogate the file system to deduce the exact
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timestamp resolution; return the answer directly. */
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{
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time_t s = src_s & ~ (res == 2 * BILLION ? 1 : 0);
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if (src_s < dst_s || (src_s == dst_s && src_ns <= dst_ns))
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return 1;
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if (dst_s < s
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|| (dst_s == s && dst_ns < src_ns - src_ns % res))
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return -1;
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}
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/* Determine the actual timestamp resolution for the
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destination file system (after truncation due to
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SYSCALL_RESOLUTION) by setting the access timestamp of the
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destination to the existing access time, except with
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trailing nonzero digits. */
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timespec[0].tv_sec = dst_a_s;
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timespec[0].tv_nsec = dst_a_ns;
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timespec[1].tv_sec = dst_m_s | (res == 2 * BILLION);
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timespec[1].tv_nsec = dst_m_ns + res / 9;
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if (utimensat (dfd, dst_name, timespec, AT_SYMLINK_NOFOLLOW))
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return -2;
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/* Read the modification time that was set. */
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{
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int stat_result
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= fstatat (dfd, dst_name, &dst_status, AT_SYMLINK_NOFOLLOW);
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if (stat_result
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| (dst_status.st_mtime ^ dst_m_s)
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| (get_stat_mtime_ns (&dst_status) ^ dst_m_ns))
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{
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/* The modification time changed, or we can't tell whether
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it changed. Change it back as best we can. */
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timespec[1].tv_sec = dst_m_s;
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timespec[1].tv_nsec = dst_m_ns;
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utimensat (dfd, dst_name, timespec, AT_SYMLINK_NOFOLLOW);
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}
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if (stat_result != 0)
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return -2;
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}
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/* Determine the exact resolution from the modification time
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that was read back. */
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{
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int old_res = res;
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int a = (BILLION * (dst_status.st_mtime & 1)
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+ get_stat_mtime_ns (&dst_status));
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res = SYSCALL_RESOLUTION;
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for (a /= res; a % 10 == 0; a /= 10)
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{
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if (res == BILLION)
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{
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res *= 2;
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break;
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}
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res *= 10;
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if (res == old_res)
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break;
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}
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}
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}
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dst_res->resolution = res;
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dst_res->exact = true;
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}
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/* Truncate the source's timestamp according to the resolution. */
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src_s &= ~ (res == 2 * BILLION ? 1 : 0);
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src_ns -= src_ns % res;
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}
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/* Compare the timestamps and return -1, 0, 1 accordingly. */
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return (_GL_CMP (dst_s, src_s)
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+ ((dst_s == src_s ? ~0 : 0) & _GL_CMP (dst_ns, src_ns)));
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}
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