redict/src/localtime.c

/*
 * Copyright (c) 2018, Salvatore Sanfilippo <antirez at gmail dot com>
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *   * Redistributions of source code must retain the above copyright notice,
 *     this list of conditions and the following disclaimer.
 *   * Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *   * Neither the name of Redis nor the names of its contributors may be used
 *     to endorse or promote products derived from this software without
 *     specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 */

#include <time.h>

/* This is a safe version of localtime() which contains no locks and is
 * fork() friendly. Even the _r version of localtime() cannot be used safely
 * in Redis. Another thread may be calling localtime() while the main thread
 * forks(). Later when the child process calls localtime() again, for instance
 * in order to log something to the Redis log, it may deadlock: in the copy
 * of the address space of the forked process the lock will never be released.
 *
 * This function takes the timezone 'tz' as argument, and the 'dst' flag is
 * used to check if daylight saving time is currently in effect. The caller
 * of this function should obtain such information calling tzset() ASAP in the
 * main() function, and later accessing the globals 'timezone' and 'daylight'.
 *
 * Note that this function does not work for dates < 1/1/1970, it is solely
 * designed to work with what time(NULL) may return, and to support Redis
 * logging of the dates, it's not really a complete implementation. */
static int is_leap_year(time_t year) {
    if (year % 4) return 0;
    else if (year % 100) return 1;
    else if (year % 400) return 0;
    else return 1;
}

void nolocks_localtime(struct tm *tmp, time_t t, time_t tz, int dst) {
    const time_t secs_min = 60;
    const time_t secs_hour = 3600;
    const time_t secs_day = 3600*24;

    t += tz;                            /* Adjust for timezone. */
    time_t days = t / secs_day;         /* Days passed since epoch. */
    time_t seconds = t % secs_day;      /* Remaining seconds. */

    tmp->tm_isdst = dst;
    tmp->tm_hour = seconds / secs_hour;
    tmp->tm_min = (seconds % secs_hour) / secs_min;
    tmp->tm_sec = (seconds % secs_hour) % secs_min;

    /* 1/1/1970 was a Thursday, that is, day 4 from the POV of the tm structure
     * where sunday = 0, so to calculate the day of the week we have to add 4
     * and take the modulo by 7. */
    tmp->tm_wday = (days+4)%7;
}