lib/time_rz.c

   1 /* Time zone functions such as tzalloc and localtime_rz
   2
   3    Copyright 2015-2020 Free Software Foundation, Inc.
   4
   5    This program is free software; you can redistribute it and/or modify
   6    it under the terms of the GNU General Public License as published by
   7    the Free Software Foundation; either version 2, or (at your option)
   8    any later version.
   9
  10    This program is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  13    GNU General Public License for more details.
  14
  15    You should have received a copy of the GNU General Public License along
  16    with this program; if not, see <https://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert.  */
  19
  20 /* Although this module is not thread-safe, any races should be fairly
  21    rare and reasonably benign.  For complete thread-safety, use a C
  22    library with a working timezone_t type, so that this module is not
  23    needed.  */
  24
  25 #include <config.h>
  26
  27 #include <time.h>
  28
  29 #include <errno.h>
  30 #include <stdbool.h>
  31 #include <stddef.h>
  32 #include <stdlib.h>
  33 #include <string.h>
  34
  35 #include "flexmember.h"
  36 #include "idx.h"
  37 #include "time-internal.h"
  38
  39 /* The approximate size to use for small allocation requests.  This is
  40    the largest "small" request for the GNU C library malloc.  */
  41 enum { DEFAULT_MXFAST = 64 * sizeof (size_t) / 4 };
  42
  43 /* Minimum size of the ABBRS member of struct tm_zone.  ABBRS is larger
  44    only in the unlikely case where an abbreviation longer than this is
  45    used.  */
  46 enum { ABBR_SIZE_MIN = DEFAULT_MXFAST - offsetof (struct tm_zone, abbrs) };
  47
  48 /* Magic cookie timezone_t value, for local time.  It differs from
  49    NULL and from all other timezone_t values.  Only the address
  50    matters; the pointer is never dereferenced.  */
  51 static timezone_t const local_tz = (timezone_t) 1;
  52
  53 /* Copy to ABBRS the abbreviation at ABBR with size ABBR_SIZE (this
  54    includes its trailing null byte).  Append an extra null byte to
  55    mark the end of ABBRS.  */
  56 static void
  57 extend_abbrs (char *abbrs, char const *abbr, size_t abbr_size)
  58 {
  59   memcpy (abbrs, abbr, abbr_size);
  60   abbrs[abbr_size] = '\0';
  61 }
  62
  63 /* Return a newly allocated time zone for NAME, or NULL on failure.
  64    A null NAME stands for wall clock time (which is like unset TZ).  */
  65 timezone_t
  66 tzalloc (char const *name)
  67 {
  68   size_t name_size = name ? strlen (name) + 1 : 0;
  69   size_t abbr_size = name_size < ABBR_SIZE_MIN ? ABBR_SIZE_MIN : name_size + 1;
  70   timezone_t tz = malloc (FLEXSIZEOF (struct tm_zone, abbrs, abbr_size));
  71   if (tz)
  72     {
  73       tz->next = NULL;
  74 #if HAVE_TZNAME && !HAVE_TM_ZONE
  75       tz->tzname_copy[0] = tz->tzname_copy[1] = NULL;
  76 #endif
  77       tz->tz_is_set = !!name;
  78       tz->abbrs[0] = '\0';
  79       if (name)
  80         extend_abbrs (tz->abbrs, name, name_size);
  81     }
  82   return tz;
  83 }
  84
  85 /* Save into TZ any nontrivial time zone abbreviation used by TM, and
  86    update *TM (if HAVE_TM_ZONE) or *TZ (if !HAVE_TM_ZONE &&
  87    HAVE_TZNAME) if they use the abbreviation.  Return true if
  88    successful, false (setting errno) otherwise.  */
  89 static bool
  90 save_abbr (timezone_t tz, struct tm *tm)
  91 {
  92 #if HAVE_TM_ZONE || HAVE_TZNAME
  93   char const *zone = NULL;
  94   char *zone_copy = (char *) "";
  95
  96 # if HAVE_TZNAME
  97   int tzname_index = -1;
  98 # endif
  99
 100 # if HAVE_TM_ZONE
 101   zone = tm->tm_zone;
 102 # endif
 103
 104 # if HAVE_TZNAME
 105   if (! (zone && *zone) && 0 <= tm->tm_isdst)
 106     {
 107       tzname_index = tm->tm_isdst != 0;
 108       zone = tzname[tzname_index];
 109     }
 110 # endif
 111
 112   /* No need to replace null zones, or zones within the struct tm.  */
 113   if (!zone || ((char *) tm <= zone && zone < (char *) (tm + 1)))
 114     return true;
 115
 116   if (*zone)
 117     {
 118       zone_copy = tz->abbrs;
 119
 120       while (strcmp (zone_copy, zone) != 0)
 121         {
 122           if (! (*zone_copy || (zone_copy == tz->abbrs && tz->tz_is_set)))
 123             {
 124               idx_t zone_size = strlen (zone) + 1;
 125               if (zone_size < tz->abbrs + ABBR_SIZE_MIN - zone_copy)
 126                 extend_abbrs (zone_copy, zone, zone_size);
 127               else
 128                 {
 129                   tz = tz->next = tzalloc (zone);
 130                   if (!tz)
 131                     return false;
 132                   tz->tz_is_set = 0;
 133                   zone_copy = tz->abbrs;
 134                 }
 135               break;
 136             }
 137
 138           zone_copy += strlen (zone_copy) + 1;
 139           if (!*zone_copy && tz->next)
 140             {
 141               tz = tz->next;
 142               zone_copy = tz->abbrs;
 143             }
 144         }
 145     }
 146
 147   /* Replace the zone name so that its lifetime matches that of TZ.  */
 148 # if HAVE_TM_ZONE
 149   tm->tm_zone = zone_copy;
 150 # else
 151   if (0 <= tzname_index)
 152     tz->tzname_copy[tzname_index] = zone_copy;
 153 # endif
 154 #endif
 155
 156   return true;
 157 }
 158
 159 /* Free a time zone.  */
 160 void
 161 tzfree (timezone_t tz)
 162 {
 163   if (tz != local_tz)
 164     while (tz)
 165       {
 166         timezone_t next = tz->next;
 167         free (tz);
 168         tz = next;
 169       }
 170 }
 171
 172 /* Get and set the TZ environment variable.  These functions can be
 173    overridden by programs like Emacs that manage their own environment.  */
 174
 175 #ifndef getenv_TZ
 176 static char *
 177 getenv_TZ (void)
 178 {
 179   return getenv ("TZ");
 180 }
 181 #endif
 182
 183 #ifndef setenv_TZ
 184 static int
 185 setenv_TZ (char const *tz)
 186 {
 187   return tz ? setenv ("TZ", tz, 1) : unsetenv ("TZ");
 188 }
 189 #endif
 190
 191 /* Change the environment to match the specified timezone_t value.
 192    Return true if successful, false (setting errno) otherwise.  */
 193 static bool
 194 change_env (timezone_t tz)
 195 {
 196   if (setenv_TZ (tz->tz_is_set ? tz->abbrs : NULL) != 0)
 197     return false;
 198   tzset ();
 199   return true;
 200 }
 201
 202 /* Temporarily set the time zone to TZ, which must not be null.
 203    Return LOCAL_TZ if the time zone setting is already correct.
 204    Otherwise return a newly allocated time zone representing the old
 205    setting, or NULL (setting errno) on failure.  */
 206 static timezone_t
 207 set_tz (timezone_t tz)
 208 {
 209   char *env_tz = getenv_TZ ();
 210   if (env_tz
 211       ? tz->tz_is_set && strcmp (tz->abbrs, env_tz) == 0
 212       : !tz->tz_is_set)
 213     return local_tz;
 214   else
 215     {
 216       timezone_t old_tz = tzalloc (env_tz);
 217       if (!old_tz)
 218         return old_tz;
 219       if (! change_env (tz))
 220         {
 221           int saved_errno = errno;
 222           tzfree (old_tz);
 223           errno = saved_errno;
 224           return NULL;
 225         }
 226       return old_tz;
 227     }
 228 }
 229
 230 /* Restore an old setting returned by set_tz.  It must not be null.
 231    Return true (preserving errno) if successful, false (setting errno)
 232    otherwise.  */
 233 static bool
 234 revert_tz (timezone_t tz)
 235 {
 236   if (tz == local_tz)
 237     return true;
 238   else
 239     {
 240       int saved_errno = errno;
 241       bool ok = change_env (tz);
 242       if (!ok)
 243         saved_errno = errno;
 244       tzfree (tz);
 245       errno = saved_errno;
 246       return ok;
 247     }
 248 }
 249
 250 /* Use time zone TZ to compute localtime_r (T, TM).  */
 251 struct tm *
 252 localtime_rz (timezone_t tz, time_t const *t, struct tm *tm)
 253 {
 254 #ifdef HAVE_LOCALTIME_INFLOOP_BUG
 255   /* The -67768038400665599 comes from:
 256      https://lists.gnu.org/r/bug-gnulib/2017-07/msg00142.html
 257      On affected platforms the greatest POSIX-compatible time_t value
 258      that could return nonnull is 67768036191766798 (when
 259      TZ="XXX24:59:59" it resolves to the year 2**31 - 1 + 1900, on
 260      12-31 at 23:59:59), so test for that too while we're in the
 261      neighborhood.  */
 262   if (! (-67768038400665599 <= *t && *t <= 67768036191766798))
 263     {
 264       errno = EOVERFLOW;
 265       return NULL;
 266     }
 267 #endif
 268
 269   if (!tz)
 270     return gmtime_r (t, tm);
 271   else
 272     {
 273       timezone_t old_tz = set_tz (tz);
 274       if (old_tz)
 275         {
 276           bool abbr_saved = localtime_r (t, tm) && save_abbr (tz, tm);
 277           if (revert_tz (old_tz) && abbr_saved)
 278             return tm;
 279         }
 280       return NULL;
 281     }
 282 }
 283
 284 /* Use time zone TZ to compute mktime (TM).  */
 285 time_t
 286 mktime_z (timezone_t tz, struct tm *tm)
 287 {
 288   if (!tz)
 289     return timegm (tm);
 290   else
 291     {
 292       timezone_t old_tz = set_tz (tz);
 293       if (old_tz)
 294         {
 295           struct tm tm_1;
 296           tm_1.tm_sec = tm->tm_sec;
 297           tm_1.tm_min = tm->tm_min;
 298           tm_1.tm_hour = tm->tm_hour;
 299           tm_1.tm_mday = tm->tm_mday;
 300           tm_1.tm_mon = tm->tm_mon;
 301           tm_1.tm_year = tm->tm_year;
 302           tm_1.tm_yday = -1;
 303           tm_1.tm_isdst = tm->tm_isdst;
 304           time_t t = mktime (&tm_1);
 305           bool ok = 0 <= tm_1.tm_yday;
 306 #if HAVE_TM_ZONE || HAVE_TZNAME
 307           ok = ok && save_abbr (tz, &tm_1);
 308 #endif
 309           if (revert_tz (old_tz) && ok)
 310             {
 311               *tm = tm_1;
 312               return t;
 313             }
 314         }
 315       return -1;
 316     }
 317 }