postgresql/src/timezone/pgtz.c

/*-------------------------------------------------------------------------
 *
 * pgtz.c
 *	  Timezone Library Integration Functions
 *
 * Portions Copyright (c) 1996-2011, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *	  src/timezone/pgtz.c
 *
 *-------------------------------------------------------------------------
 */
#define NO_REDEFINE_TIMEFUNCS

#include "postgres.h"

#include <ctype.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <time.h>

#include "miscadmin.h"
#include "pgtz.h"
#include "storage/fd.h"
#include "tzfile.h"
#include "utils/datetime.h"
#include "utils/guc.h"
#include "utils/hsearch.h"

/* Current session timezone (controlled by TimeZone GUC) */
pg_tz	   *session_timezone = NULL;

/* Current log timezone (controlled by log_timezone GUC) */
pg_tz	   *log_timezone = NULL;

/* Fallback GMT timezone for last-ditch error message formatting */
pg_tz	   *gmt_timezone = NULL;
static pg_tz gmt_timezone_data;


static bool scan_directory_ci(const char *dirname,
				  const char *fname, int fnamelen,
				  char *canonname, int canonnamelen);
static const char *identify_system_timezone(void);
static pg_tz *get_pg_tz_for_zone(const char *tzname);
static pg_tz *select_default_timezone(void);


/*
 * Return full pathname of timezone data directory
 */
static const char *
pg_TZDIR(void)
{
#ifndef SYSTEMTZDIR
	/* normal case: timezone stuff is under our share dir */
	static bool done_tzdir = false;
	static char tzdir[MAXPGPATH];

	if (done_tzdir)
		return tzdir;

	get_share_path(my_exec_path, tzdir);
	strlcpy(tzdir + strlen(tzdir), "/timezone", MAXPGPATH - strlen(tzdir));

	done_tzdir = true;
	return tzdir;
#else
	/* we're configured to use system's timezone database */
	return SYSTEMTZDIR;
#endif
}


/*
 * Given a timezone name, open() the timezone data file.  Return the
 * file descriptor if successful, -1 if not.
 *
 * The input name is searched for case-insensitively (we assume that the
 * timezone database does not contain case-equivalent names).
 *
 * If "canonname" is not NULL, then on success the canonical spelling of the
 * given name is stored there (it is assumed to be > TZ_STRLEN_MAX bytes!).
 */
int
pg_open_tzfile(const char *name, char *canonname)
{
	const char *fname;
	char		fullname[MAXPGPATH];
	int			fullnamelen;
	int			orignamelen;

	/*
	 * Loop to split the given name into directory levels; for each level,
	 * search using scan_directory_ci().
	 */
	strcpy(fullname, pg_TZDIR());
	orignamelen = fullnamelen = strlen(fullname);
	fname = name;
	for (;;)
	{
		const char *slashptr;
		int			fnamelen;

		slashptr = strchr(fname, '/');
		if (slashptr)
			fnamelen = slashptr - fname;
		else
			fnamelen = strlen(fname);
		if (fullnamelen + 1 + fnamelen >= MAXPGPATH)
			return -1;			/* not gonna fit */
		if (!scan_directory_ci(fullname, fname, fnamelen,
							   fullname + fullnamelen + 1,
							   MAXPGPATH - fullnamelen - 1))
			return -1;
		fullname[fullnamelen++] = '/';
		fullnamelen += strlen(fullname + fullnamelen);
		if (slashptr)
			fname = slashptr + 1;
		else
			break;
	}

	if (canonname)
		strlcpy(canonname, fullname + orignamelen + 1, TZ_STRLEN_MAX + 1);

	return open(fullname, O_RDONLY | PG_BINARY, 0);
}


/*
 * Scan specified directory for a case-insensitive match to fname
 * (of length fnamelen --- fname may not be null terminated!).	If found,
 * copy the actual filename into canonname and return true.
 */
static bool
scan_directory_ci(const char *dirname, const char *fname, int fnamelen,
				  char *canonname, int canonnamelen)
{
	bool		found = false;
	DIR		   *dirdesc;
	struct dirent *direntry;

	dirdesc = AllocateDir(dirname);
	if (!dirdesc)
	{
		ereport(LOG,
				(errcode_for_file_access(),
				 errmsg("could not open directory \"%s\": %m", dirname)));
		return false;
	}

	while ((direntry = ReadDir(dirdesc, dirname)) != NULL)
	{
		/*
		 * Ignore . and .., plus any other "hidden" files.	This is a security
		 * measure to prevent access to files outside the timezone directory.
		 */
		if (direntry->d_name[0] == '.')
			continue;

		if (strlen(direntry->d_name) == fnamelen &&
			pg_strncasecmp(direntry->d_name, fname, fnamelen) == 0)
		{
			/* Found our match */
			strlcpy(canonname, direntry->d_name, canonnamelen);
			found = true;
			break;
		}
	}

	FreeDir(dirdesc);

	return found;
}


/*
 * The following block of code attempts to determine which timezone in our
 * timezone database is the best match for the active system timezone.
 *
 * On most systems, we rely on trying to match the observable behavior of
 * the C library's localtime() function.  The database zone that matches
 * furthest into the past is the one to use.  Often there will be several
 * zones with identical rankings (since the zic database assigns multiple
 * names to many zones).  We break ties arbitrarily by preferring shorter,
 * then alphabetically earlier zone names.
 *
 * Win32's native knowledge about timezones appears to be too incomplete
 * and too different from the zic database for the above matching strategy
 * to be of any use. But there is just a limited number of timezones
 * available, so we can rely on a handmade mapping table instead.
 */

#ifndef WIN32

#define T_DAY	((time_t) (60*60*24))
#define T_WEEK	((time_t) (60*60*24*7))
#define T_MONTH ((time_t) (60*60*24*31))

#define MAX_TEST_TIMES (52*100) /* 100 years */

struct tztry
{
	int			n_test_times;
	time_t		test_times[MAX_TEST_TIMES];
};

static void scan_available_timezones(char *tzdir, char *tzdirsub,
						 struct tztry * tt,
						 int *bestscore, char *bestzonename);


/*
 * Get GMT offset from a system struct tm
 */
static int
get_timezone_offset(struct tm * tm)
{
#if defined(HAVE_STRUCT_TM_TM_ZONE)
	return tm->tm_gmtoff;
#elif defined(HAVE_INT_TIMEZONE)
	return -TIMEZONE_GLOBAL;
#else
#error No way to determine TZ? Can this happen?
#endif
}

/*
 * Convenience subroutine to convert y/m/d to time_t (NOT pg_time_t)
 */
static time_t
build_time_t(int year, int month, int day)
{
	struct tm	tm;

	memset(&tm, 0, sizeof(tm));
	tm.tm_mday = day;
	tm.tm_mon = month - 1;
	tm.tm_year = year - 1900;

	return mktime(&tm);
}

/*
 * Does a system tm value match one we computed ourselves?
 */
static bool
compare_tm(struct tm * s, struct pg_tm * p)
{
	if (s->tm_sec != p->tm_sec ||
		s->tm_min != p->tm_min ||
		s->tm_hour != p->tm_hour ||
		s->tm_mday != p->tm_mday ||
		s->tm_mon != p->tm_mon ||
		s->tm_year != p->tm_year ||
		s->tm_wday != p->tm_wday ||
		s->tm_yday != p->tm_yday ||
		s->tm_isdst != p->tm_isdst)
		return false;
	return true;
}

/*
 * See how well a specific timezone setting matches the system behavior
 *
 * We score a timezone setting according to the number of test times it
 * matches.  (The test times are ordered later-to-earlier, but this routine
 * doesn't actually know that; it just scans until the first non-match.)
 *
 * We return -1 for a completely unusable setting; this is worse than the
 * score of zero for a setting that works but matches not even the first
 * test time.
 */
static int
score_timezone(const char *tzname, struct tztry * tt)
{
	int			i;
	pg_time_t	pgtt;
	struct tm  *systm;
	struct pg_tm *pgtm;
	char		cbuf[TZ_STRLEN_MAX + 1];
	pg_tz		tz;


	/*
	 * Load timezone directly. Don't use pg_tzset, because we don't want all
	 * timezones loaded in the cache at startup.
	 */
	if (tzload(tzname, NULL, &tz.state, TRUE) != 0)
	{
		if (tzname[0] == ':' || tzparse(tzname, &tz.state, FALSE) != 0)
		{
			return -1;			/* can't handle the TZ name at all */
		}
	}

	/* Reject if leap seconds involved */
	if (!tz_acceptable(&tz))
	{
		elog(DEBUG4, "Reject TZ \"%s\": uses leap seconds", tzname);
		return -1;
	}

	/* Check for match at all the test times */
	for (i = 0; i < tt->n_test_times; i++)
	{
		pgtt = (pg_time_t) (tt->test_times[i]);
		pgtm = pg_localtime(&pgtt, &tz);
		if (!pgtm)
			return -1;			/* probably shouldn't happen */
		systm = localtime(&(tt->test_times[i]));
		if (!systm)
		{
			elog(DEBUG4, "TZ \"%s\" scores %d: at %ld %04d-%02d-%02d %02d:%02d:%02d %s, system had no data",
				 tzname, i, (long) pgtt,
				 pgtm->tm_year + 1900, pgtm->tm_mon + 1, pgtm->tm_mday,
				 pgtm->tm_hour, pgtm->tm_min, pgtm->tm_sec,
				 pgtm->tm_isdst ? "dst" : "std");
			return i;
		}
		if (!compare_tm(systm, pgtm))
		{
			elog(DEBUG4, "TZ \"%s\" scores %d: at %ld %04d-%02d-%02d %02d:%02d:%02d %s versus %04d-%02d-%02d %02d:%02d:%02d %s",
				 tzname, i, (long) pgtt,
				 pgtm->tm_year + 1900, pgtm->tm_mon + 1, pgtm->tm_mday,
				 pgtm->tm_hour, pgtm->tm_min, pgtm->tm_sec,
				 pgtm->tm_isdst ? "dst" : "std",
				 systm->tm_year + 1900, systm->tm_mon + 1, systm->tm_mday,
				 systm->tm_hour, systm->tm_min, systm->tm_sec,
				 systm->tm_isdst ? "dst" : "std");
			return i;
		}
		if (systm->tm_isdst >= 0)
		{
			/* Check match of zone names, too */
			if (pgtm->tm_zone == NULL)
				return -1;		/* probably shouldn't happen */
			memset(cbuf, 0, sizeof(cbuf));
			strftime(cbuf, sizeof(cbuf) - 1, "%Z", systm);		/* zone abbr */
			if (strcmp(cbuf, pgtm->tm_zone) != 0)
			{
				elog(DEBUG4, "TZ \"%s\" scores %d: at %ld \"%s\" versus \"%s\"",
					 tzname, i, (long) pgtt,
					 pgtm->tm_zone, cbuf);
				return i;
			}
		}
	}

	elog(DEBUG4, "TZ \"%s\" gets max score %d", tzname, i);
	return i;
}


/*
 * Try to identify a timezone name (in our terminology) that best matches the
 * observed behavior of the system timezone library.  We cannot assume that
 * the system TZ environment setting (if indeed there is one) matches our
 * terminology, so we ignore it and just look at what localtime() returns.
 */
static const char *
identify_system_timezone(void)
{
	static char resultbuf[TZ_STRLEN_MAX + 1];
	time_t		tnow;
	time_t		t;
	struct tztry tt;
	struct tm  *tm;
	int			thisyear;
	int			bestscore;
	char		tmptzdir[MAXPGPATH];
	int			std_ofs;
	char		std_zone_name[TZ_STRLEN_MAX + 1],
				dst_zone_name[TZ_STRLEN_MAX + 1];
	char		cbuf[TZ_STRLEN_MAX + 1];

	/* Initialize OS timezone library */
	tzset();

	/*
	 * Set up the list of dates to be probed to see how well our timezone
	 * matches the system zone.  We first probe January and July of the
	 * current year; this serves to quickly eliminate the vast majority of the
	 * TZ database entries.  If those dates match, we probe every week for 100
	 * years backwards from the current July.  (Weekly resolution is good
	 * enough to identify DST transition rules, since everybody switches on
	 * Sundays.)  This is sufficient to cover most of the Unix time_t range,
	 * and we don't want to look further than that since many systems won't
	 * have sane TZ behavior further back anyway.  The further back the zone
	 * matches, the better we score it.  This may seem like a rather random
	 * way of doing things, but experience has shown that system-supplied
	 * timezone definitions are likely to have DST behavior that is right for
	 * the recent past and not so accurate further back. Scoring in this way
	 * allows us to recognize zones that have some commonality with the zic
	 * database, without insisting on exact match. (Note: we probe Thursdays,
	 * not Sundays, to avoid triggering DST-transition bugs in localtime
	 * itself.)
	 */
	tnow = time(NULL);
	tm = localtime(&tnow);
	if (!tm)
		return NULL;			/* give up if localtime is broken... */
	thisyear = tm->tm_year + 1900;

	t = build_time_t(thisyear, 1, 15);

	/*
	 * Round back to GMT midnight Thursday.  This depends on the knowledge
	 * that the time_t origin is Thu Jan 01 1970.  (With a different origin
	 * we'd be probing some other day of the week, but it wouldn't matter
	 * anyway unless localtime() had DST-transition bugs.)
	 */
	t -= (t % T_WEEK);

	tt.n_test_times = 0;
	tt.test_times[tt.n_test_times++] = t;

	t = build_time_t(thisyear, 7, 15);
	t -= (t % T_WEEK);

	tt.test_times[tt.n_test_times++] = t;

	while (tt.n_test_times < MAX_TEST_TIMES)
	{
		t -= T_WEEK;
		tt.test_times[tt.n_test_times++] = t;
	}

	/* Search for the best-matching timezone file */
	strcpy(tmptzdir, pg_TZDIR());
	bestscore = -1;
	resultbuf[0] = '\0';
	scan_available_timezones(tmptzdir, tmptzdir + strlen(tmptzdir) + 1,
							 &tt,
							 &bestscore, resultbuf);
	if (bestscore > 0)
	{
		/* Ignore zic's rather silly "Factory" time zone; use GMT instead */
		if (strcmp(resultbuf, "Factory") == 0)
			return NULL;
		return resultbuf;
	}

	/*
	 * Couldn't find a match in the database, so next we try constructed zone
	 * names (like "PST8PDT").
	 *
	 * First we need to determine the names of the local standard and daylight
	 * zones.  The idea here is to scan forward from today until we have seen
	 * both zones, if both are in use.
	 */
	memset(std_zone_name, 0, sizeof(std_zone_name));
	memset(dst_zone_name, 0, sizeof(dst_zone_name));
	std_ofs = 0;

	tnow = time(NULL);

	/*
	 * Round back to a GMT midnight so results don't depend on local time of
	 * day
	 */
	tnow -= (tnow % T_DAY);

	/*
	 * We have to look a little further ahead than one year, in case today is
	 * just past a DST boundary that falls earlier in the year than the next
	 * similar boundary.  Arbitrarily scan up to 14 months.
	 */
	for (t = tnow; t <= tnow + T_MONTH * 14; t += T_MONTH)
	{
		tm = localtime(&t);
		if (!tm)
			continue;
		if (tm->tm_isdst < 0)
			continue;
		if (tm->tm_isdst == 0 && std_zone_name[0] == '\0')
		{
			/* found STD zone */
			memset(cbuf, 0, sizeof(cbuf));
			strftime(cbuf, sizeof(cbuf) - 1, "%Z", tm); /* zone abbr */
			strcpy(std_zone_name, cbuf);
			std_ofs = get_timezone_offset(tm);
		}
		if (tm->tm_isdst > 0 && dst_zone_name[0] == '\0')
		{
			/* found DST zone */
			memset(cbuf, 0, sizeof(cbuf));
			strftime(cbuf, sizeof(cbuf) - 1, "%Z", tm); /* zone abbr */
			strcpy(dst_zone_name, cbuf);
		}
		/* Done if found both */
		if (std_zone_name[0] && dst_zone_name[0])
			break;
	}

	/* We should have found a STD zone name by now... */
	if (std_zone_name[0] == '\0')
	{
		ereport(LOG,
				(errmsg("could not determine system time zone"),
				 errdetail("The PostgreSQL time zone will be set to \"%s\".",
						   "GMT"),
		errhint("You can specify the correct timezone in postgresql.conf.")));
		return NULL;			/* go to GMT */
	}

	/* If we found DST then try STD<ofs>DST */
	if (dst_zone_name[0] != '\0')
	{
		snprintf(resultbuf, sizeof(resultbuf), "%s%d%s",
				 std_zone_name, -std_ofs / 3600, dst_zone_name);
		if (score_timezone(resultbuf, &tt) > 0)
			return resultbuf;
	}

	/* Try just the STD timezone (works for GMT at least) */
	strcpy(resultbuf, std_zone_name);
	if (score_timezone(resultbuf, &tt) > 0)
		return resultbuf;

	/* Try STD<ofs> */
	snprintf(resultbuf, sizeof(resultbuf), "%s%d",
			 std_zone_name, -std_ofs / 3600);
	if (score_timezone(resultbuf, &tt) > 0)
		return resultbuf;

	/*
	 * Did not find the timezone.  Fallback to use a GMT zone.	Note that the
	 * zic timezone database names the GMT-offset zones in POSIX style: plus
	 * is west of Greenwich.  It's unfortunate that this is opposite of SQL
	 * conventions.  Should we therefore change the names? Probably not...
	 */
	snprintf(resultbuf, sizeof(resultbuf), "Etc/GMT%s%d",
			 (-std_ofs > 0) ? "+" : "", -std_ofs / 3600);

	ereport(LOG,
			(errmsg("could not recognize system time zone"),
			 errdetail("The PostgreSQL time zone will be set to \"%s\".",
					   resultbuf),
	   errhint("You can specify the correct timezone in postgresql.conf.")));
	return resultbuf;
}

/*
 * Recursively scan the timezone database looking for the best match to
 * the system timezone behavior.
 *
 * tzdir points to a buffer of size MAXPGPATH.	On entry, it holds the
 * pathname of a directory containing TZ files.  We internally modify it
 * to hold pathnames of sub-directories and files, but must restore it
 * to its original contents before exit.
 *
 * tzdirsub points to the part of tzdir that represents the subfile name
 * (ie, tzdir + the original directory name length, plus one for the
 * first added '/').
 *
 * tt tells about the system timezone behavior we need to match.
 *
 * *bestscore and *bestzonename on entry hold the best score found so far
 * and the name of the best zone.  We overwrite them if we find a better
 * score.  bestzonename must be a buffer of length TZ_STRLEN_MAX + 1.
 */
static void
scan_available_timezones(char *tzdir, char *tzdirsub, struct tztry * tt,
						 int *bestscore, char *bestzonename)
{
	int			tzdir_orig_len = strlen(tzdir);
	DIR		   *dirdesc;
	struct dirent *direntry;

	dirdesc = AllocateDir(tzdir);
	if (!dirdesc)
	{
		ereport(LOG,
				(errcode_for_file_access(),
				 errmsg("could not open directory \"%s\": %m", tzdir)));
		return;
	}

	while ((direntry = ReadDir(dirdesc, tzdir)) != NULL)
	{
		struct stat statbuf;

		/* Ignore . and .., plus any other "hidden" files */
		if (direntry->d_name[0] == '.')
			continue;

		snprintf(tzdir + tzdir_orig_len, MAXPGPATH - tzdir_orig_len,
				 "/%s", direntry->d_name);

		if (stat(tzdir, &statbuf) != 0)
		{
			ereport(LOG,
					(errcode_for_file_access(),
					 errmsg("could not stat \"%s\": %m", tzdir)));
			tzdir[tzdir_orig_len] = '\0';
			continue;
		}

		if (S_ISDIR(statbuf.st_mode))
		{
			/* Recurse into subdirectory */
			scan_available_timezones(tzdir, tzdirsub, tt,
									 bestscore, bestzonename);
		}
		else
		{
			/* Load and test this file */
			int			score = score_timezone(tzdirsub, tt);

			if (score > *bestscore)
			{
				*bestscore = score;
				strlcpy(bestzonename, tzdirsub, TZ_STRLEN_MAX + 1);
			}
			else if (score == *bestscore)
			{
				/* Consider how to break a tie */
				if (strlen(tzdirsub) < strlen(bestzonename) ||
					(strlen(tzdirsub) == strlen(bestzonename) &&
					 strcmp(tzdirsub, bestzonename) < 0))
					strlcpy(bestzonename, tzdirsub, TZ_STRLEN_MAX + 1);
			}
		}

		/* Restore tzdir */
		tzdir[tzdir_orig_len] = '\0';
	}

	FreeDir(dirdesc);
}
#else							/* WIN32 */

static const struct
{
	const char *stdname;		/* Windows name of standard timezone */
	const char *dstname;		/* Windows name of daylight timezone */
	const char *pgtzname;		/* Name of pgsql timezone to map to */
}	win32_tzmap[] =

{
	/*
	 * This list was built from the contents of the registry at
	 * HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Time
	 * Zones on Windows 2003 R2.
	 *
	 * The zones have been matched to zic timezones by looking at the cities
	 * listed in the win32 display name (in the comment here) in most cases.
	 */
	{
		"Afghanistan Standard Time", "Afghanistan Daylight Time",
		"Asia/Kabul"
	},							/* (GMT+04:30) Kabul */
	{
		"Alaskan Standard Time", "Alaskan Daylight Time",
		"US/Alaska"
	},							/* (GMT-09:00) Alaska */
	{
		"Arab Standard Time", "Arab Daylight Time",
		"Asia/Kuwait"
	},							/* (GMT+03:00) Kuwait, Riyadh */
	{
		"Arabian Standard Time", "Arabian Daylight Time",
		"Asia/Muscat"
	},							/* (GMT+04:00) Abu Dhabi, Muscat */
	{
		"Arabic Standard Time", "Arabic Daylight Time",
		"Asia/Baghdad"
	},							/* (GMT+03:00) Baghdad */
	{
		"Argentina Standard Time", "Argentina Daylight Time",
		"America/Buenos_Aires"
	},							/* (GMT-03:00) Buenos Aires */
	{
		"Armenian Standard Time", "Armenian Daylight Time",
		"Asia/Yerevan"
	},							/* (GMT+04:00) Yerevan */
	{
		"Atlantic Standard Time", "Atlantic Daylight Time",
		"Canada/Atlantic"
	},							/* (GMT-04:00) Atlantic Time (Canada) */
	{
		"AUS Central Standard Time", "AUS Central Daylight Time",
		"Australia/Darwin"
	},							/* (GMT+09:30) Darwin */
	{
		"AUS Eastern Standard Time", "AUS Eastern Daylight Time",
		"Australia/Canberra"
	},							/* (GMT+10:00) Canberra, Melbourne, Sydney */
	{
		"Azerbaijan Standard Time", "Azerbaijan Daylight Time",
		"Asia/Baku"
	},							/* (GMT+04:00) Baku */
	{
		"Azores Standard Time", "Azores Daylight Time",
		"Atlantic/Azores"
	},							/* (GMT-01:00) Azores */
	{
		"Bangladesh Standard Time", "Bangladesh Daylight Time",
		"Asia/Dhaka"
	},							/* (GMT+06:00) Dhaka */
	{
		"Canada Central Standard Time", "Canada Central Daylight Time",
		"Canada/Saskatchewan"
	},							/* (GMT-06:00) Saskatchewan */
	{
		"Cape Verde Standard Time", "Cape Verde Daylight Time",
		"Atlantic/Cape_Verde"
	},							/* (GMT-01:00) Cape Verde Is. */
	{
		"Caucasus Standard Time", "Caucasus Daylight Time",
		"Asia/Baku"
	},							/* (GMT+04:00) Baku, Tbilisi, Yerevan */
	{
		"Cen. Australia Standard Time", "Cen. Australia Daylight Time",
		"Australia/Adelaide"
	},							/* (GMT+09:30) Adelaide */
	{
		"Central America Standard Time", "Central America Daylight Time",
		"CST6CDT"
	},							/* (GMT-06:00) Central America */
	{
		"Central Asia Standard Time", "Central Asia Daylight Time",
		"Asia/Dhaka"
	},							/* (GMT+06:00) Astana, Dhaka */
	{
		"Central Brazilian Standard Time", "Central Brazilian Daylight Time",
		"America/Cuiaba"
	},							/* (GMT-04:00) Cuiaba */
	{
		"Central Europe Standard Time", "Central Europe Daylight Time",
		"Europe/Belgrade"
	},							/* (GMT+01:00) Belgrade, Bratislava, Budapest,
								 * Ljubljana, Prague */
	{
		"Central European Standard Time", "Central European Daylight Time",
		"Europe/Sarajevo"
	},							/* (GMT+01:00) Sarajevo, Skopje, Warsaw,
								 * Zagreb */
	{
		"Central Pacific Standard Time", "Central Pacific Daylight Time",
		"Pacific/Noumea"
	},							/* (GMT+11:00) Magadan, Solomon Is., New
								 * Caledonia */
	{
		"Central Standard Time", "Central Daylight Time",
		"US/Central"
	},							/* (GMT-06:00) Central Time (US & Canada) */
	{
		"Central Standard Time (Mexico)", "Central Daylight Time (Mexico)",
		"America/Mexico_City"
	},							/* (GMT-06:00) Guadalajara, Mexico City,
								 * Monterrey - New */
	{
		"China Standard Time", "China Daylight Time",
		"Asia/Hong_Kong"
	},							/* (GMT+08:00) Beijing, Chongqing, Hong Kong,
								 * Urumqi */
	{
		"Dateline Standard Time", "Dateline Daylight Time",
		"Etc/GMT+12"
	},							/* (GMT-12:00) International Date Line West */
	{
		"E. Africa Standard Time", "E. Africa Daylight Time",
		"Africa/Nairobi"
	},							/* (GMT+03:00) Nairobi */
	{
		"E. Australia Standard Time", "E. Australia Daylight Time",
		"Australia/Brisbane"
	},							/* (GMT+10:00) Brisbane */
	{
		"E. Europe Standard Time", "E. Europe Daylight Time",
		"Europe/Bucharest"
	},							/* (GMT+02:00) Bucharest */
	{
		"E. South America Standard Time", "E. South America Daylight Time",
		"America/Araguaina"
	},							/* (GMT-03:00) Brasilia */
	{
		"Eastern Standard Time", "Eastern Daylight Time",
		"US/Eastern"
	},							/* (GMT-05:00) Eastern Time (US & Canada) */
	{
		"Egypt Standard Time", "Egypt Daylight Time",
		"Africa/Cairo"
	},							/* (GMT+02:00) Cairo */
	{
		"Ekaterinburg Standard Time", "Ekaterinburg Daylight Time",
		"Asia/Yekaterinburg"
	},							/* (GMT+05:00) Ekaterinburg */
	{
		"Fiji Standard Time", "Fiji Daylight Time",
		"Pacific/Fiji"
	},							/* (GMT+12:00) Fiji, Kamchatka, Marshall Is. */
	{
		"FLE Standard Time", "FLE Daylight Time",
		"Europe/Helsinki"
	},							/* (GMT+02:00) Helsinki, Kyiv, Riga, Sofia,
								 * Tallinn, Vilnius */
	{
		"Georgian Standard Time", "Georgian Daylight Time",
		"Asia/Tbilisi"
	},							/* (GMT+03:00) Tbilisi */
	{
		"GMT Standard Time", "GMT Daylight Time",
		"Europe/London"
	},							/* (GMT) Greenwich Mean Time : Dublin,
								 * Edinburgh, Lisbon, London */
	{
		"Greenland Standard Time", "Greenland Daylight Time",
		"America/Godthab"
	},							/* (GMT-03:00) Greenland */
	{
		"Greenwich Standard Time", "Greenwich Daylight Time",
		"Africa/Casablanca"
	},							/* (GMT) Casablanca, Monrovia */
	{
		"GTB Standard Time", "GTB Daylight Time",
		"Europe/Athens"
	},							/* (GMT+02:00) Athens, Istanbul, Minsk */
	{
		"Hawaiian Standard Time", "Hawaiian Daylight Time",
		"US/Hawaii"
	},							/* (GMT-10:00) Hawaii */
	{
		"India Standard Time", "India Daylight Time",
		"Asia/Calcutta"
	},							/* (GMT+05:30) Chennai, Kolkata, Mumbai, New
								 * Delhi */
	{
		"Iran Standard Time", "Iran Daylight Time",
		"Asia/Tehran"
	},							/* (GMT+03:30) Tehran */
	{
		"Jerusalem Standard Time", "Jerusalem Daylight Time",
		"Asia/Jerusalem"
	},							/* (GMT+02:00) Jerusalem */
	{
		"Jordan Standard Time", "Jordan Daylight Time",
		"Asia/Amman"
	},							/* (GMT+02:00) Amman */
	{
		"Kamchatka Standard Time", "Kamchatka Daylight Time",
		"Asia/Kamchatka"
	},							/* (GMT+12:00) Petropavlovsk-Kamchatsky */
	{
		"Korea Standard Time", "Korea Daylight Time",
		"Asia/Seoul"
	},							/* (GMT+09:00) Seoul */
	{
		"Mauritius Standard Time", "Mauritius Daylight Time",
		"Indian/Mauritius"
	},							/* (GMT+04:00) Port Louis */
	{
		"Mexico Standard Time", "Mexico Daylight Time",
		"America/Mexico_City"
	},							/* (GMT-06:00) Guadalajara, Mexico City,
								 * Monterrey */
	{
		"Mexico Standard Time 2", "Mexico Daylight Time 2",
		"America/Chihuahua"
	},							/* (GMT-07:00) Chihuahua, La Paz, Mazatlan */
	{
		"Mid-Atlantic Standard Time", "Mid-Atlantic Daylight Time",
		"Atlantic/South_Georgia"
	},							/* (GMT-02:00) Mid-Atlantic */
	{
		"Middle East Standard Time", "Middle East Daylight Time",
		"Asia/Beirut"
	},							/* (GMT+02:00) Beirut */
	{
		"Montevideo Standard Time", "Montevideo Daylight Time",
		"America/Montevideo"
	},							/* (GMT-03:00) Montevideo */
	{
		"Morocco Standard Time", "Morocco Daylight Time",
		"Africa/Casablanca"
	},							/* (GMT) Casablanca */
	{
		"Mountain Standard Time", "Mountain Daylight Time",
		"US/Mountain"
	},							/* (GMT-07:00) Mountain Time (US & Canada) */
	{
		"Mountain Standard Time (Mexico)", "Mountain Daylight Time (Mexico)",
		"America/Chihuahua"
	},							/* (GMT-07:00) Chihuahua, La Paz, Mazatlan -
								 * New */
	{
		"Myanmar Standard Time", "Myanmar Daylight Time",
		"Asia/Rangoon"
	},							/* (GMT+06:30) Rangoon */
	{
		"N. Central Asia Standard Time", "N. Central Asia Daylight Time",
		"Asia/Novosibirsk"
	},							/* (GMT+06:00) Novosibirsk */
	{
		"Namibia Standard Time", "Namibia Daylight Time",
		"Africa/Windhoek"
	},							/* (GMT+02:00) Windhoek */
	{
		"Nepal Standard Time", "Nepal Daylight Time",
		"Asia/Katmandu"
	},							/* (GMT+05:45) Kathmandu */
	{
		"New Zealand Standard Time", "New Zealand Daylight Time",
		"Pacific/Auckland"
	},							/* (GMT+12:00) Auckland, Wellington */
	{
		"Newfoundland Standard Time", "Newfoundland Daylight Time",
		"Canada/Newfoundland"
	},							/* (GMT-03:30) Newfoundland */
	{
		"North Asia East Standard Time", "North Asia East Daylight Time",
		"Asia/Irkutsk"
	},							/* (GMT+08:00) Irkutsk, Ulaan Bataar */
	{
		"North Asia Standard Time", "North Asia Daylight Time",
		"Asia/Krasnoyarsk"
	},							/* (GMT+07:00) Krasnoyarsk */
	{
		"Pacific SA Standard Time", "Pacific SA Daylight Time",
		"America/Santiago"
	},							/* (GMT-04:00) Santiago */
	{
		"Pacific Standard Time", "Pacific Daylight Time",
		"US/Pacific"
	},							/* (GMT-08:00) Pacific Time (US & Canada);
								 * Tijuana */
	{
		"Pacific Standard Time (Mexico)", "Pacific Daylight Time (Mexico)",
		"America/Tijuana"
	},							/* (GMT-08:00) Tijuana, Baja California */
	{
		"Pakistan Standard Time", "Pakistan Daylight Time",
		"Asia/Karachi"
	},							/* (GMT+05:00) Islamabad, Karachi */
	{
		"Paraguay Standard Time", "Paraguay Daylight Time",
		"America/Asuncion"
	},							/* (GMT-04:00) Asuncion */
	{
		"Romance Standard Time", "Romance Daylight Time",
		"Europe/Brussels"
	},							/* (GMT+01:00) Brussels, Copenhagen, Madrid,
								 * Paris */
	{
		"Russian Standard Time", "Russian Daylight Time",
		"Europe/Moscow"
	},							/* (GMT+03:00) Moscow, St. Petersburg,
								 * Volgograd */
	{
		"SA Eastern Standard Time", "SA Eastern Daylight Time",
		"America/Buenos_Aires"
	},							/* (GMT-03:00) Buenos Aires, Georgetown */
	{
		"SA Pacific Standard Time", "SA Pacific Daylight Time",
		"America/Bogota"
	},							/* (GMT-05:00) Bogota, Lima, Quito */
	{
		"SA Western Standard Time", "SA Western Daylight Time",
		"America/Caracas"
	},							/* (GMT-04:00) Caracas, La Paz */
	{
		"Samoa Standard Time", "Samoa Daylight Time",
		"Pacific/Midway"
	},							/* (GMT-11:00) Midway Island, Samoa */
	{
		"SE Asia Standard Time", "SE Asia Daylight Time",
		"Asia/Bangkok"
	},							/* (GMT+07:00) Bangkok, Hanoi, Jakarta */
	{
		"Malay Peninsula Standard Time", "Malay Peninsula Daylight Time",
		"Asia/Kuala_Lumpur"
	},							/* (GMT+08:00) Kuala Lumpur, Singapore */
	{
		"South Africa Standard Time", "South Africa Daylight Time",
		"Africa/Harare"
	},							/* (GMT+02:00) Harare, Pretoria */
	{
		"Sri Lanka Standard Time", "Sri Lanka Daylight Time",
		"Asia/Colombo"
	},							/* (GMT+06:00) Sri Jayawardenepura */
	{
		"Taipei Standard Time", "Taipei Daylight Time",
		"Asia/Taipei"
	},							/* (GMT+08:00) Taipei */
	{
		"Tasmania Standard Time", "Tasmania Daylight Time",
		"Australia/Hobart"
	},							/* (GMT+10:00) Hobart */
	{
		"Tokyo Standard Time", "Tokyo Daylight Time",
		"Asia/Tokyo"
	},							/* (GMT+09:00) Osaka, Sapporo, Tokyo */
	{
		"Tonga Standard Time", "Tonga Daylight Time",
		"Pacific/Tongatapu"
	},							/* (GMT+13:00) Nuku'alofa */
	{
		"Ulaanbaatar Standard Time", "Ulaanbaatar Daylight Time",
		"Asia/Ulaanbaatar",
	},							/* (GMT+08:00) Ulaanbaatar */
	{
		"US Eastern Standard Time", "US Eastern Daylight Time",
		"US/Eastern"
	},							/* (GMT-05:00) Indiana (East) */
	{
		"US Mountain Standard Time", "US Mountain Daylight Time",
		"US/Arizona"
	},							/* (GMT-07:00) Arizona */
	{
		"Coordinated Universal Time", "Coordinated Universal Time",
		"UTC"
	},							/* (GMT) Coordinated Universal Time */
	{
		"UTC+12", "UTC+12",
		"Etc/GMT+12"
	},							/* (GMT+12:00) Coordinated Universal Time+12 */
	{
		"UTC-02", "UTC-02",
		"Etc/GMT-02"
	},							/* (GMT-02:00) Coordinated Universal Time-02 */
	{
		"UTC-11", "UTC-11",
		"Etc/GMT-11"
	},							/* (GMT-11:00) Coordinated Universal Time-11 */
	{
		"Venezuela Standard Time", "Venezuela Daylight Time",
		"America/Caracas",
	},							/* (GMT-04:30) Caracas */
	{
		"Vladivostok Standard Time", "Vladivostok Daylight Time",
		"Asia/Vladivostok"
	},							/* (GMT+10:00) Vladivostok */
	{
		"W. Australia Standard Time", "W. Australia Daylight Time",
		"Australia/Perth"
	},							/* (GMT+08:00) Perth */
#ifdef NOT_USED
	/* Could not find a match for this one (just a guess). Excluded for now. */
	{
		"W. Central Africa Standard Time", "W. Central Africa Daylight Time",
		"WAT"
	},							/* (GMT+01:00) West Central Africa */
#endif
	{
		"W. Europe Standard Time", "W. Europe Daylight Time",
		"CET"
	},							/* (GMT+01:00) Amsterdam, Berlin, Bern, Rome,
								 * Stockholm, Vienna */
	{
		"West Asia Standard Time", "West Asia Daylight Time",
		"Asia/Karachi"
	},							/* (GMT+05:00) Islamabad, Karachi, Tashkent */
	{
		"West Pacific Standard Time", "West Pacific Daylight Time",
		"Pacific/Guam"
	},							/* (GMT+10:00) Guam, Port Moresby */
	{
		"Yakutsk Standard Time", "Yakutsk Daylight Time",
		"Asia/Yakutsk"
	},							/* (GMT+09:00) Yakutsk */
	{
		NULL, NULL, NULL
	}
};

static const char *
identify_system_timezone(void)
{
	int			i;
	char		tzname[128];
	char		localtzname[256];
	time_t		t = time(NULL);
	struct tm  *tm = localtime(&t);
	HKEY		rootKey;
	int			idx;

	if (!tm)
	{
		ereport(LOG,
		  (errmsg("could not identify system time zone: localtime() failed"),
		   errdetail("The PostgreSQL time zone will be set to \"%s\".",
					 "GMT"),
		errhint("You can specify the correct timezone in postgresql.conf.")));
		return NULL;			/* go to GMT */
	}

	memset(tzname, 0, sizeof(tzname));
	strftime(tzname, sizeof(tzname) - 1, "%Z", tm);

	for (i = 0; win32_tzmap[i].stdname != NULL; i++)
	{
		if (strcmp(tzname, win32_tzmap[i].stdname) == 0 ||
			strcmp(tzname, win32_tzmap[i].dstname) == 0)
		{
			elog(DEBUG4, "TZ \"%s\" matches system time zone \"%s\"",
				 win32_tzmap[i].pgtzname, tzname);
			return win32_tzmap[i].pgtzname;
		}
	}

	/*
	 * Localized Windows versions return localized names for the timezone.
	 * Scan the registry to find the English name, and then try matching
	 * against our table again.
	 */
	memset(localtzname, 0, sizeof(localtzname));
	if (RegOpenKeyEx(HKEY_LOCAL_MACHINE,
			   "SOFTWARE\\Microsoft\\Windows NT\\CurrentVersion\\Time Zones",
					 0,
					 KEY_READ,
					 &rootKey) != ERROR_SUCCESS)
	{
		ereport(LOG,
				(errmsg("could not open registry key to identify system time zone: error code %lu",
						GetLastError()),
				 errdetail("The PostgreSQL time zone will be set to \"%s\".",
						   "GMT"),
		errhint("You can specify the correct timezone in postgresql.conf.")));
		return NULL;			/* go to GMT */
	}

	for (idx = 0;; idx++)
	{
		char		keyname[256];
		char		zonename[256];
		DWORD		namesize;
		FILETIME	lastwrite;
		HKEY		key;
		LONG		r;

		memset(keyname, 0, sizeof(keyname));
		namesize = sizeof(keyname);
		if ((r = RegEnumKeyEx(rootKey,
							  idx,
							  keyname,
							  &namesize,
							  NULL,
							  NULL,
							  NULL,
							  &lastwrite)) != ERROR_SUCCESS)
		{
			if (r == ERROR_NO_MORE_ITEMS)
				break;
			ereport(LOG,
					(errmsg_internal("could not enumerate registry subkeys to identify system time zone: %d", (int) r)));
			break;
		}

		if ((r = RegOpenKeyEx(rootKey, keyname, 0, KEY_READ, &key)) != ERROR_SUCCESS)
		{
			ereport(LOG,
					(errmsg_internal("could not open registry subkey to identify system time zone: %d", (int) r)));
			break;
		}

		memset(zonename, 0, sizeof(zonename));
		namesize = sizeof(zonename);
		if ((r = RegQueryValueEx(key, "Std", NULL, NULL, (unsigned char *) zonename, &namesize)) != ERROR_SUCCESS)
		{
			ereport(LOG,
					(errmsg_internal("could not query value for key \"std\" to identify system time zone \"%s\": %d",
									 keyname, (int) r)));
			RegCloseKey(key);
			continue;			/* Proceed to look at the next timezone */
		}
		if (strcmp(tzname, zonename) == 0)
		{
			/* Matched zone */
			strcpy(localtzname, keyname);
			RegCloseKey(key);
			break;
		}
		memset(zonename, 0, sizeof(zonename));
		namesize = sizeof(zonename);
		if ((r = RegQueryValueEx(key, "Dlt", NULL, NULL, (unsigned char *) zonename, &namesize)) != ERROR_SUCCESS)
		{
			ereport(LOG,
					(errmsg_internal("could not query value for key \"dlt\" to identify system time zone \"%s\": %d",
									 keyname, (int) r)));
			RegCloseKey(key);
			continue;			/* Proceed to look at the next timezone */
		}
		if (strcmp(tzname, zonename) == 0)
		{
			/* Matched DST zone */
			strcpy(localtzname, keyname);
			RegCloseKey(key);
			break;
		}

		RegCloseKey(key);
	}

	RegCloseKey(rootKey);

	if (localtzname[0])
	{
		/* Found a localized name, so scan for that one too */
		for (i = 0; win32_tzmap[i].stdname != NULL; i++)
		{
			if (strcmp(localtzname, win32_tzmap[i].stdname) == 0 ||
				strcmp(localtzname, win32_tzmap[i].dstname) == 0)
			{
				elog(DEBUG4, "TZ \"%s\" matches localized system time zone \"%s\" (\"%s\")",
					 win32_tzmap[i].pgtzname, tzname, localtzname);
				return win32_tzmap[i].pgtzname;
			}
		}
	}

	ereport(LOG,
			(errmsg("could not find a match for system time zone \"%s\"",
					tzname),
			 errdetail("The PostgreSQL time zone will be set to \"%s\".",
					   "GMT"),
	   errhint("You can specify the correct timezone in postgresql.conf.")));
	return NULL;				/* go to GMT */
}
#endif   /* WIN32 */



/*
 * We keep loaded timezones in a hashtable so we don't have to
 * load and parse the TZ definition file every time one is selected.
 * Because we want timezone names to be found case-insensitively,
 * the hash key is the uppercased name of the zone.
 */
typedef struct
{
	/* tznameupper contains the all-upper-case name of the timezone */
	char		tznameupper[TZ_STRLEN_MAX + 1];
	pg_tz		tz;
} pg_tz_cache;

static HTAB *timezone_cache = NULL;


static bool
init_timezone_hashtable(void)
{
	HASHCTL		hash_ctl;

	MemSet(&hash_ctl, 0, sizeof(hash_ctl));

	hash_ctl.keysize = TZ_STRLEN_MAX + 1;
	hash_ctl.entrysize = sizeof(pg_tz_cache);

	timezone_cache = hash_create("Timezones",
								 4,
								 &hash_ctl,
								 HASH_ELEM);
	if (!timezone_cache)
		return false;

	return true;
}

/*
 * Load a timezone from file or from cache.
 * Does not verify that the timezone is acceptable!
 */
struct pg_tz *
pg_tzset(const char *name)
{
	pg_tz_cache *tzp;
	struct state tzstate;
	char		uppername[TZ_STRLEN_MAX + 1];
	char		canonname[TZ_STRLEN_MAX + 1];
	char	   *p;

	if (strlen(name) > TZ_STRLEN_MAX)
		return NULL;			/* not going to fit */

	if (!timezone_cache)
		if (!init_timezone_hashtable())
			return NULL;

	/*
	 * Upcase the given name to perform a case-insensitive hashtable search.
	 * (We could alternatively downcase it, but we prefer upcase so that we
	 * can get consistently upcased results from tzparse() in case the name is
	 * a POSIX-style timezone spec.)
	 */
	p = uppername;
	while (*name)
		*p++ = pg_toupper((unsigned char) *name++);
	*p = '\0';

	tzp = (pg_tz_cache *) hash_search(timezone_cache,
									  uppername,
									  HASH_FIND,
									  NULL);
	if (tzp)
	{
		/* Timezone found in cache, nothing more to do */
		return &tzp->tz;
	}

	if (tzload(uppername, canonname, &tzstate, TRUE) != 0)
	{
		if (uppername[0] == ':' || tzparse(uppername, &tzstate, FALSE) != 0)
		{
			/* Unknown timezone. Fail our call instead of loading GMT! */
			return NULL;
		}
		/* For POSIX timezone specs, use uppercase name as canonical */
		strcpy(canonname, uppername);
	}

	/* Save timezone in the cache */
	tzp = (pg_tz_cache *) hash_search(timezone_cache,
									  uppername,
									  HASH_ENTER,
									  NULL);

	/* hash_search already copied uppername into the hash key */
	strcpy(tzp->tz.TZname, canonname);
	memcpy(&tzp->tz.state, &tzstate, sizeof(tzstate));

	return &tzp->tz;
}


/*
 * Check whether timezone is acceptable.
 *
 * What we are doing here is checking for leap-second-aware timekeeping.
 * We need to reject such TZ settings because they'll wreak havoc with our
 * date/time arithmetic.
 *
 * NB: this must NOT ereport(ERROR).  The caller must get control back so that
 * it can restore the old value of TZ if we don't like the new one.
 */
bool
tz_acceptable(pg_tz *tz)
{
	struct pg_tm *tt;
	pg_time_t	time2000;

	/*
	 * To detect leap-second timekeeping, run pg_localtime for what should be
	 * GMT midnight, 2000-01-01.  Insist that the tm_sec value be zero; any
	 * other result has to be due to leap seconds.
	 */
	time2000 = (POSTGRES_EPOCH_JDATE - UNIX_EPOCH_JDATE) * SECS_PER_DAY;
	tt = pg_localtime(&time2000, tz);
	if (!tt || tt->tm_sec != 0)
		return false;

	return true;
}


/*
 * Get a pg_tz struct for the given timezone name.	Returns NULL if name
 * is invalid or not an "acceptable" zone.
 */
static pg_tz *
get_pg_tz_for_zone(const char *tzname)
{
	pg_tz	   *tz;

	if (!tzname || !tzname[0])
		return NULL;

	tz = pg_tzset(tzname);
	if (!tz)
		return NULL;

	if (!tz_acceptable(tz))
		return NULL;

	return tz;
}

/*
 * Identify a suitable default timezone setting based on the environment.
 *
 * We first look to the TZ environment variable.  If not found or not
 * recognized by our own code, we see if we can identify the timezone
 * from the behavior of the system timezone library.  When all else fails,
 * fall back to GMT.
 */
static pg_tz *
select_default_timezone(void)
{
	pg_tz	   *def_tz;

	def_tz = get_pg_tz_for_zone(getenv("TZ"));
	if (def_tz)
		return def_tz;

	def_tz = get_pg_tz_for_zone(identify_system_timezone());
	if (def_tz)
		return def_tz;

	def_tz = get_pg_tz_for_zone("GMT");
	if (def_tz)
		return def_tz;

	ereport(FATAL,
			(errmsg("could not select a suitable default time zone"),
			 errdetail("It appears that your GMT time zone uses leap seconds. PostgreSQL does not support leap seconds.")));
	return NULL;				/* keep compiler quiet */
}


/*
 * Pre-initialize timezone library
 *
 * This is called before GUC variable initialization begins.  Its purpose
 * is to ensure that elog.c has a pgtz variable available to format timestamps
 * with, in case log_line_prefix is set to a value requiring that.	We cannot
 * set log_timezone yet.
 */
void
pg_timezone_pre_initialize(void)
{
	/*
	 * We can't use tzload() because we may not know where PGSHAREDIR is (in
	 * particular this is true in an EXEC_BACKEND subprocess). Since this
	 * timezone variable will only be used for emergency fallback purposes, it
	 * seems OK to just use the "lastditch" case provided by tzparse().
	 */
	if (tzparse("GMT", &gmt_timezone_data.state, TRUE) != 0)
		elog(FATAL, "could not initialize GMT time zone");
	strcpy(gmt_timezone_data.TZname, "GMT");
	gmt_timezone = &gmt_timezone_data;
}

/*
 * Initialize timezone library
 *
 * This is called after initial loading of postgresql.conf.  If no TimeZone
 * setting was found therein, we try to derive one from the environment.
 * Likewise for log_timezone.
 *
 * Note: this is also called from ProcessConfigFile, to re-establish valid
 * GUC settings if the GUCs have been reset to default following their
 * removal from postgresql.conf.
 */
void
pg_timezone_initialize(void)
{
	pg_tz	   *def_tz = NULL;

	/*
	 * Make sure that session_timezone and log_timezone are set.
	 * (session_timezone could still be NULL even if a timezone value was set
	 * in postgresql.conf, if that setting was interval-based rather than
	 * timezone-based.)
	 */
	if (!session_timezone)
	{
		def_tz = select_default_timezone();
		session_timezone = def_tz;
	}
	if (!log_timezone)
	{
		/* Don't duplicate work */
		if (!def_tz)
			def_tz = select_default_timezone();
		log_timezone = def_tz;
	}

	/*
	 * Now, set the timezone and log_timezone GUCs if they're still default.
	 * (This will redundantly call pg_tzset().)
	 *
	 * We choose to label these values PGC_S_ENV_VAR, rather than
	 * PGC_S_DYNAMIC_DEFAULT which would be functionally equivalent, because
	 * they came either from getenv("TZ") or from libc behavior that's
	 * determined by process environment of some kind.
	 *
	 * Note: in the case where a setting has just been removed from
	 * postgresql.conf, this code will not do what you might expect, namely
	 * call select_default_timezone() and install that value as the setting.
	 * Rather, the previously active setting --- typically the one from
	 * postgresql.conf --- will be reinstalled, relabeled as PGC_S_ENV_VAR. If
	 * we did try to install the "correct" default value, the effect would be
	 * that each postmaster child would independently run an extremely
	 * expensive search of the timezone database, bringing the database to its
	 * knees for possibly multiple seconds.  This is so unpleasant, and could
	 * so easily be triggered quite unintentionally, that it seems better to
	 * violate the principle of least astonishment.
	 */
	if (GetConfigOptionResetString("timezone") == NULL)
		SetConfigOption("timezone", pg_get_timezone_name(session_timezone),
						PGC_POSTMASTER, PGC_S_ENV_VAR);

	if (GetConfigOptionResetString("log_timezone") == NULL)
		SetConfigOption("log_timezone", pg_get_timezone_name(log_timezone),
						PGC_POSTMASTER, PGC_S_ENV_VAR);
}


/*
 * Functions to enumerate available timezones
 *
 * Note that pg_tzenumerate_next() will return a pointer into the pg_tzenum
 * structure, so the data is only valid up to the next call.
 *
 * All data is allocated using palloc in the current context.
 */
#define MAX_TZDIR_DEPTH 10

struct pg_tzenum
{
	int			baselen;
	int			depth;
	DIR		   *dirdesc[MAX_TZDIR_DEPTH];
	char	   *dirname[MAX_TZDIR_DEPTH];
	struct pg_tz tz;
};

/* typedef pg_tzenum is declared in pgtime.h */

pg_tzenum *
pg_tzenumerate_start(void)
{
	pg_tzenum  *ret = (pg_tzenum *) palloc0(sizeof(pg_tzenum));
	char	   *startdir = pstrdup(pg_TZDIR());

	ret->baselen = strlen(startdir) + 1;
	ret->depth = 0;
	ret->dirname[0] = startdir;
	ret->dirdesc[0] = AllocateDir(startdir);
	if (!ret->dirdesc[0])
		ereport(ERROR,
				(errcode_for_file_access(),
				 errmsg("could not open directory \"%s\": %m", startdir)));
	return ret;
}

void
pg_tzenumerate_end(pg_tzenum *dir)
{
	while (dir->depth >= 0)
	{
		FreeDir(dir->dirdesc[dir->depth]);
		pfree(dir->dirname[dir->depth]);
		dir->depth--;
	}
	pfree(dir);
}

pg_tz *
pg_tzenumerate_next(pg_tzenum *dir)
{
	while (dir->depth >= 0)
	{
		struct dirent *direntry;
		char		fullname[MAXPGPATH];
		struct stat statbuf;

		direntry = ReadDir(dir->dirdesc[dir->depth], dir->dirname[dir->depth]);

		if (!direntry)
		{
			/* End of this directory */
			FreeDir(dir->dirdesc[dir->depth]);
			pfree(dir->dirname[dir->depth]);
			dir->depth--;
			continue;
		}

		if (direntry->d_name[0] == '.')
			continue;

		snprintf(fullname, MAXPGPATH, "%s/%s",
				 dir->dirname[dir->depth], direntry->d_name);
		if (stat(fullname, &statbuf) != 0)
			ereport(ERROR,
					(errcode_for_file_access(),
					 errmsg("could not stat \"%s\": %m", fullname)));

		if (S_ISDIR(statbuf.st_mode))
		{
			/* Step into the subdirectory */
			if (dir->depth >= MAX_TZDIR_DEPTH - 1)
				ereport(ERROR,
					 (errmsg_internal("timezone directory stack overflow")));
			dir->depth++;
			dir->dirname[dir->depth] = pstrdup(fullname);
			dir->dirdesc[dir->depth] = AllocateDir(fullname);
			if (!dir->dirdesc[dir->depth])
				ereport(ERROR,
						(errcode_for_file_access(),
						 errmsg("could not open directory \"%s\": %m",
								fullname)));

			/* Start over reading in the new directory */
			continue;
		}

		/*
		 * Load this timezone using tzload() not pg_tzset(), so we don't fill
		 * the cache
		 */
		if (tzload(fullname + dir->baselen, dir->tz.TZname, &dir->tz.state,
				   TRUE) != 0)
		{
			/* Zone could not be loaded, ignore it */
			continue;
		}

		if (!tz_acceptable(&dir->tz))
		{
			/* Ignore leap-second zones */
			continue;
		}

		/* Timezone loaded OK. */
		return &dir->tz;
	}

	/* Nothing more found */
	return NULL;
}