Make timetz_zone() stable, and correct a bug for DYNTZ abbreviations.

Historically, timetz_zone() has used time(NULL) as the reference point
for deciding whether DST is active.  That means its result can change
intra-statement, requiring it to be marked VOLATILE (cf. 35979e6).
But that definition is pretty inconsistent with the way we deal with
timestamps elsewhere.  Let's make it use the transaction start time
("now()") as the reference point instead.  That lets it be marked
STABLE, and also saves a kernel call per invocation.

While at it, remove the function's use of pg_time_t and pg_localtime.
Those are inconsistent with the other code in this area, which indeed
created a bug: timetz_zone() delivered completely wrong answers if
the zone was specified by a dynamic TZ abbreviation.  (We need to do
something about that in the back branches, but the fix will look
different from this.)

Aleksander Alekseev and Tom Lane

Discussion: https://postgr.es/m/CAJ7c6TOMG8zSNEZtCn5SPe+cCk3Lfxb71ZaQwT2F4T7PJ_t=KA@mail.gmail.com

Author: tglsfdc

src/backend/utils/adt/date.c

@@ -1481,9 +1481,7 @@ float_time_overflows(int hour, int min, double sec)
 /* time2tm()
  * Convert time data type to POSIX time structure.
  *
- * For dates within the range of pg_time_t, convert to the local time zone.
- * If out of this range, leave as UTC (in practice that could only happen
- * if pg_time_t is just 32 bits) - thomas 97/05/27
+ * Note that only the hour/min/sec/fractional-sec fields are filled in.
  */
 int
 time2tm(TimeADT time, struct pg_tm *tm, fsec_t *fsec)
@@ -3029,7 +3027,7 @@ extract_timetz(PG_FUNCTION_ARGS)
 
 /* timetz_zone()
  * Encode time with time zone type with specified time zone.
- * Applies DST rules as of the current date.
+ * Applies DST rules as of the transaction start time.
  */
 Datum
 timetz_zone(PG_FUNCTION_ARGS)
@@ -3068,25 +3066,27 @@ timetz_zone(PG_FUNCTION_ARGS)
 	}
 	else if (type == DYNTZ)
 	{
-		/* dynamic-offset abbreviation, resolve using current time */
-		pg_time_t	now = (pg_time_t) time(NULL);
-		struct pg_tm *tm;
+		/* dynamic-offset abbreviation, resolve using transaction start time */
+		TimestampTz now = GetCurrentTransactionStartTimestamp();
+		int			isdst;
 
-		tm = pg_localtime(&now, tzp);
-		tz = DetermineTimeZoneAbbrevOffset(tm, tzname, tzp);
+		tz = DetermineTimeZoneAbbrevOffsetTS(now, tzname, tzp, &isdst);
 	}
 	else
 	{
 		/* try it as a full zone name */
 		tzp = pg_tzset(tzname);
 		if (tzp)
 		{
-			/* Get the offset-from-GMT that is valid today for the zone */
-			pg_time_t	now = (pg_time_t) time(NULL);
-			struct pg_tm *tm;
+			/* Get the offset-from-GMT that is valid now for the zone */
+			TimestampTz now = GetCurrentTransactionStartTimestamp();
+			struct pg_tm tm;
+			fsec_t		fsec;
 
-			tm = pg_localtime(&now, tzp);
-			tz = -tm->tm_gmtoff;
+			if (timestamp2tm(now, &tz, &tm, &fsec, NULL, tzp) != 0)
+				ereport(ERROR,
+						(errcode(ERRCODE_DATETIME_VALUE_OUT_OF_RANGE),
+						 errmsg("timestamp out of range")));
 		}
 		else
 		{

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202109032
+#define CATALOG_VERSION_NO	202109061
 
 #endif

src/include/catalog/pg_proc.dat

@@ -5953,7 +5953,7 @@
   proname => 'timestamp_larger', prorettype => 'timestamp',
   proargtypes => 'timestamp timestamp', prosrc => 'timestamp_larger' },
 { oid => '2037', descr => 'adjust time with time zone to new zone',
-  proname => 'timezone', provolatile => 'v', prorettype => 'timetz',
+  proname => 'timezone', provolatile => 's', prorettype => 'timetz',
   proargtypes => 'text timetz', prosrc => 'timetz_zone' },
 { oid => '2038', descr => 'adjust time with time zone to new zone',
   proname => 'timezone', prorettype => 'timetz',