Fix some odd behaviors when using a SQL-style simple GMT offset timezone.

Formerly, when using a SQL-spec timezone setting with a fixed GMT offset
(called a "brute force" timezone in the code), the session_timezone
variable was not updated to match the nominal timezone; rather, all code
was expected to ignore session_timezone if HasCTZSet was true.  This is
of course obviously fragile, though a search of the code finds only
timeofday() failing to honor the rule.  A bigger problem was that
DetermineTimeZoneOffset() supposed that if its pg_tz parameter was
pointer-equal to session_timezone, then HasCTZSet should override the
parameter.  This would cause datetime input containing an explicit zone
name to be treated as referencing the brute-force zone instead, if the
zone name happened to match the session timezone that had prevailed
before installing the brute-force zone setting (as reported in bug #8572).
The same malady could affect AT TIME ZONE operators.

To fix, set up session_timezone so that it matches the brute-force zone
specification, which we can do using the POSIX timezone definition syntax
"<abbrev>offset", and get rid of the bogus lookaside check in
DetermineTimeZoneOffset().  Aside from fixing the erroneous behavior in
datetime parsing and AT TIME ZONE, this will cause the timeofday() function
to print its result in the user-requested time zone rather than some
previously-set zone.  It might also affect results in third-party
extensions, if there are any that make use of session_timezone without
considering HasCTZSet, but in all cases the new behavior should be saner
than before.

Back-patch to all supported branches.

src/backend/commands/variable.c

@@ -327,6 +327,7 @@ check_timezone(char **newval, void **extra, GucSource source)
 #else
 		myextra.CTimeZone = -interval->time;
 #endif
+		myextra.session_timezone = pg_tzset_offset(myextra.CTimeZone);
 		myextra.HasCTZSet = true;
 
 		pfree(interval);
@@ -341,6 +342,7 @@ check_timezone(char **newval, void **extra, GucSource source)
 		{
 			/* Here we change from SQL to Unix sign convention */
 			myextra.CTimeZone = -hours * SECS_PER_HOUR;
+			myextra.session_timezone = pg_tzset_offset(myextra.CTimeZone);
 			myextra.HasCTZSet = true;
 		}
 		else

src/backend/utils/adt/datetime.c

@@ -1465,12 +1465,6 @@ DetermineTimeZoneOffset(struct pg_tm * tm, pg_tz *tzp)
 				after_isdst;
 	int			res;
 
-	if (tzp == session_timezone && HasCTZSet)
-	{
-		tm->tm_isdst = 0;		/* for lack of a better idea */
-		return CTimeZone;
-	}
-
 	/*
 	 * First, generate the pg_time_t value corresponding to the given
 	 * y/m/d/h/m/s taken as GMT time.  If this overflows, punt and decide the

src/include/pgtime.h

@@ -68,6 +68,7 @@ extern pg_tz *log_timezone;
 
 extern void pg_timezone_initialize(void);
 extern pg_tz *pg_tzset(const char *tzname);
+extern pg_tz *pg_tzset_offset(long gmtoffset);
 
 extern pg_tzenum *pg_tzenumerate_start(void);
 extern pg_tz *pg_tzenumerate_next(pg_tzenum *dir);

src/test/regress/expected/horology.out

@@ -2935,3 +2935,33 @@ DETAIL:  Value must be an integer.
 SELECT to_timestamp('10000000000', 'FMYYYY');
 ERROR:  value for "YYYY" in source string is out of range
 DETAIL:  Value must be in the range -2147483648 to 2147483647.
+--
+-- Check behavior with SQL-style fixed-GMT-offset time zone (cf bug #8572)
+--
+SET TIME ZONE 'America/New_York';
+SET TIME ZONE '-1.5';
+SHOW TIME ZONE;
+       TimeZone       
+----------------------
+ @ 1 hour 30 mins ago
+(1 row)
+
+SELECT '2012-12-12 12:00'::timestamptz;
+           timestamptz           
+---------------------------------
+ Wed Dec 12 12:00:00 2012 -01:30
+(1 row)
+
+SELECT '2012-12-12 12:00 America/New_York'::timestamptz;
+           timestamptz           
+---------------------------------
+ Wed Dec 12 15:30:00 2012 -01:30
+(1 row)
+
+SELECT to_char('2012-12-12 12:00'::timestamptz, 'YYYY-MM-DD HH:MI:SS TZ');
+       to_char        
+----------------------
+ 2012-12-12 12:00:00 
+(1 row)
+
+RESET TIME ZONE;

src/test/regress/sql/horology.sql

@@ -461,3 +461,19 @@ SELECT to_timestamp('199711xy', 'YYYYMMDD');
 
 -- Input that doesn't fit in an int:
 SELECT to_timestamp('10000000000', 'FMYYYY');
+
+--
+-- Check behavior with SQL-style fixed-GMT-offset time zone (cf bug #8572)
+--
+
+SET TIME ZONE 'America/New_York';
+SET TIME ZONE '-1.5';
+
+SHOW TIME ZONE;
+
+SELECT '2012-12-12 12:00'::timestamptz;
+SELECT '2012-12-12 12:00 America/New_York'::timestamptz;
+
+SELECT to_char('2012-12-12 12:00'::timestamptz, 'YYYY-MM-DD HH:MI:SS TZ');
+
+RESET TIME ZONE;

src/timezone/pgtz.c

@@ -288,6 +288,46 @@ pg_tzset(const char *name)
 	return &tzp->tz;
 }
 
+/*
+ * Load a fixed-GMT-offset timezone.
+ * This is used for SQL-spec SET TIME ZONE INTERVAL 'foo' cases.
+ * It's otherwise equivalent to pg_tzset().
+ *
+ * The GMT offset is specified in seconds, positive values meaning west of
+ * Greenwich (ie, POSIX not ISO sign convention).  However, we use ISO
+ * sign convention in the displayable abbreviation for the zone.
+ */
+pg_tz *
+pg_tzset_offset(long gmtoffset)
+{
+	long		absoffset = (gmtoffset < 0) ? -gmtoffset : gmtoffset;
+	char		offsetstr[64];
+	char		tzname[128];
+
+	snprintf(offsetstr, sizeof(offsetstr),
+			 "%02ld", absoffset / SECSPERHOUR);
+	absoffset %= SECSPERHOUR;
+	if (absoffset != 0)
+	{
+		snprintf(offsetstr + strlen(offsetstr),
+				 sizeof(offsetstr) - strlen(offsetstr),
+				 ":%02ld", absoffset / SECSPERMIN);
+		absoffset %= SECSPERMIN;
+		if (absoffset != 0)
+			snprintf(offsetstr + strlen(offsetstr),
+					 sizeof(offsetstr) - strlen(offsetstr),
+					 ":%02ld", absoffset);
+	}
+	if (gmtoffset > 0)
+		snprintf(tzname, sizeof(tzname), "<-%s>+%s",
+				 offsetstr, offsetstr);
+	else
+		snprintf(tzname, sizeof(tzname), "<+%s>-%s",
+				 offsetstr, offsetstr);
+
+	return pg_tzset(tzname);
+}
+
 
 /*
  * Initialize timezone library