Const-ify a couple of datetime parsing subroutines.

More could be done in this line, but I just grabbed some low-hanging
fruit.  Principal objective was to remove the need for several ugly
unconstify() usages in formatting.c.

Author: tglsfdc

src/backend/utils/adt/datetime.c

@@ -3145,7 +3145,7 @@ DecodeNumberField(int len, char *str, int fmask,
  * Return 0 if okay (and set *tzp), a DTERR code if not okay.
  */
 int
-DecodeTimezone(char *str, int *tzp)
+DecodeTimezone(const char *str, int *tzp)
 {
 	int			tz;
 	int			hr,
@@ -3223,7 +3223,7 @@ DecodeTimezone(char *str, int *tzp)
  *	will be related in format.
  */
 int
-DecodeTimezoneAbbrev(int field, char *lowtoken,
+DecodeTimezoneAbbrev(int field, const char *lowtoken,
 					 int *offset, pg_tz **tz)
 {
 	int			type;
@@ -3278,7 +3278,7 @@ DecodeTimezoneAbbrev(int field, char *lowtoken,
  *	will be related in format.
  */
 int
-DecodeSpecial(int field, char *lowtoken, int *val)
+DecodeSpecial(int field, const char *lowtoken, int *val)
 {
 	int			type;
 	const datetkn *tp;
@@ -3985,7 +3985,7 @@ DecodeISO8601Interval(char *str,
  *	will be related in format.
  */
 int
-DecodeUnits(int field, char *lowtoken, int *val)
+DecodeUnits(int field, const char *lowtoken, int *val)
 {
 	int			type;
 	const datetkn *tp;

src/backend/utils/adt/formatting.c

@@ -4246,7 +4246,7 @@ to_timestamp(PG_FUNCTION_ARGS)
 	/* Use the specified time zone, if any. */
 	if (tm.tm_zone)
 	{
-		int			dterr = DecodeTimezone(unconstify(char *, tm.tm_zone), &tz);
+		int			dterr = DecodeTimezone(tm.tm_zone, &tz);
 
 		if (dterr)
 			DateTimeParseError(dterr, text_to_cstring(date_txt), "timestamptz");
@@ -4343,7 +4343,7 @@ parse_datetime(text *date_txt, text *fmt, Oid collid, bool strict,
 
 				if (tm.tm_zone)
 				{
-					int			dterr = DecodeTimezone(unconstify(char *, tm.tm_zone), tz);
+					int			dterr = DecodeTimezone(tm.tm_zone, tz);
 
 					if (dterr)
 						DateTimeParseError(dterr, text_to_cstring(date_txt), "timestamptz");
@@ -4429,7 +4429,7 @@ parse_datetime(text *date_txt, text *fmt, Oid collid, bool strict,
 
 			if (tm.tm_zone)
 			{
-				int			dterr = DecodeTimezone(unconstify(char *, tm.tm_zone), tz);
+				int			dterr = DecodeTimezone(tm.tm_zone, tz);
 
 				if (dterr)
 					RETURN_ERROR(DateTimeParseError(dterr, text_to_cstring(date_txt), "timetz"));

src/include/utils/datetime.h

@@ -296,7 +296,7 @@ extern int	ParseDateTime(const char *timestr, char *workbuf, size_t buflen,
 extern int	DecodeDateTime(char **field, int *ftype,
 						   int nf, int *dtype,
 						   struct pg_tm *tm, fsec_t *fsec, int *tzp);
-extern int	DecodeTimezone(char *str, int *tzp);
+extern int	DecodeTimezone(const char *str, int *tzp);
 extern int	DecodeTimeOnly(char **field, int *ftype,
 						   int nf, int *dtype,
 						   struct pg_tm *tm, fsec_t *fsec, int *tzp);
@@ -322,10 +322,10 @@ extern void EncodeSpecialTimestamp(Timestamp dt, char *str);
 extern int	ValidateDate(int fmask, bool isjulian, bool is2digits, bool bc,
 						 struct pg_tm *tm);
 
-extern int	DecodeTimezoneAbbrev(int field, char *lowtoken,
+extern int	DecodeTimezoneAbbrev(int field, const char *lowtoken,
 								 int *offset, pg_tz **tz);
-extern int	DecodeSpecial(int field, char *lowtoken, int *val);
-extern int	DecodeUnits(int field, char *lowtoken, int *val);
+extern int	DecodeSpecial(int field, const char *lowtoken, int *val);
+extern int	DecodeUnits(int field, const char *lowtoken, int *val);
 
 extern int	j2day(int date);