Fix handling of format string text characters in to_timestamp()/to_date()

cf984672 introduced improvement of handling of spaces and separators in
to_timestamp()/to_date() functions.  In particular, now we're skipping spaces
both before and after fields.  That may cause format string text character to
consume part of field in the situations, when it didn't happen before cf984672.
This commit cause format string text character consume input string characters
only when since previous field (or string beginning) number of skipped input
string characters is not greater than number of corresponding format string
characters (that is we didn't skip any extra characters in input string).

src/backend/utils/adt/formatting.c

@@ -3061,9 +3061,24 @@ DCH_from_char(FormatNode *node, char *in, TmFromChar *out)
 			 * Text character, so consume one character from input string.
 			 * Notice we don't insist that the consumed character match the
 			 * format's character.
-			 * Text field ignores FX mode.
 			 */
-			s += pg_mblen(s);
+			if (!fx_mode)
+			{
+				/*
+				 * In non FX mode we might have skipped some extra characters
+				 * (more than specified in format string) before.  In this
+				 * case we don't skip input string character, because it might
+				 * be part of field.
+				 */
+				if (extra_skip > 0)
+					extra_skip--;
+				else
+					s += pg_mblen(s);
+			}
+			else
+			{
+				s += pg_mblen(s);
+			}
 			continue;
 		}
 

src/test/regress/expected/horology.out

@@ -3128,6 +3128,21 @@ SELECT to_date('2011   12 18', 'YYYY  MM DD');
  12-18-2011
 (1 row)
 
+SELECT to_date('2011 12 18', 'YYYYxMMxDD');
+  to_date   
+------------
+ 12-18-2011
+(1 row)
+
+SELECT to_date('2011x 12x 18', 'YYYYxMMxDD');
+  to_date   
+------------
+ 12-18-2011
+(1 row)
+
+SELECT to_date('2011 x12 x18', 'YYYYxMMxDD');
+ERROR:  invalid value "x1" for "MM"
+DETAIL:  Value must be an integer.
 --
 -- Check errors for some incorrect usages of to_timestamp() and to_date()
 --

src/test/regress/sql/horology.sql

@@ -495,6 +495,10 @@ SELECT to_date('2011 12 18', 'YYYY  MM DD');
 SELECT to_date('2011  12 18', 'YYYY  MM DD');
 SELECT to_date('2011   12 18', 'YYYY  MM DD');
 
+SELECT to_date('2011 12 18', 'YYYYxMMxDD');
+SELECT to_date('2011x 12x 18', 'YYYYxMMxDD');
+SELECT to_date('2011 x12 x18', 'YYYYxMMxDD');
+
 --
 -- Check errors for some incorrect usages of to_timestamp() and to_date()
 --