patch for preventing the specification of conflicting transaction read/write options

diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 62fde67..28c987f 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -146,6 +146,9 @@ static void processCASbits(int cas_bits, int location, const char *constrType,
 			   bool *deferrable, bool *initdeferred, bool *not_valid,
 			   core_yyscan_t yyscanner);
 
+static void
+check_trans_mode(List *list, DefElem *elem, core_yyscan_t yyscanner);
+
 %}
 
 %pure-parser
@@ -7510,9 +7513,16 @@ transaction_mode_list:
 			transaction_mode_item
 					{ $$ = list_make1($1); }
 			| transaction_mode_list ',' transaction_mode_item
-					{ $$ = lappend($1, $3); }
+					{
+						check_trans_mode((List *)$1, (DefElem *)$3, yyscanner);
+						$$ = lappend($1, $3);
+					}
+
 			| transaction_mode_list transaction_mode_item
-					{ $$ = lappend($1, $2); }
+					{
+						check_trans_mode((List *)$1, (DefElem *)$2, yyscanner);
+						$$ = lappend($1, $2);
+					}
 		;
 
 transaction_mode_list_or_empty:
@@ -13215,6 +13225,57 @@ parser_init(base_yy_extra_type *yyext)
 }
 
 /*
+ * checks for conflicting transaction modes by looking up current
+ * element in the given list.
+ */
+static void
+check_trans_mode(List *list, DefElem *elem, core_yyscan_t yyscanner)
+{
+	ListCell *lc;
+	A_Const *elem_arg;
+
+	elem_arg =(A_Const *) elem->arg;
+
+	/* cannot specify conflicting value for transaction mode. */
+	foreach (lc, list)
+	{
+		DefElem *next;
+		A_Const *arg;
+
+		next = (DefElem*)lfirst (lc);
+		arg = (A_Const *)next->arg;
+
+		/* check for duplicate value in remaining list */
+		if (strcmp(elem->defname, next->defname) == 0)
+		{
+			/*
+			 * isolation level values are stored
+			 * as string whereas other modes are
+			 * stored as integer values.
+			 */
+			if (strcmp(elem->defname,"transaction_isolation") == 0)
+			{
+				/* check for conflicting values */
+				if (strcmp(elem_arg->val.val.str,arg->val.val.str)!= 0)
+							ereport(ERROR,
+								(errcode(ERRCODE_SYNTAX_ERROR),
+								 errmsg("conflicting options"),
+									 parser_errposition(arg->location)));
+			}
+			else
+			{
+				/* check for conflicting values */
+				if (elem_arg->val.val.ival != arg->val.val.ival)
+							ereport(ERROR,
+								(errcode(ERRCODE_SYNTAX_ERROR),
+								 errmsg("conflicting options"),
+									 parser_errposition(arg->location)));
+			}
+		}
+	}
+}
+
+/*
  * Must undefine this stuff before including scan.c, since it has different
  * definitions for these macros.
  */