Add XMLText function (SQL/XML X038)

This function implements the standard XMLTest function, which
converts text into xml text nodes. It uses the libxml2 function
xmlEncodeSpecialChars to escape predefined entities (&"<>), so
that those do not cause any conflict when concatenating the text
node output with existing xml documents.

This also adds a note in  features.sgml about not supporting
XML(SEQUENCE). The SQL specification defines a RETURNING clause
to a set of XML functions, where RETURNING CONTENT or RETURNING
SEQUENCE can be defined. Since PostgreSQL doesn't support
XML(SEQUENCE) all of these functions operate with an
implicit RETURNING CONTENT.

Author: Jim Jones <jim.jones@uni-muenster.de>
Reviewed-by: Vik Fearing <vik@postgresfriends.org>
Discussion: https://postgr.es/m/86617a66-ec95-581f-8d54-08059cca8885@uni-muenster.de

src/backend/catalog/sql_features.txt

@@ -633,7 +633,7 @@ X034	XMLAgg			YES
 X035	XMLAgg: ORDER BY option			YES	
 X036	XMLComment			YES	
 X037	XMLPI			YES	
-X038	XMLText			NO	
+X038	XMLText			YES	supported except for RETURNING
 X040	Basic table mapping			YES	
 X041	Basic table mapping: null absent			YES	
 X042	Basic table mapping: null as nil			YES	

src/backend/utils/adt/xml.c

@@ -47,6 +47,7 @@
 
 #ifdef USE_LIBXML
 #include <libxml/chvalid.h>
+#include <libxml/entities.h>
 #include <libxml/parser.h>
 #include <libxml/parserInternals.h>
 #include <libxml/tree.h>
@@ -513,6 +514,27 @@ xmlcomment(PG_FUNCTION_ARGS)
 }
 
 
+Datum
+xmltext(PG_FUNCTION_ARGS)
+{
+#ifdef USE_LIBXML
+	text	   *arg = PG_GETARG_TEXT_PP(0);
+	text	   *result;
+	xmlChar    *xmlbuf = NULL;
+
+	xmlbuf = xmlEncodeSpecialChars(NULL, xml_text2xmlChar(arg));
+
+	Assert(xmlbuf);
+
+	result = cstring_to_text_with_len((const char *) xmlbuf, xmlStrlen(xmlbuf));
+	xmlFree(xmlbuf);
+	PG_RETURN_XML_P(result);
+#else
+	NO_XML_SUPPORT();
+	return 0;
+#endif							/* not USE_LIBXML */
+}
+
 
 /*
  * TODO: xmlconcat needs to merge the notations and unparsed entities