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

doc/src/sgml/features.sgml

@@ -199,6 +199,15 @@
        standard.
       </para>
      </listitem>
+
+     <listitem>
+      <para>
+       <productname>PostgreSQL</productname> does not support the
+       <literal>RETURNING CONTENT</literal> or <literal>RETURNING SEQUENCE</literal>
+       clauses, functions which are defined to have these in the specification
+       are implicitly returning content.
+      </para>
+     </listitem>
     </itemizedlist>
    </para>
 

doc/src/sgml/func.sgml

@@ -14180,6 +14180,36 @@ CREATE TYPE rainbow AS ENUM ('red', 'orange', 'yellow', 'green', 'blue', 'purple
     documents for processing in client applications.
    </para>
 
+  <sect3 id="functions-producing-xml-xmltext">
+    <title><literal>xmltext</literal></title>
+
+    <indexterm>
+     <primary>xmltext</primary>
+    </indexterm>
+
+<synopsis>
+<function>xmltext</function> ( <type>text</type> ) <returnvalue>xml</returnvalue>
+</synopsis>
+
+    <para>
+     The function <function>xmltext</function> returns an XML value with a single
+     text node containing the input argument as its content. Predefined entities
+     like ampersand (<literal><![CDATA[&]]></literal>), left and right angle brackets
+     (<literal><![CDATA[< >]]></literal>), and quotation marks (<literal><![CDATA[""]]></literal>)
+     are escaped.
+    </para>
+
+    <para>
+     Example:
+<screen><![CDATA[
+SELECT xmltext('< foo & bar >');
+         xmltext
+-------------------------
+ &lt; foo &amp; bar &gt;
+]]></screen>
+    </para>
+   </sect3>
+
    <sect3 id="functions-producing-xml-xmlcomment">
     <title><literal>xmlcomment</literal></title>
 

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

src/include/catalog/pg_proc.dat

@@ -8793,6 +8793,9 @@
 { oid => '2922', descr => 'serialize an XML value to a character string',
   proname => 'text', prorettype => 'text', proargtypes => 'xml',
   prosrc => 'xmltotext' },
+{ oid => '3813', descr => 'generate XML text node',
+  proname => 'xmltext', proisstrict => 't', prorettype => 'xml',
+  proargtypes => 'text', prosrc => 'xmltext' },
 
 { oid => '2923', descr => 'map table contents to XML',
   proname => 'table_to_xml', procost => '100', provolatile => 's',

src/test/regress/expected/xml.out

@@ -1785,3 +1785,39 @@ SELECT * FROM XMLTABLE('.' PASSING XMLELEMENT(NAME a) columns a varchar(20) PATH
  <foo/> | &lt;foo/&gt;
 (1 row)
 
+SELECT xmltext(NULL);
+ xmltext 
+---------
+ 
+(1 row)
+
+SELECT xmltext('');
+ xmltext 
+---------
+ 
+(1 row)
+
+SELECT xmltext('  ');
+ xmltext 
+---------
+   
+(1 row)
+
+SELECT xmltext('foo `$_-+?=*^%!|/\()[]{}');
+         xmltext          
+--------------------------
+ foo `$_-+?=*^%!|/\()[]{}
+(1 row)
+
+SELECT xmltext('foo & <"bar">');
+              xmltext              
+-----------------------------------
+ foo &amp; &lt;&quot;bar&quot;&gt;
+(1 row)
+
+SELECT xmltext('x'|| '<P>73</P>'::xml || .42 || true || 'j'::char);
+             xmltext             
+---------------------------------
+ x&lt;P&gt;73&lt;/P&gt;0.42truej
+(1 row)
+

src/test/regress/expected/xml_1.out

@@ -1402,3 +1402,26 @@ DETAIL:  This functionality requires the server to be built with libxml support.
 SELECT * FROM XMLTABLE('.' PASSING XMLELEMENT(NAME a) columns a varchar(20) PATH '"<foo/>"', b xml PATH '"<foo/>"');
 ERROR:  unsupported XML feature
 DETAIL:  This functionality requires the server to be built with libxml support.
+SELECT xmltext(NULL);
+ xmltext 
+---------
+ 
+(1 row)
+
+SELECT xmltext('');
+ERROR:  unsupported XML feature
+DETAIL:  This functionality requires the server to be built with libxml support.
+SELECT xmltext('  ');
+ERROR:  unsupported XML feature
+DETAIL:  This functionality requires the server to be built with libxml support.
+SELECT xmltext('foo `$_-+?=*^%!|/\()[]{}');
+ERROR:  unsupported XML feature
+DETAIL:  This functionality requires the server to be built with libxml support.
+SELECT xmltext('foo & <"bar">');
+ERROR:  unsupported XML feature
+DETAIL:  This functionality requires the server to be built with libxml support.
+SELECT xmltext('x'|| '<P>73</P>'::xml || .42 || true || 'j'::char);
+ERROR:  unsupported XML feature
+LINE 1: SELECT xmltext('x'|| '<P>73</P>'::xml || .42 || true || 'j':...
+                             ^
+DETAIL:  This functionality requires the server to be built with libxml support.

src/test/regress/expected/xml_2.out

@@ -1765,3 +1765,39 @@ SELECT * FROM XMLTABLE('.' PASSING XMLELEMENT(NAME a) columns a varchar(20) PATH
  <foo/> | &lt;foo/&gt;
 (1 row)
 
+SELECT xmltext(NULL);
+ xmltext 
+---------
+ 
+(1 row)
+
+SELECT xmltext('');
+ xmltext 
+---------
+ 
+(1 row)
+
+SELECT xmltext('  ');
+ xmltext 
+---------
+   
+(1 row)
+
+SELECT xmltext('foo `$_-+?=*^%!|/\()[]{}');
+         xmltext          
+--------------------------
+ foo `$_-+?=*^%!|/\()[]{}
+(1 row)
+
+SELECT xmltext('foo & <"bar">');
+              xmltext              
+-----------------------------------
+ foo &amp; &lt;&quot;bar&quot;&gt;
+(1 row)
+
+SELECT xmltext('x'|| '<P>73</P>'::xml || .42 || true || 'j'::char);
+             xmltext             
+---------------------------------
+ x&lt;P&gt;73&lt;/P&gt;0.42truej
+(1 row)
+

src/test/regress/sql/xml.sql

@@ -660,3 +660,10 @@ SELECT * FROM XMLTABLE('*' PASSING '<e>pre<!--c1--><?pi arg?><![CDATA[&ent1]]><n
 \x
 
 SELECT * FROM XMLTABLE('.' PASSING XMLELEMENT(NAME a) columns a varchar(20) PATH '"<foo/>"', b xml PATH '"<foo/>"');
+
+SELECT xmltext(NULL);
+SELECT xmltext('');
+SELECT xmltext('  ');
+SELECT xmltext('foo `$_-+?=*^%!|/\()[]{}');
+SELECT xmltext('foo & <"bar">');
+SELECT xmltext('x'|| '<P>73</P>'::xml || .42 || true || 'j'::char);