Ensure that xpath() escapes special characters in string values.

Without this it's possible for the output to not be legal XML, as
illustrated by the added regression test cases.

NB: this change will need to be called out as an incompatibility in the
9.2 release notes, since it's possible somebody was relying on the old
behavior, even though it's clearly wrong.

Florian Pflug, reviewed by Radoslaw Smogura

src/backend/utils/adt/xml.c

@@ -3537,7 +3537,11 @@ xml_xmlnodetoxmltype(xmlNodePtr cur)
 		str = xmlXPathCastNodeToString(cur);
 		PG_TRY();
 		{
-			result = (xmltype *) cstring_to_text((char *) str);
+			/* Here we rely on XML having the same representation as TEXT */
+			char   *escaped = escape_xml((char *) str);
+
+			result = (xmltype *) cstring_to_text(escaped);
+			pfree(escaped);
 		}
 		PG_CATCH();
 		{

src/test/regress/expected/xml.out

@@ -589,6 +589,18 @@ SELECT xpath('//b', '<a>one <b>two</b> three <b>etc</b></a>');
  {<b>two</b>,<b>etc</b>}
 (1 row)
 
+SELECT xpath('//text()', '<root>&lt;</root>');
+ xpath  
+--------
+ {&lt;}
+(1 row)
+
+SELECT xpath('//@value', '<root value="&lt;"/>');
+ xpath  
+--------
+ {&lt;}
+(1 row)
+
 -- Test xmlexists and xpath_exists
 SELECT xmlexists('//town[text() = ''Toronto'']' PASSING BY REF '<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>');
  xmlexists 

src/test/regress/expected/xml_1.out

@@ -504,6 +504,18 @@ LINE 1: SELECT xpath('//b', '<a>one <b>two</b> three <b>etc</b></a>'...
                             ^
 DETAIL:  This functionality requires the server to be built with libxml support.
 HINT:  You need to rebuild PostgreSQL using --with-libxml.
+SELECT xpath('//text()', '<root>&lt;</root>');
+ERROR:  unsupported XML feature
+LINE 1: SELECT xpath('//text()', '<root>&lt;</root>');
+                                 ^
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.
+SELECT xpath('//@value', '<root value="&lt;"/>');
+ERROR:  unsupported XML feature
+LINE 1: SELECT xpath('//@value', '<root value="&lt;"/>');
+                                 ^
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.
 -- Test xmlexists and xpath_exists
 SELECT xmlexists('//town[text() = ''Toronto'']' PASSING BY REF '<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>');
 ERROR:  unsupported XML feature

src/test/regress/sql/xml.sql

@@ -175,6 +175,8 @@ SELECT xpath('', '<!-- error -->');
 SELECT xpath('//text()', '<local:data xmlns:local="http://127.0.0.1"><local:piece id="1">number one</local:piece><local:piece id="2" /></local:data>');
 SELECT xpath('//loc:piece/@id', '<local:data xmlns:local="http://127.0.0.1"><local:piece id="1">number one</local:piece><local:piece id="2" /></local:data>', ARRAY[ARRAY['loc', 'http://127.0.0.1']]);
 SELECT xpath('//b', '<a>one <b>two</b> three <b>etc</b></a>');
+SELECT xpath('//text()', '<root>&lt;</root>');
+SELECT xpath('//@value', '<root value="&lt;"/>');
 
 -- Test xmlexists and xpath_exists
 SELECT xmlexists('//town[text() = ''Toronto'']' PASSING BY REF '<towns><town>Bidford-on-Avon</town><town>Cwmbran</town><town>Bristol</town></towns>');