Set correct context for XPath evaluation

According to the SQL standard, the context of XMLTABLE's XPath
row_expression is the document node of the XML input document, not the
root node.  This becomes visible when a relative path rather than
absolute is used as row expression.  Absolute paths is what was used in
original tests and docs (and the most common form used in examples
throughout the interwebs), which explains why this wasn't noticed
before.

Other functions such as xpath() and xpath_exists() also have this
problem.  While not specified by the SQL standard, it would be pretty
odd to leave those functions to behave differently than XMLTABLE, so
change them too.  However, this is a backwards-incompatible change.

No backpatch, out of fear of breaking code depending on the original
broken behavior.

Author: Markus Winand
Reported-By: Markus Winand
Reviewed-by: Álvaro Herrera
Discussion: https://postgr.es/m/0684A598-002C-42A2-AE12-F024A324EAE4@winand.at

src/backend/utils/adt/xml.c

@@ -3934,10 +3934,7 @@ xpath_internal(text *xpath_expr_text, xmltype *data, ArrayType *namespaces,
 		if (xpathctx == NULL || xmlerrcxt->err_occurred)
 			xml_ereport(xmlerrcxt, ERROR, ERRCODE_OUT_OF_MEMORY,
 						"could not allocate XPath context");
-		xpathctx->node = xmlDocGetRootElement(doc);
+		xpathctx->node = (xmlNodePtr) doc;
-		if (xpathctx->node == NULL || xmlerrcxt->err_occurred)
-			xml_ereport(xmlerrcxt, ERROR, ERRCODE_INTERNAL_ERROR,
-						"could not find root XML element");
 
 		/* register namespaces, if any */
 		if (ns_count > 0)
@@ -4276,10 +4273,7 @@ XmlTableSetDocument(TableFuncScanState *state, Datum value)
 		if (xpathcxt == NULL || xtCxt->xmlerrcxt->err_occurred)
 			xml_ereport(xtCxt->xmlerrcxt, ERROR, ERRCODE_OUT_OF_MEMORY,
 						"could not allocate XPath context");
-		xpathcxt->node = xmlDocGetRootElement(doc);
+		xpathcxt->node = (xmlNodePtr) doc;
-		if (xpathcxt->node == NULL || xtCxt->xmlerrcxt->err_occurred)
-			xml_ereport(xtCxt->xmlerrcxt, ERROR, ERRCODE_INTERNAL_ERROR,
-						"could not find root XML element");
 	}
 	PG_CATCH();
 	{

src/test/regress/expected/xml.out

@@ -670,6 +670,12 @@ SELECT xpath('/nosuchtag', '<root/>');
  {}
 (1 row)
 
+SELECT xpath('root', '<root/>');
+   xpath   
+-----------
+ {<root/>}
+(1 row)
+
 -- Round-trip non-ASCII data through xpath().
 DO $$
 DECLARE
@@ -1212,7 +1218,7 @@ SELECT * FROM xmltable('/root' passing '<root><element>a1a<!-- aaaa -->a2a<?aaaa
 SELECT * FROM xmltable('/root' passing '<root><element>a1a<!-- aaaa -->a2a<?aaaaa?> <!--z-->  bbbb<x>xxx</x>cccc</element></root>' COLUMNS element text PATH 'element/text()'); -- should fail
 ERROR:  more than one value returned by column XPath expression
 -- CDATA test
-select * from xmltable('r' passing '<d><r><c><![CDATA[<hello> &"<>!<a>foo</a>]]></c></r><r><c>2</c></r></d>' columns c text);
+select * from xmltable('d/r' passing '<d><r><c><![CDATA[<hello> &"<>!<a>foo</a>]]></c></r><r><c>2</c></r></d>' columns c text);
             c            
 -------------------------
  <hello> &"<>!<a>foo</a>

src/test/regress/expected/xml_1.out

@@ -576,6 +576,12 @@ LINE 1: SELECT xpath('/nosuchtag', '<root/>');
                                    ^
 DETAIL:  This functionality requires the server to be built with libxml support.
 HINT:  You need to rebuild PostgreSQL using --with-libxml.
+SELECT xpath('root', '<root/>');
+ERROR:  unsupported XML feature
+LINE 1: SELECT xpath('root', '<root/>');
+                             ^
+DETAIL:  This functionality requires the server to be built with libxml support.
+HINT:  You need to rebuild PostgreSQL using --with-libxml.
 -- Round-trip non-ASCII data through xpath().
 DO $$
 DECLARE
@@ -1067,10 +1073,10 @@ LINE 1: SELECT * FROM xmltable('/root' passing '<root><element>a1a<!...
 DETAIL:  This functionality requires the server to be built with libxml support.
 HINT:  You need to rebuild PostgreSQL using --with-libxml.
 -- CDATA test
-select * from xmltable('r' passing '<d><r><c><![CDATA[<hello> &"<>!<a>foo</a>]]></c></r><r><c>2</c></r></d>' columns c text);
+select * from xmltable('d/r' passing '<d><r><c><![CDATA[<hello> &"<>!<a>foo</a>]]></c></r><r><c>2</c></r></d>' columns c text);
 ERROR:  unsupported XML feature
-LINE 1: select * from xmltable('r' passing '<d><r><c><![CDATA[<hello...
+LINE 1: select * from xmltable('d/r' passing '<d><r><c><![CDATA[<hel...
-                                           ^
+                                             ^
 DETAIL:  This functionality requires the server to be built with libxml support.
 HINT:  You need to rebuild PostgreSQL using --with-libxml.
 -- XML builtin entities

src/test/regress/expected/xml_2.out

@@ -650,6 +650,12 @@ SELECT xpath('/nosuchtag', '<root/>');
  {}
 (1 row)
 
+SELECT xpath('root', '<root/>');
+   xpath   
+-----------
+ {<root/>}
+(1 row)
+
 -- Round-trip non-ASCII data through xpath().
 DO $$
 DECLARE
@@ -1192,7 +1198,7 @@ SELECT * FROM xmltable('/root' passing '<root><element>a1a<!-- aaaa -->a2a<?aaaa
 SELECT * FROM xmltable('/root' passing '<root><element>a1a<!-- aaaa -->a2a<?aaaaa?> <!--z-->  bbbb<x>xxx</x>cccc</element></root>' COLUMNS element text PATH 'element/text()'); -- should fail
 ERROR:  more than one value returned by column XPath expression
 -- CDATA test
-select * from xmltable('r' passing '<d><r><c><![CDATA[<hello> &"<>!<a>foo</a>]]></c></r><r><c>2</c></r></d>' columns c text);
+select * from xmltable('d/r' passing '<d><r><c><![CDATA[<hello> &"<>!<a>foo</a>]]></c></r><r><c>2</c></r></d>' columns c text);
             c            
 -------------------------
  <hello> &"<>!<a>foo</a>

src/test/regress/sql/xml.sql

@@ -188,6 +188,7 @@ SELECT xpath('count(//*)=0', '<root><sub/><sub/></root>');
 SELECT xpath('count(//*)=3', '<root><sub/><sub/></root>');
 SELECT xpath('name(/*)', '<root><sub/><sub/></root>');
 SELECT xpath('/nosuchtag', '<root/>');
+SELECT xpath('root', '<root/>');
 
 -- Round-trip non-ASCII data through xpath().
 DO $$
@@ -423,7 +424,7 @@ SELECT * FROM xmltable('/root' passing '<root><element>a1a<!-- aaaa -->a2a<?aaaa
 SELECT * FROM xmltable('/root' passing '<root><element>a1a<!-- aaaa -->a2a<?aaaaa?> <!--z-->  bbbb<x>xxx</x>cccc</element></root>' COLUMNS element text PATH 'element/text()'); -- should fail
 
 -- CDATA test
-select * from xmltable('r' passing '<d><r><c><![CDATA[<hello> &"<>!<a>foo</a>]]></c></r><r><c>2</c></r></d>' columns c text);
+select * from xmltable('d/r' passing '<d><r><c><![CDATA[<hello> &"<>!<a>foo</a>]]></c></r><r><c>2</c></r></d>' columns c text);
 
 -- XML builtin entities
 SELECT * FROM xmltable('/x/a' PASSING '<x><a><ent>&apos;</ent></a><a><ent>&quot;</ent></a><a><ent>&amp;</ent></a><a><ent>&lt;</ent></a><a><ent>&gt;</ent></a></x>' COLUMNS ent text);