xml.result, xml.test:

  Adding test.
item_xmlfunc.cc:
  Bug #18171 XML: ExtractValue: the XPath position() function crashes the server!
  Disallowing use of position() and last() without context.

mysql-test/r/xml.result

@@ -609,3 +609,7 @@ extractvalue('<a>Jack</a>','/a[contains(../a,"j")]' collate latin1_bin)
 select extractvalue('<a>Jack</a>' collate latin1_bin,'/a[contains(../a,"j")]');
 extractvalue('<a>Jack</a>' collate latin1_bin,'/a[contains(../a,"j")]')
 
+select extractValue('<e>1</e>','position()');
+ERROR HY000: XPATH syntax error: ''
+select extractValue('<e>1</e>','last()');
+ERROR HY000: XPATH syntax error: ''

mysql-test/t/xml.test

@@ -277,3 +277,12 @@ select extractvalue('<a>Jack</a>','/a[contains(../a,"J")]');
 select extractvalue('<a>Jack</a>','/a[contains(../a,"j")]');
 select extractvalue('<a>Jack</a>','/a[contains(../a,"j")]' collate latin1_bin);
 select extractvalue('<a>Jack</a>' collate latin1_bin,'/a[contains(../a,"j")]');
+
+#
+# Bug #18171 XML: ExtractValue: the XPath position()
+# function crashes the server!
+#
+--error 1105
+select extractValue('<e>1</e>','position()');
+--error 1105
+select extractValue('<e>1</e>','last()');

sql/item_xmlfunc.cc

@@ -1141,13 +1141,15 @@ static Item *create_func_round(MY_XPATH *xpath, Item **args, uint nargs)
 
 static Item *create_func_last(MY_XPATH *xpath, Item **args, uint nargs)
 {
-  return new Item_func_xpath_count(xpath->context, xpath->pxml);
+  return xpath->context ? 
+         new Item_func_xpath_count(xpath->context, xpath->pxml) : NULL;
 }
 
 
 static Item *create_func_position(MY_XPATH *xpath, Item **args, uint nargs)
 {
-  return new Item_func_xpath_position(xpath->context, xpath->pxml);
+  return xpath->context ? 
+         new Item_func_xpath_position(xpath->context, xpath->pxml) : NULL;
 }