MDEV-15118 ExtractValue(xml,something_complex) does not work

Item_xml_str_func::fix_fields() used a local "String tmp" as a buffer
for args[1]->val_str(). "tmp" was freed at the end of fix_fields(),
while Items created during my_xpath_parse() still pointed to its fragments.

Adding a new member Item_xml_str_func::m_xpath_query and store the result
of args[1]->val_str() into it.

mysql-test/r/xml.result

@@ -1268,5 +1268,17 @@ c1	c2
 2	b2
 DROP TABLE t1;
 #
+# MDEV-15118 ExtractValue(xml,something_complex) does not work
+#
+CREATE TABLE t1 (a TEXT);
+INSERT INTO t1 VALUES (CONCAT('<a>aaa</a>'));
+SELECT ExtractValue(a, '/a') AS a FROM t1;
+a
+aaa
+SELECT ExtractValue(a, FROM_BASE64(TO_BASE64('/a'))) AS a FROM t1;
+a
+aaa
+DROP TABLE t1;
+#
 # End of 10.0 tests
 #

mysql-test/t/xml.test

@@ -748,6 +748,15 @@ SELECT *,IF(@i:=c1,ExtractValue('<a><b>b1</b><b>b2</b></a>','//b[$@i]'),0) AS xp
 SELECT * FROM t1 WHERE c2=IF(@i:=c1,ExtractValue('<a><b>b1</b><b>b2</b></a>','//b[$@i]'),0);
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-15118 ExtractValue(xml,something_complex) does not work
+--echo #
+
+CREATE TABLE t1 (a TEXT);
+INSERT INTO t1 VALUES (CONCAT('<a>aaa</a>'));
+SELECT ExtractValue(a, '/a') AS a FROM t1;
+SELECT ExtractValue(a, FROM_BASE64(TO_BASE64('/a'))) AS a FROM t1;
+DROP TABLE t1;
 
 --echo #
 --echo # End of 10.0 tests

sql/item_xmlfunc.cc

@@ -2634,7 +2634,7 @@ void Item_xml_str_func::fix_length_and_dec()
 
 bool Item_xml_str_func::fix_fields(THD *thd, Item **ref)
 {
-  String *xp, tmp;
+  String *xp;
   MY_XPATH xpath;
   int rc;
 
@@ -2662,7 +2662,13 @@ bool Item_xml_str_func::fix_fields(THD *thd, Item **ref)
     return true;
   }
 
-  if (!(xp= args[1]->val_str(&tmp)))
+  /*
+    Get the XPath query text from args[1] and cache it in m_xpath_query.
+    Its fragments will be referenced by items created during my_xpath_parse(),
+    e.g. by Item_nodeset_func_axisbyname::node_name.
+  */
+  if (!(xp= args[1]->val_str(&m_xpath_query)) ||
+      (xp != &m_xpath_query && m_xpath_query.copy(*xp)))
     return false; // Will return NULL
   my_xpath_init(&xpath);
   xpath.cs= collation.collation;

sql/item_xmlfunc.h

@@ -67,6 +67,7 @@ class Item_xml_str_func: public Item_str_func
       return parse(res, cache);
     }
   };
+  String m_xpath_query; // XPath query text
   Item *nodeset_func;
   XML xml;
   bool get_xml(XML *xml, bool cache= false)