MDEV-16471 mysqldump throws "Variable 'sql_mode' can't be set to the value of 'NULL' (1231)"

mysql-test/main/sql_mode.result

@@ -780,3 +780,27 @@ END;
 $$
 ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'error;
 END' at line 4
+#
+# End of 10.2 tests
+#
+#
+# Start of 10.3 tests
+#
+#
+# MDEV-16471 mysqldump throws "Variable 'sql_mode' can't be set to the value of 'NULL' (1231)"
+#
+SET sql_mode='ORACLE,EMPTY_STRING_IS_NULL';
+SELECT @@sql_mode;
+@@sql_mode
+PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,ORACLE,NO_KEY_OPTIONS,NO_TABLE_OPTIONS,NO_FIELD_OPTIONS,NO_AUTO_CREATE_USER,EMPTY_STRING_IS_NULL,SIMULTANEOUS_ASSIGNMENT
+SELECT '' AS empty;
+empty
+NULL
+SET sql_mode='';
+SELECT @@sql_mode;
+@@sql_mode
+
+SET sql_mode=DEFAULT;
+#
+# End of 10.3 tests
+#

mysql-test/main/sql_mode.test

@@ -554,3 +554,26 @@ BEGIN
 END;
 $$
 DELIMITER ;$$
+
+--echo #
+--echo # End of 10.2 tests
+--echo #
+
+--echo #
+--echo # Start of 10.3 tests
+--echo #
+
+--echo #
+--echo # MDEV-16471 mysqldump throws "Variable 'sql_mode' can't be set to the value of 'NULL' (1231)"
+--echo #
+
+SET sql_mode='ORACLE,EMPTY_STRING_IS_NULL';
+SELECT @@sql_mode;
+SELECT '' AS empty;
+SET sql_mode='';
+SELECT @@sql_mode;
+SET sql_mode=DEFAULT;
+
+--echo #
+--echo # End of 10.3 tests
+--echo #

sql/item.cc

@@ -202,6 +202,19 @@ String *Item::val_str_ascii(String *str)
 }
 
 
+String *Item::val_str_ascii_revert_empty_string_is_null(THD *thd, String *str)
+{
+  String *res= val_str_ascii(str);
+  if (!res && (thd->variables.sql_mode & MODE_EMPTY_STRING_IS_NULL))
+  {
+    null_value= false;
+    str->set("", 0, &my_charset_latin1);
+    return str;
+  }
+  return res;
+}
+
+
 String *Item::val_str(String *str, String *converter, CHARSET_INFO *cs)
 {
   String *res= val_str(str);

sql/item.h

@@ -1171,7 +1171,13 @@ class Item: public Value_source,
       Similar to val_str()
   */
   virtual String *val_str_ascii(String *str);
-  
+
+  /*
+    Returns the result of val_str_ascii(), translating NULLs back
+    to empty strings (if MODE_EMPTY_STRING_IS_NULL is set).
+  */
+  String *val_str_ascii_revert_empty_string_is_null(THD *thd, String *str);
+
   /*
     Returns the val_str() value converted to the given character set.
   */

sql/sys_vars.ic

@@ -1354,7 +1354,8 @@ public:
 
     if (var->value->result_type() == STRING_RESULT)
     {
-      if (!(res=var->value->val_str_ascii(&str)))
+      if (!(res= var->value->val_str_ascii_revert_empty_string_is_null(thd,
+                                                                       &str)))
         return true;
       else
       {