BUG#20075747 RND_INIT() ON AN OPEN SCAN IS USED TO REPOSITION THE CURSOR

 - rnd_init() on an open scan is used to reposition the "cursor"
   back to first row
 - Allow rnd_init() on an already open scan. Close the scan and let it reopen.

Author: blaudden

mysql-test/suite/ndb/r/ndbinfo.result

@@ -1328,3 +1328,11 @@ select count(*) from information_schema.tables
 count(*)
 0
 
+set @@optimizer_switch='block_nested_loop=off';
+create temporary table old_count
+       select counter_name, sum(val) as val
+       from ndbinfo.counters
+       where block_name='DBSPJ'
+       group by counter_name;
+drop table old_count;
+set @@optimizer_switch=default;

mysql-test/suite/ndb/t/ndbinfo.test

@@ -705,4 +705,18 @@ select type_name from ndb$dict_obj_types where type_id = 3;
 select count(*) from information_schema.tables
   where table_schema = 'ndbinfo' and engine != 'ndbinfo';
 
+#
+# BUG#20075747 RND_INIT() ON AN OPEN SCAN IS USED TO REPOSITION THE CURSOR
+# - rnd_init() on an open scan is used to reposition the "cursor"
+#   back to first row
+#
+set @@optimizer_switch='block_nested_loop=off';
+create temporary table old_count
+       select counter_name, sum(val) as val
+       from ndbinfo.counters
+       where block_name='DBSPJ'
+       group by counter_name;
+drop table old_count;
+set @@optimizer_switch=default;
+
 --source ndbinfo_drop.inc

sql/ha_ndbinfo.cc

@@ -1,5 +1,5 @@
 /*
-   Copyright (c) 2009, 2014, Oracle and/or its affiliates. All rights reserved.
+   Copyright (c) 2009, 2015, Oracle and/or its affiliates. All rights reserved.
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
@@ -475,6 +475,33 @@ int ha_ndbinfo::rnd_init(bool scan)
   }
 
   assert(is_open());
+
+  if (m_impl.m_scan_op)
+  {
+    /*
+      It should be impossible to come here with an already open
+      scan, assumption is that rnd_end() would be called to indicate
+      that the previous scan should be closed or perhaps like it says
+      in decsription of rnd_init() that it "may be called two times". Once
+      to open the cursor and once to position te cursor at first row.
+
+      Unfortunately the assumption and description of rnd_init() is not
+      correct. The rnd_init function is used on an open scan to reposition
+      it back to first row. For ha_ndbinfo this means closing
+      the scan and letting it be reopened.
+    */
+    assert(scan); // "only makes sense if scan=1" (from rnd_init() description)
+
+    DBUG_PRINT("info", ("Closing scan to position it back to first row"));
+
+    // Release the scan operation
+    g_ndbinfo->releaseScanOperation(m_impl.m_scan_op);
+    m_impl.m_scan_op = NULL;
+
+    // Release pointers to the columns
+    m_impl.m_columns.clear();
+  }
+
   assert(m_impl.m_scan_op == NULL); // No scan already ongoing
 
   if (m_impl.m_first_use)