FETCHING VARIABLE

Analysis:
--------
Concurrent execution of a stored program having cursor and
a FLUSH TABLE operation may cause the 'mysqld' to crash.
The crash can be observed, when the events occurs in the
following sequence:

a) Cursor OPEN executes the SELECT statement for which the
   table is opened.
b) Flush table operation is triggered and finds that a table
   share object is present. Its version is marked as zero to
   ensure that the share is removed when it is no longer
   referenced.
c) Since the share version is old and is referenced, the
   flush table operation waits until the flush request is
   granted.
d) The SELECT statement execution for cursor OPEN closes all
   tables except the internal temporary table used by cursor
   for saving the materialized records.
e) While closing the table, since the table share is an old
   version and there is a pending flush request, the flush
   request is granted. Thus the table share is deleted by
   awakening the FLUSH TABLE operation.
f) During the cursor FETCH operation, the column type is checked
   for field conversion. To perform the check, the table share of
   orig_table in the field definition of the cursor temporary
   table is accessed. Since the share was deleted by FLUSH
   operation, accessing the invalid memory may cause the
   server to crash.

Fix:
---
In case of cursors, since all tables other than the temporary
table are closed, the orig_table in the field definition for
the internal temporary table is set to NULL. This is done 
once the metadata of the temporary table for the CURSOR is 
sent.