Bug#20789 Merge Subtable Rename Causes Crash

- When a MyISAM table which belongs to a merge table union and is 
  renamed the associated file descriptors are closed on windows.
  This causes a server crash next time an insert or update is 
  performed on the merge table.
- This patch prevents the system from crashing on windows by
  checking for bad file descriptors every time the MyISAM table
  is locked by associated the merge table.


myisam/mi_locking.c:
  - Added check for bad file descriptors when table is part of merge union.
  - This patch prevents the server from crash on windows.
myisam/myisamdef.h:
  Added boolean value to indicate that this myisam table is part of
  a merge union.
myisammrg/myrg_locking.c:
  Added paramter owned_by_merge=TRUE for mi_lock_database through MYRG_TABLE struct.

1 files changed, 3 insertions, 0 deletions

diff --git a/myisam/myisamdef.h b/myisam/myisamdef.h
index d589173f0e7..ea45915a088 100644
--- a/myisam/myisamdef.h
+++ b/myisam/myisamdef.h
@@ -278,6 +278,9 @@ struct st_myisam_info {
   my_bool page_changed;		/* If info->buff can't be used for rnext */
   my_bool buff_used;		/* If info->buff has to be reread for rnext */
   my_bool once_flags;           /* For MYISAMMRG */
+#ifdef __WIN__
+  my_bool owned_by_merge;                       /* This MyISAM table is part of a merge union */
+#endif
 #ifdef THREAD
   THR_LOCK_DATA lock;
 #endif