Backport of:

------------------------------------------------------------
revno: 2617.23.18
committer: Davi Arnaut <Davi.Arnaut@Sun.COM>
branch nick: 4284-6.0
timestamp: Mon 2009-03-02 18:18:26 -0300
message:
Bug#989: If DROP TABLE while there's an active transaction, wrong binlog order
WL#4284: Transactional DDL locking

This is a prerequisite patch:

These changes are intended to split lock requests from granted
locks and to allow the memory and lifetime of granted locks to
be managed within the MDL subsystem. Furthermore, tickets can
now be shared and therefore are used to satisfy multiple lock
requests, but only shared locks can be recursive.

The problem is that the MDL subsystem morphs lock requests into
granted locks locks but does not manage the memory and lifetime
of lock requests, and hence, does not manage the memory of
granted locks either. This can be problematic because it puts the
burden of tracking references on the users of the subsystem and
it can't be easily done in transactional contexts where the locks
have to be kept around for the duration of a transaction.

Another issue is that recursive locks (when the context trying to
acquire a lock already holds a lock on the same object) requires
that each time the lock is granted, a unique lock request/granted
lock structure structure must be kept around until the lock is
released. This can lead to memory leaks in transactional contexts
as locks taken during the transaction should only be released at
the end of the transaction. This also leads to unnecessary wake
ups (broadcasts) in the MDL subsystem if the context still holds
a equivalent of the lock being released.

These issues are exacerbated due to the fact that WL#4284 low-level
design says that the implementation should "2) Store metadata locks
in transaction memory root, rather than statement memory root" but
this is not possible because a memory root, as implemented in mysys,
requires all objects allocated from it to be freed all at once.

This patch combines review input and significant code contributions
from Konstantin Osipov (kostja) and Dmitri Lenev (dlenev).

1 files changed, 5 insertions, 4 deletions

diff --git a/sql/table.h b/sql/table.h
index 918fb8dfff0..3b125217338 100644
--- a/sql/table.h
+++ b/sql/table.h
@@ -30,7 +30,8 @@ class st_select_lex;
 class partition_info;
 class COND_EQUAL;
 class Security_context;
-struct MDL_LOCK_DATA;
+struct MDL_LOCK_REQUEST;
+struct MDL_LOCK_TICKET;
 
 /*************************************************************************/
 
@@ -813,7 +814,7 @@ public:
   partition_info *part_info;            /* Partition related information */
   bool no_partitions_used; /* If true, all partitions have been pruned away */
 #endif
-  MDL_LOCK_DATA *mdl_lock_data;
+  MDL_LOCK_TICKET *mdl_lock_ticket;
 
   bool fill_item_list(List<Item> *item_list) const;
   void reset_item_list(List<Item> *item_list) const;
@@ -1416,7 +1417,7 @@ struct TABLE_LIST
   uint table_open_method;
   enum enum_schema_table_state schema_table_state;
 
-  MDL_LOCK_DATA *mdl_lock_data;
+  MDL_LOCK_REQUEST *mdl_lock_request;
 
   void calc_md5(char *buffer);
   void set_underlying_merge();
@@ -1785,6 +1786,6 @@ static inline void dbug_tmp_restore_column_maps(MY_BITMAP *read_set,
 size_t max_row_length(TABLE *table, const uchar *data);
 
 
-void alloc_mdl_locks(TABLE_LIST *table_list, MEM_ROOT *root);
+void alloc_mdl_requests(TABLE_LIST *table_list, MEM_ROOT *root);
 
 #endif /* TABLE_INCLUDED */