A pre-requisite for patch fixing bug #52044 "FLUSH TABLES

WITH READ LOCK and FLUSH TABLES <list> WITH READ LOCK are
incompatible", which adds information about waits caused by 
FLUSH TABLES statement to deadlock detector in MDL subsystem.

Remove API supporting caching of pointers to TABLE_SHARE 
object in MDL subsystem and all code related to it. 

The problem was that locking requirements of code 
implementing this API conflicted with locking requirements 
of code which adds information about waits caused by flushes 
to deadlock detector in MDL subsystem (the former needed to
lock LOCK_open or its future equivalent while having 
write-lock on MDL_lock's rwlock, and the latter needs to be 
able to read-lock MDL_lock rwlock while owning LOCK_open or 
its future equivalent).

Since caching of pointers to TABLE_SHARE objects in MDL 
subsystem didn't bring expected performance benefits we 
decided to remove caching API rather than try to come up 
with some complex solution for this problem.

sql/mdl.cc

@@ -284,8 +284,6 @@ public:
 public:
   /** The key of the object (data) being protected. */
   MDL_key key;
-  /** A cached reference to the TABLE_SHARE. Protected by LOCK_open. */
-  void   *m_cached_object;
   /**
     Read-write lock protecting this lock context.
 
@@ -362,7 +360,6 @@ public:
 
   MDL_lock(const MDL_key *key_arg)
   : key(key_arg),
-    m_cached_object(NULL),
     m_ref_usage(0),
     m_ref_release(0),
     m_is_destroyed(FALSE)
@@ -370,8 +367,6 @@ public:
     mysql_prlock_init(key_MDL_lock_rwlock, &m_rwlock);
   }
 
-  /* Overridden for TABLE objects, to support TABLE_SHARE cache in MDL. */
-  virtual void release_cached_object() {}
   virtual ~MDL_lock()
   {
     mysql_prlock_destroy(&m_rwlock);
@@ -460,25 +455,6 @@ private:
 };
 
 
-/** 
-  A lock implementation for MDL_key::TABLE.
-*/
-
-class MDL_table_lock: public MDL_object_lock
-{
-public:
-  MDL_table_lock(const MDL_key *key_arg)
-    : MDL_object_lock(key_arg)
-  { }
-#ifdef DISABLED_UNTIL_GRL_IS_MADE_PART_OF_MDL
-  virtual void release_cached_object()
-  {
-    tdc_release_cached_share(&m_cached_object);
-  }
-#endif
-};
-
-
 static MDL_map mdl_locks;
 
 extern "C"
@@ -695,8 +671,6 @@ void MDL_map::remove(MDL_lock *lock)
 {
   uint ref_usage, ref_release;
 
-  lock->release_cached_object();
-
   /*
     Destroy the MDL_lock object, but ensure that anyone that is
     holding a reference to the object is not remaining, if so he
@@ -860,8 +834,6 @@ inline MDL_lock *MDL_lock::create(const MDL_key *mdl_key)
     case MDL_key::GLOBAL:
     case MDL_key::SCHEMA:
       return new MDL_scoped_lock(mdl_key);
-    case MDL_key::TABLE:
-      return new MDL_table_lock(mdl_key);
     default:
       return new MDL_object_lock(mdl_key);
   }
@@ -1682,9 +1654,6 @@ MDL_context::try_acquire_lock_impl(MDL_request *mdl_request,
 
     m_tickets.push_front(ticket);
 
-    if (ticket->get_type() == MDL_EXCLUSIVE)
-      ticket->clear_cached_object();
-
     mdl_request->ticket= ticket;
   }
   else
@@ -1886,9 +1855,6 @@ MDL_context::acquire_lock(MDL_request *mdl_request, ulong lock_wait_timeout)
   */
   DBUG_ASSERT(wait_status == MDL_wait::GRANTED);
 
-  if (ticket->get_type() == MDL_EXCLUSIVE)
-    ticket->clear_cached_object();
-
   m_tickets.push_front(ticket);
 
   mdl_request->ticket= ticket;
@@ -2451,75 +2417,6 @@ bool MDL_ticket::has_pending_conflicting_lock() const
 }
 
 
-/**
-  Associate pointer to an opaque object with a lock.
-
-  @param cached_object Pointer to the object
-  @param release_hook  Cleanup function to be called when MDL subsystem
-                       decides to remove lock or associate another object.
-
-  This is used to cache a pointer to TABLE_SHARE in the lock
-  structure. Such caching can save one acquisition of LOCK_open
-  and one table definition cache lookup for every table.
-
-  Since the pointer may be stored only inside an acquired lock,
-  the caching is only effective when there is more than one lock
-  granted on a given table.
-
-  This function has the following usage pattern:
-    - try to acquire an MDL lock
-    - when done, call for get_cached_object(). If it returns NULL, our
-      thread has the only lock on this table.
-    - look up TABLE_SHARE in the table definition cache
-    - call mdl_set_cache_object() to assign the share to the opaque pointer.
-
- The release hook is invoked when the last shared metadata
- lock on this name is released.
-*/
-
-void
-MDL_ticket::set_cached_object(void *cached_object)
-{
-  DBUG_ENTER("MDL_ticket::set_cached_object");
-  DBUG_PRINT("enter", ("db=%s name=%s cached_object=%p",
-                        m_lock->key.db_name(), m_lock->key.name(),
-                        cached_object));
-  mysql_mutex_assert_owner(&LOCK_open);
-  DBUG_ASSERT(m_lock->key.mdl_namespace() == MDL_key::TABLE);
-  DBUG_ASSERT(!m_lock->m_cached_object);
-
-  m_lock->m_cached_object= cached_object;
-
-  DBUG_VOID_RETURN;
-}
-
-
-/**
-  A helper function to flush the table share cached in MDL.
-  @pre The ticket is acquired.
-*/
-
-void MDL_ticket::clear_cached_object()
-{
-  m_lock->release_cached_object();
-}
-
-
-/**
-  Get a pointer to an opaque object that associated with the lock.
-
-  @param ticket  Lock ticket for the lock which the object is associated to.
-
-  @return Pointer to an opaque object associated with the lock.
-*/
-
-void *MDL_ticket::get_cached_object()
-{
-  mysql_mutex_assert_owner(&LOCK_open);
-  return m_lock->m_cached_object;
-}
-
-
 /**
   Releases metadata locks that were acquired after a specific savepoint.
 

sql/mdl.h

@@ -398,9 +398,6 @@ public:
 public:
   bool has_pending_conflicting_lock() const;
 
-  void *get_cached_object();
-  void set_cached_object(void *cached_object);
-  void clear_cached_object();
   MDL_context *get_ctx() const { return m_ctx; }
   bool is_upgradable_or_exclusive() const
   {
@@ -728,7 +725,6 @@ extern "C" const char *set_thd_proc_info(void *thd_arg, const char *info,
                                          const char *calling_function,
                                          const char *calling_file,
                                          const unsigned int calling_line);
-extern void tdc_release_cached_share(void *ptr);
 #ifndef DBUG_OFF
 extern mysql_mutex_t LOCK_open;
 #endif