global.cc

Go to the documentation of this file.

/* 
    Copyright (C) 2011 Brian Aker
    Copyright (C) 2000-2006 MySQL AB

   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
   the Free Software Foundation; version 2 of the License.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA */


#include <config.h>

#include <fcntl.h>

#include <drizzled/error.h>
#include <drizzled/thr_lock.h>
#include <drizzled/session.h>
#include <drizzled/session/times.h>
#include <drizzled/sql_base.h>
#include <drizzled/lock.h>
#include <drizzled/pthread_globals.h>
#include <drizzled/internal/my_sys.h>
#include <drizzled/pthread_globals.h>
#include <drizzled/plugin/storage_engine.h>
#include <drizzled/util/test.h>
#include <drizzled/open_tables_state.h>
#include <drizzled/table/cache.h>

#include <set>
#include <vector>
#include <algorithm>
#include <functional>

#include <boost/thread/shared_mutex.hpp>
#include <boost/thread/condition_variable.hpp>

using namespace std;

namespace drizzled
{

static boost::mutex LOCK_global_read_lock;
static boost::condition_variable_any COND_global_read_lock;

static void print_lock_error(int error, const char *);

/*
  Lock tables.

  SYNOPSIS
    lockTables()
    tables                      An array of pointers to the tables to lock.
    count                       The number of tables to lock.
    flags                       Options:
      DRIZZLE_LOCK_IGNORE_GLOBAL_READ_LOCK      Ignore a global read lock
      DRIZZLE_LOCK_IGNORE_FLUSH                 Ignore a flush tables.
      DRIZZLE_LOCK_NOTIFY_IF_NEED_REOPEN        Instead of reopening altered
                                              or dropped tables by itself,
                                              lockTables() should
                                              notify upper level and rely
                                              on caller doing this.

  RETURN
    A lock structure pointer on success.
    NULL on error or if some tables should be reopen.
*/

/* Map the return value of thr_lock to an error from errmsg.txt */
static drizzled::error_t thr_lock_errno_to_mysql[]=
{ EE_OK, EE_ERROR_FIRST, ER_LOCK_WAIT_TIMEOUT, ER_LOCK_DEADLOCK };


static void reset_lock_data_and_free(DrizzleLock*& lock)
{
  lock->reset();
  delete lock;
  lock= NULL;
}

DrizzleLock *Session::lockTables(Table **tables, uint32_t count, uint32_t flags)
{
  DrizzleLock *sql_lock;
  Table *write_lock_used;
  vector<plugin::StorageEngine *> involved_engines;

  do
  {
    if (! (sql_lock= get_lock_data(tables, count, true, &write_lock_used)))
      break;

    if (global_read_lock && write_lock_used and (not (flags & DRIZZLE_LOCK_IGNORE_GLOBAL_READ_LOCK)))
    {
      /*
  Someone has issued LOCK ALL TABLES FOR READ and we want a write lock
  Wait until the lock is gone
      */
      if (wait_if_global_read_lock(1, 1))
      {
        /* Clear the lock type of all lock data to avoid reusage. */
        reset_lock_data_and_free(sql_lock);
  break;
      }

      if (open_tables.version != g_refresh_version)
      {
        /* Clear the lock type of all lock data to avoid reusage. */
        reset_lock_data_and_free(sql_lock);
  break;
      }
    }
    
    set_proc_info("Notify start statement");
    /*
     * Here, we advise all storage engines involved in the
     * statement that we are starting a new statement
     */
    if (sql_lock->sizeTable())
    {
      size_t num_tables= sql_lock->sizeTable();
      plugin::StorageEngine *engine;
      std::set<size_t> involved_slots;

      for (size_t x= 1; x <= num_tables; x++, tables++)
      {
        engine= (*tables)->cursor->getEngine();

        if (involved_slots.count(engine->getId()) > 0)
          continue; /* already added to involved engines */

        involved_engines.push_back(engine);
        involved_slots.insert(engine->getId());
      }

      for_each(involved_engines.begin(),
               involved_engines.end(),
               bind2nd(mem_fun(&plugin::StorageEngine::startStatement), this));
    }

    set_proc_info("External lock");
    /*
     * Here, the call to lock_external() informs the
     * all engines for all tables used in this statement
     * of the type of lock that Drizzle intends to take on a 
     * specific table.
     */
    if (sql_lock->sizeTable() && lock_external(sql_lock->getTable(), sql_lock->sizeTable()))
    {
      /* Clear the lock type of all lock data to avoid reusage. */
      reset_lock_data_and_free(sql_lock);
      break;
    }
    set_proc_info("Table lock");
    /* Copy the lock data array. thr_multi_lock() reorders its contens. */
    memcpy(sql_lock->getLocks() + sql_lock->sizeLock(),
           sql_lock->getLocks(),
           sql_lock->sizeLock() * sizeof(*sql_lock->getLocks()));

    /* Lock on the copied half of the lock data array. */
    drizzled::error_t rc;
    rc= thr_lock_errno_to_mysql[(int) thr_multi_lock(*this,
                                                     sql_lock->getLocks() +
                                                     sql_lock->sizeLock(),
                                                     sql_lock->sizeLock(),
                                                     this->lock_id)];
    if (rc)                                 /* a timeout or a deadlock */
    {
      if (sql_lock->sizeTable())
        unlock_external(sql_lock->getTable(), sql_lock->sizeTable());
      reset_lock_data_and_free(sql_lock);
      my_error(rc, MYF(0));
    }
  } while(0);

  set_proc_info(0);
  if (getKilled())
  {
    send_kill_message();
    if (sql_lock)
    {
      unlockTables(sql_lock);
      sql_lock= NULL;
    }
  }

  times.set_time_after_lock();

  return sql_lock;
}


int Session::lock_external(Table **tables, uint32_t count)
{
  int lock_type,error;
  for (uint32_t i= 1 ; i <= count ; i++, tables++)
  {
    assert((*tables)->reginfo.lock_type >= TL_READ);
    lock_type=F_WRLCK;        /* Lock exclusive */
    if ((*tables)->db_stat & HA_READ_ONLY ||
      ((*tables)->reginfo.lock_type >= TL_READ &&
      (*tables)->reginfo.lock_type <= TL_READ_NO_INSERT))
      lock_type=F_RDLCK;

    if ((error=(*tables)->cursor->ha_external_lock(this,lock_type)))
    {
      print_lock_error(error, (*tables)->cursor->getEngine()->getName().c_str());
      while (--i)
      {
        tables--;
        (*tables)->cursor->ha_external_lock(this, F_UNLCK);
        (*tables)->current_lock=F_UNLCK;
      }
      return error;
    }
    else
    {
      (*tables)->db_stat &= ~ HA_BLOCK_LOCK;
      (*tables)->current_lock= lock_type;
    }
  }
  return 0;
}


void Session::unlockTables(DrizzleLock *sql_lock)
{
  if (sql_lock->sizeLock())
    sql_lock->unlock(sql_lock->sizeLock());
  if (sql_lock->sizeTable())
    unlock_external(sql_lock->getTable(), sql_lock->sizeTable());
  delete sql_lock;
}

void Session::unlockSomeTables(Table **table, uint32_t count)
{
  DrizzleLock *sql_lock;
  Table *write_lock_used;
  if ((sql_lock= get_lock_data(table, count, false,
                               &write_lock_used)))
    unlockTables(sql_lock);
}


void Session::unlockReadTables(DrizzleLock *sql_lock)
{
  uint32_t i,found;

  /* Move all write locks first */
  THR_LOCK_DATA **lock_local= sql_lock->getLocks();
  for (i=found=0 ; i < sql_lock->sizeLock(); i++)
  {
    if (sql_lock->getLocks()[i]->type >= TL_WRITE_ALLOW_READ)
    {
      std::swap(*lock_local, sql_lock->getLocks()[i]);
      lock_local++;
      found++;
    }
  }
  /* unlock the read locked tables */
  if (i != found)
  {
    thr_multi_unlock(lock_local, i - found);
    sql_lock->setLock(found);
  }

  /* Then do the same for the external locks */
  /* Move all write locked tables first */
  Table **table= sql_lock->getTable();
  for (i=found=0 ; i < sql_lock->sizeTable() ; i++)
  {
    assert(sql_lock->getTable()[i]->lock_position == i);
    if ((uint32_t) sql_lock->getTable()[i]->reginfo.lock_type >= TL_WRITE_ALLOW_READ)
    {
      std::swap(*table, sql_lock->getTable()[i]);
      table++;
      found++;
    }
  }
  /* Unlock all read locked tables */
  if (i != found)
  {
    unlock_external(table, i - found);
    sql_lock->resizeTable(found);
  }
  /* Fix the lock positions in Table */
  table= sql_lock->getTable();
  found= 0;
  for (i= 0; i < sql_lock->sizeTable(); i++)
  {
    Table *tbl= *table;
    tbl->lock_position= table - sql_lock->getTable();
    tbl->lock_data_start= found;
    found+= tbl->lock_count;
    table++;
  }
}


void Session::removeLock(Table *table)
{
  unlockSomeTables(&table, /* table count */ 1);
}


void Session::abortLock(Table *table)
{
  DrizzleLock *locked;
  Table *write_lock_used;

  if ((locked= get_lock_data(&table, 1, false,
                             &write_lock_used)))
  {
    for (uint32_t x= 0; x < locked->sizeLock(); x++)
      locked->getLocks()[x]->lock->abort_locks();
    delete locked;
  }
}


bool Session::abortLockForThread(Table *table)
{
  bool result= false;
  Table* write_lock_used;
  if (DrizzleLock* locked= get_lock_data(&table, 1, false, &write_lock_used))
  {
    for (uint32_t i= 0; i < locked->sizeLock(); i++)
    {
      if (locked->getLocks()[i]->lock->abort_locks_for_thread(table->in_use->thread_id))
        result= true;
    }
    delete locked;
  }
  return result;
}

int Session::unlock_external(Table **table, uint32_t count)
{
  int error;

  int error_code=0;
  do
  {
    if ((*table)->current_lock != F_UNLCK)
    {
      (*table)->current_lock = F_UNLCK;
      if ((error=(*table)->cursor->ha_external_lock(this, F_UNLCK)))
      {
  error_code=error;
  print_lock_error(error_code, (*table)->cursor->getEngine()->getName().c_str());
      }
    }
    table++;
  } while (--count);
  return error_code;
}


DrizzleLock *Session::get_lock_data(Table **table_ptr, uint32_t count,
                                    bool should_lock, Table **write_lock_used)
{
  uint32_t lock_count;
  THR_LOCK_DATA **locks, **locks_buf, **locks_start;
  Table **to, **table_buf;

  *write_lock_used=0;
  for (uint32_t i= lock_count= 0 ; i < count ; i++)
  {
    Table *t= table_ptr[i];

    if (! (t->getEngine()->check_flag(HTON_BIT_SKIP_STORE_LOCK)))
    {
      lock_count++;
    }
  }

  /*
    Allocating twice the number of pointers for lock data for use in
    thr_mulit_lock(). This function reorders the lock data, but cannot
    update the table values. So the second part of the array is copied
    from the first part immediately before calling thr_multi_lock().
  */
  DrizzleLock *sql_lock= new DrizzleLock(lock_count);

  if (not sql_lock)
    return NULL;

  locks= locks_buf= sql_lock->getLocks();
  to= table_buf= sql_lock->getTable();

  for (uint32_t i= 0; i < count ; i++)
  {
    Table *table;
    thr_lock_type lock_type;

    if (table_ptr[i]->getEngine()->check_flag(HTON_BIT_SKIP_STORE_LOCK))
      continue;

    table= table_ptr[i];
    lock_type= table->reginfo.lock_type;
    assert (lock_type != TL_WRITE_DEFAULT);
    if (lock_type >= TL_WRITE_ALLOW_WRITE)
    {
      *write_lock_used=table;
      if (table->db_stat & HA_READ_ONLY)
      {
  my_error(ER_OPEN_AS_READONLY, MYF(0), table->getAlias());
        /* Clear the lock type of the lock data that are stored already. */
        sql_lock->setLock(locks - sql_lock->getLocks());
        reset_lock_data_and_free(sql_lock);
  return NULL;
      }
    }
    locks_start= locks;
    locks= table->cursor->store_lock(this, locks, should_lock ? lock_type : TL_IGNORE);
    if (should_lock)
    {
      table->lock_position=   (uint32_t) (to - table_buf);
      table->lock_data_start= (uint32_t) (locks_start - locks_buf);
      table->lock_count=      (uint32_t) (locks - locks_start);
      assert(table->lock_count == 1);
    }
    *to++= table;
  }
  /*
    We do not use 'tables', because there are cases where store_lock()
    returns less locks than lock_count() claimed. This can happen when
    a FLUSH TABLES tries to abort locks from a MERGE table of another
    thread. When that thread has just opened the table, but not yet
    attached its children, it cannot return the locks. lock_count()
    always returns the number of locks that an attached table has.
    This is done to avoid the reverse situation: If lock_count() would
    return 0 for a non-attached MERGE table, and that table becomes
    attached between the calls to lock_count() and store_lock(), then
    we would have allocated too little memory for the lock data. Now
    we may allocate too much, but better safe than memory overrun.
    And in the FLUSH case, the memory is released quickly anyway.
  */
  sql_lock->setLock(locks - locks_buf);

  return sql_lock;
}


int Session::lock_table_name(TableList *table_list)
{
  identifier::Table identifier(table_list->getSchemaName(), table_list->getTableName());
  {
    /* Only insert the table if we haven't insert it already */
    table::CacheRange ppp= table::getCache().equal_range(identifier.getKey());
    for (table::CacheMap::const_iterator iter= ppp.first; iter != ppp.second; ++iter)
    {
      Table *table= iter->second;
      if (table->reginfo.lock_type < TL_WRITE)
        continue;
      if (table->in_use == this)
      {
        table->getMutableShare()->resetVersion();                  // Ensure no one can use this
        table->locked_by_name= true;
        return 0;
      }
    }
  }

  table::Placeholder *table= &table_cache_insert_placeholder(identifier);
  table_list->table= reinterpret_cast<Table*>(table);

  /* Return 1 if table is in use */
  return (test(table::Cache::removeTable(*this, identifier, RTFC_NO_FLAG)));
}


void TableList::unlock_table_name()
{
  if (table)
  {
    table::remove_table(static_cast<table::Concurrent *>(table));
    locking::broadcast_refresh();
  }
}


static bool locked_named_table(TableList *table_list)
{
  for (; table_list; table_list=table_list->next_local)
  {
    Table *table= table_list->table;
    if (table)
    {
      Table *save_next= table->getNext();
      table->setNext(NULL);
      bool result= table::Cache::areTablesUsed(table_list->table, 0);
      table->setNext(save_next);
      if (result)
        return 1;
    }
  }
  return 0;         // All tables are locked
}


bool Session::wait_for_locked_table_names(TableList *table_list)
{
  bool result= false;

#if 0
  assert(ownership of table::Cache::mutex());
#endif

  while (locked_named_table(table_list))
  {
    if (getKilled())
    {
      result= true;
      break;
    }
    wait_for_condition(table::Cache::mutex(), COND_refresh);
    table::Cache::mutex().lock(); /* Wait for a table to unlock and then lock it */
  }
  return result;
}


bool Session::lock_table_names(TableList *table_list)
{
  bool got_all_locks= true;
  for (TableList* lock_table= table_list; lock_table; lock_table= lock_table->next_local)
  {
    int got_lock= lock_table_name(lock_table);
    if (got_lock < 0)
    {
      table_list->unlock_table_names(table_list);
      return true; // Fatal error
    }
    if (got_lock)
      got_all_locks= false;       // Someone is using table
  }

  /* If some table was in use, wait until we got the lock */
  if (not got_all_locks && wait_for_locked_table_names(table_list))
  {
    table_list->unlock_table_names(table_list);
    return true;
  }
  return false;
}


bool Session::lock_table_names_exclusively(TableList *table_list)
{
  if (lock_table_names(table_list))
    return true;

  /*
    Upgrade the table name locks from semi-exclusive to exclusive locks.
  */
  for (TableList *table= table_list; table; table= table->next_global)
  {
    if (table->table)
      table->table->open_placeholder= 1;
  }
  return false;
}


void TableList::unlock_table_names(TableList *last_table)
{
  for (TableList *table_iter= this; table_iter != last_table; table_iter= table_iter->next_local)
  {
    table_iter->unlock_table_name();
  }
  locking::broadcast_refresh();
}


static void print_lock_error(int error, const char *table)
{
  drizzled::error_t textno;
  switch (error) 
  {
  case HA_ERR_LOCK_WAIT_TIMEOUT:
    textno=ER_LOCK_WAIT_TIMEOUT;
    break;
  case HA_ERR_READ_ONLY_TRANSACTION:
    textno=ER_READ_ONLY_TRANSACTION;
    break;
  case HA_ERR_LOCK_DEADLOCK:
    textno=ER_LOCK_DEADLOCK;
    break;
  case HA_ERR_WRONG_COMMAND:
    textno=ER_ILLEGAL_HA;
    break;
  default:
    textno=ER_CANT_LOCK;
    break;
  }

  if ( textno == ER_ILLEGAL_HA )
    my_error(textno, MYF(ME_BELL+ME_OLDWIN+ME_WAITTANG), table);
  else
    my_error(textno, MYF(ME_BELL+ME_OLDWIN+ME_WAITTANG), error);
}


/****************************************************************************
  Handling of global read locks

  Taking the global read lock is TWO steps (2nd step is optional; without
  it, COMMIT of existing transactions will be allowed):
  lock_global_read_lock() THEN make_global_read_lock_block_commit().

  The global locks are handled through the global variables:
  global_read_lock
    count of threads which have the global read lock (i.e. have completed at
    least the first step above)
  global_read_lock_blocks_commit
    count of threads which have the global read lock and block
    commits (i.e. are in or have completed the second step above)
  waiting_for_read_lock
    count of threads which want to take a global read lock but cannot
  protect_against_global_read_lock
    count of threads which have set protection against global read lock.

  access to them is protected with a mutex LOCK_global_read_lock

  (XXX: one should never take table::Cache::mutex() if LOCK_global_read_lock is
  taken, otherwise a deadlock may occur. Other mutexes could be a
  problem too - grep the code for global_read_lock if you want to use
  any other mutex here) Also one must not hold table::Cache::mutex() when calling
  wait_if_global_read_lock(). When the thread with the global read lock
  tries to close its tables, it needs to take table::Cache::mutex() in
  close_thread_table().

  How blocking of threads by global read lock is achieved: that's
  advisory. Any piece of code which should be blocked by global read lock must
  be designed like this:
  - call to wait_if_global_read_lock(). When this returns 0, no global read
  lock is owned; if argument abort_on_refresh was 0, none can be obtained.
  - job
  - if abort_on_refresh was 0, call to session->startWaitingGlobalReadLock() to
  allow other threads to get the global read lock. I.e. removal of the
  protection.
  (Note: it's a bit like an implementation of rwlock).

  [ I am sorry to mention some SQL syntaxes below I know I shouldn't but found
  no better descriptive way ]

  Why does FLUSH TABLES WITH READ LOCK need to block COMMIT: because it's used
  to read a non-moving SHOW MASTER STATUS, and a COMMIT writes to the binary
  log.

  Why getting the global read lock is two steps and not one. Because FLUSH
  TABLES WITH READ LOCK needs to insert one other step between the two:
  flushing tables. So the order is
  1) lock_global_read_lock() (prevents any new table write locks, i.e. stalls
  all new updates)
  2) close_cached_tables() (the FLUSH TABLES), which will wait for tables
  currently opened and being updated to close (so it's possible that there is
  a moment where all new updates of server are stalled *and* FLUSH TABLES WITH
  READ LOCK is, too).
  3) session::makeGlobalReadLockBlockCommit().
  If we have merged 1) and 3) into 1), we would have had this deadlock:
  imagine thread 1 and 2, in non-autocommit mode, thread 3, and an InnoDB
  table t.
  session1: SELECT * FROM t FOR UPDATE;
  session2: UPDATE t SET a=1; # blocked by row-level locks of session1
  session3: FLUSH TABLES WITH READ LOCK; # blocked in close_cached_tables() by the
  table instance of session2
  session1: COMMIT; # blocked by session3.
  session1 blocks session2 which blocks session3 which blocks session1: deadlock.

  Note that we need to support that one thread does
  FLUSH TABLES WITH READ LOCK; and then COMMIT;
  (that's what innobackup does, for some good reason).
  So in this exceptional case the COMMIT should not be blocked by the FLUSH
  TABLES WITH READ LOCK.

****************************************************************************/

volatile uint32_t global_read_lock=0;
volatile uint32_t global_read_lock_blocks_commit=0;
static volatile uint32_t protect_against_global_read_lock=0;
static volatile uint32_t waiting_for_read_lock=0;

bool Session::lockGlobalReadLock()
{
  if (isGlobalReadLock() == Session::NONE)
  {
    const char *old_message;
    LOCK_global_read_lock.lock();
    old_message= enter_cond(COND_global_read_lock, LOCK_global_read_lock,
                            "Waiting to get readlock");

    waiting_for_read_lock++;
    boost::mutex::scoped_lock scopedLock(LOCK_global_read_lock, boost::adopt_lock_t());
    while (protect_against_global_read_lock && not getKilled())
      COND_global_read_lock.wait(scopedLock);
    waiting_for_read_lock--;
    scopedLock.release();
    if (getKilled())
    {
      exit_cond(old_message);
      return true;
    }
    setGlobalReadLock(Session::GOT_GLOBAL_READ_LOCK);
    global_read_lock++;
    exit_cond(old_message); // this unlocks LOCK_global_read_lock
  }

  /*
    We DON'T set global_read_lock_blocks_commit now, it will be set after
    tables are flushed (as the present function serves for FLUSH TABLES WITH
    READ LOCK only). Doing things in this order is necessary to avoid
    deadlocks (we must allow COMMIT until all tables are closed; we should not
    forbid it before, or we can have a 3-thread deadlock if 2 do SELECT FOR
    UPDATE and one does FLUSH TABLES WITH READ LOCK).
  */
  return false;
}


void Session::unlockGlobalReadLock(void)
{
  uint32_t tmp;

  if (not isGlobalReadLock()) // If we have no personal stake in the global lock, just return
    return;

  {
    boost::mutex::scoped_lock scopedLock(LOCK_global_read_lock);
    tmp= --global_read_lock;
    if (isGlobalReadLock() == Session::MADE_GLOBAL_READ_LOCK_BLOCK_COMMIT)
      --global_read_lock_blocks_commit;
  }
  /* Send the signal outside the mutex to avoid a context switch */
  if (not tmp)
  {
    COND_global_read_lock.notify_all();
  }
  setGlobalReadLock(Session::NONE);
}

static inline bool must_wait(bool is_not_commit)
{
  return (global_read_lock &&
          (is_not_commit ||
          global_read_lock_blocks_commit));
}

bool Session::wait_if_global_read_lock(bool abort_on_refresh, bool is_not_commit)
{
  const char *old_message= NULL;
  bool result= 0, need_exit_cond;

  /*
    Assert that we do not own table::Cache::mutex(). If we would own it, other
    threads could not close their tables. This would make a pretty
    deadlock.
  */
  safe_mutex_assert_not_owner(table::Cache::mutex().native_handle());

  LOCK_global_read_lock.lock();
  if ((need_exit_cond= must_wait(is_not_commit)))
  {
    if (isGlobalReadLock())   // This thread had the read locks
    {
      if (is_not_commit)
        my_message(ER_CANT_UPDATE_WITH_READLOCK,
                   ER(ER_CANT_UPDATE_WITH_READLOCK), MYF(0));
      LOCK_global_read_lock.unlock();
      /*
        We allow FLUSHer to COMMIT; we assume FLUSHer knows what it does.
        This allowance is needed to not break existing versions of innobackup
        which do a BEGIN; INSERT; FLUSH TABLES WITH READ LOCK; COMMIT.
      */
      return is_not_commit;
    }
    old_message= enter_cond(COND_global_read_lock, LOCK_global_read_lock,
                            "Waiting for release of readlock");

    while (must_wait(is_not_commit) && not getKilled() &&
     (!abort_on_refresh || open_tables.version == g_refresh_version))
    {
      boost::mutex::scoped_lock scoped(LOCK_global_read_lock, boost::adopt_lock_t());
      COND_global_read_lock.wait(scoped);
      scoped.release();
    }
    if (getKilled())
      result=1;
  }

  if (not abort_on_refresh && not result)
    protect_against_global_read_lock++;

  /*
    The following is only true in case of a global read locks (which is rare)
    and if old_message is set
  */
  if (unlikely(need_exit_cond))
  {
    exit_cond(old_message); // this unlocks LOCK_global_read_lock
  }
  else
  {
    LOCK_global_read_lock.unlock();
  }

  return result;
}


void Session::startWaitingGlobalReadLock()
{
  if (unlikely(isGlobalReadLock()))
    return;

  LOCK_global_read_lock.lock();
  bool tmp= (!--protect_against_global_read_lock && (waiting_for_read_lock || global_read_lock_blocks_commit));
  LOCK_global_read_lock.unlock();

  if (tmp)
    COND_global_read_lock.notify_all();
}


bool Session::makeGlobalReadLockBlockCommit()
{
  bool error;
  const char *old_message;
  /*
    If we didn't succeed lock_global_read_lock(), or if we already suceeded
    Session::makeGlobalReadLockBlockCommit(), do nothing.
  */
  if (isGlobalReadLock() != Session::GOT_GLOBAL_READ_LOCK)
    return false;
  LOCK_global_read_lock.lock();
  /* increment this BEFORE waiting on cond (otherwise race cond) */
  global_read_lock_blocks_commit++;
  old_message= enter_cond(COND_global_read_lock, LOCK_global_read_lock,
                          "Waiting for all running commits to finish");
  while (protect_against_global_read_lock && not getKilled())
  {
    boost::mutex::scoped_lock scopedLock(LOCK_global_read_lock, boost::adopt_lock_t());
    COND_global_read_lock.wait(scopedLock);
    scopedLock.release();
  }
  if ((error= test(getKilled())))
  {
    global_read_lock_blocks_commit--; // undo what we did
  }
  else
  {
    setGlobalReadLock(Session::MADE_GLOBAL_READ_LOCK_BLOCK_COMMIT);
  }

  exit_cond(old_message); // this unlocks LOCK_global_read_lock

  return error;
}


void locking::broadcast_refresh()
{
  COND_refresh.notify_all();
  COND_global_read_lock.notify_all();
}

} /* namespace drizzled */