server/dep/include/zthread/FairReadWriteLock.h

/*
 * Copyright (c) 2005, Eric Crahen
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is furnished
 * to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
 * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 *
 */

#ifndef __ZTFAIRREADWRITELOCK_H__
#define __ZTFAIRREADWRITELOCK_H__

#include "zthread/ReadWriteLock.h"
#include "zthread/Condition.h"
#include "zthread/Guard.h"
#include "zthread/Mutex.h"

namespace ZThread {

  /**
   * @class FairReadWriteLock
   *
   * @author Eric Crahen <http://www.code-foo.com>
   * @date <2003-07-16T10:26:25-0400>
   * @version 2.2.7
   *  
   * A FairReadWriteLock maintains a balance between the order read-only access 
   * and read-write access is allowed. Threads contending for the pair of Lockable
   * objects this ReadWriteLock provides will gain access to the locks in FIFO order.
   *
   * @see ReadWriteLock 
   */
  class FairReadWriteLock : public ReadWriteLock {

    Mutex _lock;
    Condition _cond;
  
    volatile int _readers;

    //! @class ReadLock
    class ReadLock : public Lockable {

      FairReadWriteLock& _rwlock;

    public:

      ReadLock(FairReadWriteLock& rwlock) : _rwlock(rwlock) {}

      virtual ~ReadLock() {}

      virtual void acquire() {

        Guard<Mutex> g(_rwlock._lock);
        ++_rwlock._readers; 

      }

      virtual bool tryAcquire(unsigned long timeout) {
      
        if(!_rwlock._lock.tryAcquire(timeout))
          return false;

        ++_rwlock._readers; 
        _rwlock._lock.release();

        return true;
      }

      virtual void release() {

        Guard<Mutex> g(_rwlock._lock);
        --_rwlock._readers; 

        if(_rwlock._readers == 0)
          _rwlock._cond.signal();

      }

    };

    //! @class WriteLock
    class WriteLock : public Lockable {

      FairReadWriteLock& _rwlock;

    public:

      WriteLock(FairReadWriteLock& rwlock) : _rwlock(rwlock) {}

      virtual ~WriteLock() {}

      virtual void acquire() {

        _rwlock._lock.acquire();

        try {

          while(_rwlock._readers > 0)
            _rwlock._cond.wait();

        } catch(...) {

          _rwlock._lock.release();
          throw;

        }

      }

      virtual bool tryAcquire(unsigned long timeout) {
      
        if(!_rwlock._lock.tryAcquire(timeout))
          return false;

        try {

          while(_rwlock._readers > 0)
            _rwlock._cond.wait(timeout);

        } catch(...) {

          _rwlock._lock.release();
          throw;

        }

        return true;

      }

      virtual void release() {
        _rwlock._lock.release();
      }

    };

    friend class ReadLock;
    friend class WriteLock;

    ReadLock _rlock;
    WriteLock _wlock;

  public:
  
    /**
     * Create a BiasedReadWriteLock
     *
     * @exception Initialization_Exception thrown if resources could not be 
     *            allocated for this object.
     */
    FairReadWriteLock() : _cond(_lock), _readers(0), _rlock(*this), _wlock(*this) {}

    //! Destroy this ReadWriteLock
    virtual ~FairReadWriteLock() {}

    /**
     * @see ReadWriteLock::getReadLock()
     */
    virtual Lockable& getReadLock() { return _rlock; }

    /**
     * @see ReadWriteLock::getWriteLock()
     */
    virtual Lockable& getWriteLock() { return _wlock; }
  
  };

}; // __ZTFAIRREADWRITELOCK_H__

#endif