base/lock.h

// Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_LOCK_H_
#define BASE_LOCK_H_

#include "base/lock_impl.h"

// A convenient wrapper for an OS specific critical section.
//
// NOTE: Although windows critical sections support recursive locks, we do not
// allow this, and we will commonly fire a DCHECK() if a thread attempts to
// acquire the lock a second time (while already holding it).
//
// Complication: UnitTest for DeathTests catch DCHECK exceptions, so we need
// to write code assuming DCHECK will throw.  This means we need to save any
// assertable value in a local until we can safely throw.

class Lock {
 public:
  Lock();
  ~Lock();
  void Acquire();
  void Release();
  // If the lock is not held, take it and return true. If the lock is already
  // held by another thread, immediately return false.
  bool Try();

  // Return the underlying lock implementation.
  // TODO(awalker): refactor lock and condition variables so that this is
  // unnecessary.
  LockImpl* lock_impl() { return &lock_; }

 private:
  LockImpl lock_;  // User-supplied underlying lock implementation.

#ifndef NDEBUG
  // All private data is implicitly protected by lock_.
  // Be VERY careful to only access members under that lock.
  int32 recursion_count_shadow_;
  bool recursion_used_;      // Allow debugging to continued after a DCHECK().
  int32 acquisition_count_;  // Number of times lock was acquired.
  int32 contention_count_;   // Number of times there was contention.
#endif  // NDEBUG

  DISALLOW_COPY_AND_ASSIGN(Lock);
};

// A helper class that acquires the given Lock while the AutoLock is in scope.
class AutoLock {
 public:
  explicit AutoLock(Lock& lock) : lock_(lock) {
    lock_.Acquire();
  }

  ~AutoLock() {
    lock_.Release();
  }

 private:
  Lock& lock_;
  DISALLOW_COPY_AND_ASSIGN(AutoLock);
};

// AutoUnlock is a helper class for ConditionVariable that will Release() the
// lock argument in the constructor, and re-Acquire() it in the destructor.
class ConditionVariable;
class AutoUnlock {
 private:  // Everything is private, so only our friend can use us.
  friend class ConditionVariable;  // The only user of this class.

  explicit AutoUnlock(Lock& lock) : lock_(&lock) {
    // We require our caller to have the lock.
    lock_->Release();
  }

  ~AutoUnlock() {
    lock_->Acquire();
  }

  Lock* lock_;
};

#endif  // BASE_LOCK_H_