third_party/WebKit/Source/wtf/ThreadRestrictionVerifier.h

/*
 * Copyright (C) 2011 Google Inc. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *     * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above
 * copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the
 * distribution.
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 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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 */

#ifndef ThreadRestrictionVerifier_h
#define ThreadRestrictionVerifier_h

#include "wtf/Assertions.h"

#if DCHECK_IS_ON()

#include "wtf/Threading.h"

namespace WTF {

// Verifies that a class is used in a way that respects its lack of
// thread-safety.  The default mode is to verify that the object will only be
// used on a single thread. The thread gets captured when setShared(true) is
// called.  The mode may be changed by calling useMutexMode (or
// turnOffVerification).
class ThreadRestrictionVerifier {
 public:
  ThreadRestrictionVerifier() : m_shared(false), m_owningThread(0) {}

  // Call onRef() before refCount is incremented in ref(). Returns whether the
  // ref() is safe.
  template <typename COUNTERTYPE>
  bool onRef(COUNTERTYPE refCount) {
    // Start thread verification as soon as the ref count gets to 2. This
    // heuristic reflects the fact that items are often created on one
    // thread and then given to another thread to be used.
    // FIXME: Make this restriction tigher. Especially as we move to more
    // common methods for sharing items across threads like
    // CrossThreadCopier.h
    // We should be able to add a "detachFromThread" method to make this
    // explicit.
    if (refCount == 1)
      setShared(true);
    return isSafeToUse();
  }

  // Call onDeref() before refCount is decremented in deref(). Returns whether
  // the deref() is safe.
  template <typename COUNTERTYPE>
  bool onDeref(COUNTERTYPE refCount) {
    bool safe = isSafeToUse();
    // Stop thread verification when the ref goes to 1 because it
    // is safe to be passed to another thread at this point.
    if (refCount == 2)
      setShared(false);
    return safe;
  }

  // Is it OK to use the object at this moment on the current thread?
  bool isSafeToUse() const {
    return !m_shared || m_owningThread == currentThread();
  }

 private:
  // Indicates that the object may (or may not) be owned by more than one place.
  void setShared(bool shared) {
    bool previouslyShared = m_shared;
    m_shared = shared;

    if (!m_shared)
      return;

    DCHECK_NE(shared, previouslyShared);
    // Capture the current thread to verify that subsequent ref/deref happen on
    // this thread.
    m_owningThread = currentThread();
  }

  bool m_shared;

  ThreadIdentifier m_owningThread;
};

}  // namespace WTF

#endif  // DCHECK_IS_ON()
#endif  // ThreadRestrictionVerifier_h