// Copyright (c) 2012 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.

// The LazyInstance<Type, Traits> class manages a single instance of Type,

// which will be lazily created on the first time it's accessed.  This class is

// useful for places you would normally use a function-level static, but you

// need to have guaranteed thread-safety.  The Type constructor will only ever

// be called once, even if two threads are racing to create the object.  Get()

// and Pointer() will always return the same, completely initialized instance.

// When the instance is constructed it is registered with AtExitManager.  The

// destructor will be called on program exit.

//

// LazyInstance is completely thread safe, assuming that you create it safely.

// The class was designed to be POD initialized, so it shouldn't require a

// static constructor.  It really only makes sense to declare a LazyInstance as

// a global variable using the LAZY_INSTANCE_INITIALIZER initializer.

//

// LazyInstance is similar to Singleton, except it does not have the singleton

// property.  You can have multiple LazyInstance's of the same type, and each

// will manage a unique instance.  It also preallocates the space for Type, as

// to avoid allocating the Type instance on the heap.  This may help with the

// performance of creating the instance, and reducing heap fragmentation.  This

// requires that Type be a complete type so we can determine the size.

//

// Example usage:

//   static LazyInstance<MyClass> my_instance = LAZY_INSTANCE_INITIALIZER;

//   void SomeMethod() {

//     my_instance.Get().SomeMethod();  // MyClass::SomeMethod()

//

//     MyClass* ptr = my_instance.Pointer();

//     ptr->DoDoDo();  // MyClass::DoDoDo

//   }

#ifndef BUTIL_LAZY_INSTANCE_H_

#define BUTIL_LAZY_INSTANCE_H_

#include <new>  // For placement new.

#include "butil/atomicops.h"

#include "butil/base_export.h"

#include "butil/debug/leak_annotations.h"

#include "butil/logging.h"

#include "butil/memory/aligned_memory.h"

#include "butil/third_party/dynamic_annotations/dynamic_annotations.h"

#include "butil/threading/thread_restrictions.h"

// LazyInstance uses its own struct initializer-list style static

// initialization, as base's LINKER_INITIALIZED requires a constructor and on

// some compilers (notably gcc 4.4) this still ends up needing runtime

// initialization.

#define LAZY_INSTANCE_INITIALIZER { 0, {{0}} }

namespace butil {

template <typename Type>

struct DefaultLazyInstanceTraits {

  static const bool kRegisterOnExit;

#ifndef NDEBUG

  static const bool kAllowedToAccessOnNonjoinableThread;

#endif

  static Type* New(void* instance) {

    DCHECK_EQ(reinterpret_cast<uintptr_t>(instance) & (ALIGNOF(Type) - 1), 0u)

        << ": Bad boy, the buffer passed to placement new is not aligned!\n"

        "This may break some stuff like SSE-based optimizations assuming the "

        "<Type> objects are word aligned.";

    // Use placement new to initialize our instance in our preallocated space.

    // The parenthesis is very important here to force POD type initialization.

    return new (instance) Type();

  }

Unexecuted instantiation: butil::DefaultLazyInstanceTraits<butil::UnixEpochSingleton>::New(void*)

Unexecuted instantiation: butil::DefaultLazyInstanceTraits<butil::Lock>::New(void*)

  static void Delete(Type* instance) {

    // Explicitly call the destructor.

    instance->~Type();

  }

};

// NOTE(gejun): BullseyeCoverage Compile C++ 8.4.4 complains about `undefined

// reference' on in-place assignments to static constants.

template <typename Type>

const bool DefaultLazyInstanceTraits<Type>::kRegisterOnExit = true;

#ifndef NDEBUG

template <typename Type>

const bool DefaultLazyInstanceTraits<Type>::kAllowedToAccessOnNonjoinableThread = false;

#endif

// We pull out some of the functionality into non-templated functions, so we

// can implement the more complicated pieces out of line in the .cc file.

namespace internal {

// Use LazyInstance<T>::Leaky for a less-verbose call-site typedef; e.g.:

// butil::LazyInstance<T>::Leaky my_leaky_lazy_instance;

// instead of:

// butil::LazyInstance<T, butil::internal::LeakyLazyInstanceTraits<T> >

// my_leaky_lazy_instance;

// (especially when T is MyLongTypeNameImplClientHolderFactory).

// Only use this internal::-qualified verbose form to extend this traits class

// (depending on its implementation details).

template <typename Type>

struct LeakyLazyInstanceTraits {

  static const bool kRegisterOnExit;

#ifndef NDEBUG

  static const bool kAllowedToAccessOnNonjoinableThread;

#endif

  static Type* New(void* instance) {

    // Instruct LeakSanitizer to ignore the designated memory leak.

    ANNOTATE_SCOPED_MEMORY_LEAK;

    return DefaultLazyInstanceTraits<Type>::New(instance);

  }

Unexecuted instantiation: butil::internal::LeakyLazyInstanceTraits<butil::UnixEpochSingleton>::New(void*)

Unexecuted instantiation: butil::internal::LeakyLazyInstanceTraits<butil::Lock>::New(void*)

  static void Delete(Type* instance) {

  }

Unexecuted instantiation: butil::internal::LeakyLazyInstanceTraits<butil::UnixEpochSingleton>::Delete(butil::UnixEpochSingleton*)

Unexecuted instantiation: butil::internal::LeakyLazyInstanceTraits<butil::Lock>::Delete(butil::Lock*)

};

template <typename Type>

const bool LeakyLazyInstanceTraits<Type>::kRegisterOnExit = false;

#ifndef NDEBUG

template <typename Type>

const bool LeakyLazyInstanceTraits<Type>::kAllowedToAccessOnNonjoinableThread = true;

#endif

// Our AtomicWord doubles as a spinlock, where a value of

// kBeingCreatedMarker means the spinlock is being held for creation.

static const subtle::AtomicWord kLazyInstanceStateCreating = 1;

// Check if instance needs to be created. If so return true otherwise

// if another thread has beat us, wait for instance to be created and

// return false.

BUTIL_EXPORT bool NeedsLazyInstance(subtle::AtomicWord* state);

// After creating an instance, call this to register the dtor to be called

// at program exit and to update the atomic state to hold the |new_instance|

BUTIL_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state,

                                      subtle::AtomicWord new_instance,

                                      void* lazy_instance,

                                      void (*dtor)(void*));

}  // namespace internal

template <typename Type, typename Traits = DefaultLazyInstanceTraits<Type> >

class LazyInstance {

 public:

  // Do not define a destructor, as doing so makes LazyInstance a

  // non-POD-struct. We don't want that because then a static initializer will

  // be created to register the (empty) destructor with atexit() under MSVC, for

  // example. We handle destruction of the contained Type class explicitly via

  // the OnExit member function, where needed.

  // ~LazyInstance() {}

  // Convenience typedef to avoid having to repeat Type for leaky lazy

  // instances.

  typedef LazyInstance<Type, internal::LeakyLazyInstanceTraits<Type> > Leaky;

  Type& Get() {

    return *Pointer();

  }

  Type* Pointer() {

#ifndef NDEBUG

    // Avoid making TLS lookup on release builds.

    if (!Traits::kAllowedToAccessOnNonjoinableThread)

      ThreadRestrictions::AssertSingletonAllowed();

#endif

    // If any bit in the created mask is true, the instance has already been

    // fully constructed.

    static const subtle::AtomicWord kLazyInstanceCreatedMask =

        ~internal::kLazyInstanceStateCreating;

    // We will hopefully have fast access when the instance is already created.

    // Since a thread sees private_instance_ == 0 or kLazyInstanceStateCreating

    // at most once, the load is taken out of NeedsInstance() as a fast-path.

    // The load has acquire memory ordering as a thread which sees

    // private_instance_ > creating needs to acquire visibility over

    // the associated data (private_buf_). Pairing Release_Store is in

    // CompleteLazyInstance().

    subtle::AtomicWord value = subtle::Acquire_Load(&private_instance_);

    if (!(value & kLazyInstanceCreatedMask) &&

        internal::NeedsLazyInstance(&private_instance_)) {

      // Create the instance in the space provided by |private_buf_|.

      value = reinterpret_cast<subtle::AtomicWord>(

          Traits::New(private_buf_.void_data()));

      internal::CompleteLazyInstance(&private_instance_, value, this,

                                     Traits::kRegisterOnExit ? OnExit : NULL);

    }

    // This annotation helps race detectors recognize correct lock-less

    // synchronization between different threads calling Pointer().

    // We suggest dynamic race detection tool that "Traits::New" above

    // and CompleteLazyInstance(...) happens before "return instance()" below.

    // See the corresponding HAPPENS_BEFORE in CompleteLazyInstance(...).

    ANNOTATE_HAPPENS_AFTER(&private_instance_);

    return instance();

  }

Unexecuted instantiation: butil::LazyInstance<butil::UnixEpochSingleton, butil::internal::LeakyLazyInstanceTraits<butil::UnixEpochSingleton> >::Pointer()

Unexecuted instantiation: butil::LazyInstance<butil::Lock, butil::internal::LeakyLazyInstanceTraits<butil::Lock> >::Pointer()

  bool operator==(Type* p) {

    switch (subtle::NoBarrier_Load(&private_instance_)) {

      case 0:

        return p == NULL;

      case internal::kLazyInstanceStateCreating:

        return static_cast<void*>(p) == private_buf_.void_data();

      default:

        return p == instance();

    }

  }

  // Effectively private: member data is only public to allow the linker to

  // statically initialize it and to maintain a POD class. DO NOT USE FROM

  // OUTSIDE THIS CLASS.

  subtle::AtomicWord private_instance_;

  // Preallocated space for the Type instance.

  butil::AlignedMemory<sizeof(Type), ALIGNOF(Type)> private_buf_;

 private:

  Type* instance() {

    return reinterpret_cast<Type*>(subtle::NoBarrier_Load(&private_instance_));

  }

Unexecuted instantiation: butil::LazyInstance<butil::UnixEpochSingleton, butil::internal::LeakyLazyInstanceTraits<butil::UnixEpochSingleton> >::instance()

Unexecuted instantiation: butil::LazyInstance<butil::Lock, butil::internal::LeakyLazyInstanceTraits<butil::Lock> >::instance()

  // Adapter function for use with AtExit.  This should be called single

  // threaded, so don't synchronize across threads.

  // Calling OnExit while the instance is in use by other threads is a mistake.

  static void OnExit(void* lazy_instance) {

    LazyInstance<Type, Traits>* me =

        reinterpret_cast<LazyInstance<Type, Traits>*>(lazy_instance);

    Traits::Delete(me->instance());

    subtle::NoBarrier_Store(&me->private_instance_, 0);

  }

Unexecuted instantiation: butil::LazyInstance<butil::UnixEpochSingleton, butil::internal::LeakyLazyInstanceTraits<butil::UnixEpochSingleton> >::OnExit(void*)

Unexecuted instantiation: butil::LazyInstance<butil::Lock, butil::internal::LeakyLazyInstanceTraits<butil::Lock> >::OnExit(void*)

};

}  // namespace butil

#endif  // BUTIL_LAZY_INSTANCE_H_