// 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>::Leaky inst = LAZY_INSTANCE_INITIALIZER;

//   void SomeMethod() {

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

//

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

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

//   }

#ifndef BASE_LAZY_INSTANCE_H_

#define BASE_LAZY_INSTANCE_H_

#include <new>  // For placement new.

#include "base/atomicops.h"

#include "base/base_export.h"

#include "base/debug/leak_annotations.h"

#include "base/logging.h"

#include "base/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}

namespace base {

template <typename Type>

struct LazyInstanceTraitsBase {

  static Type* New(void* instance) {

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

    // 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();

  }

  static void CallDestructor(Type* instance) {

    // Explicitly call the destructor.

    instance->~Type();

  }

};

// 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 {

// This traits class causes destruction the contained Type at process exit via

// AtExitManager. This is probably generally not what you want. Instead, prefer

// Leaky below.

template <typename Type>

struct DestructorAtExitLazyInstanceTraits {

  static const bool kRegisterOnExit = true;

#if DCHECK_IS_ON()

  static const bool kAllowedToAccessOnNonjoinableThread = false;

#endif

  static Type* New(void* instance) {

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

  }

  static void Delete(Type* instance) {

    LazyInstanceTraitsBase<Type>::CallDestructor(instance);

  }

};

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

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

// instead of:

// base::LazyInstance<T, base::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 = false;

#if DCHECK_IS_ON()

  static const bool kAllowedToAccessOnNonjoinableThread = true;

#endif

  static Type* New(void* instance) {

    ANNOTATE_SCOPED_MEMORY_LEAK;

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

  }

  static void Delete(Type* instance) {

  }

};

template <typename Type>

struct ErrorMustSelectLazyOrDestructorAtExitForLazyInstance {};

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

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

constexpr 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.

BASE_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|

BASE_EXPORT void CompleteLazyInstance(subtle::AtomicWord* state,

                                      subtle::AtomicWord new_instance,

                                      void (*destructor)(void*),

                                      void* destructor_arg);

// If |state| is uninitialized, constructs a value using |creator_func|, stores

// it into |state| and registers |destructor| to be called with |destructor_arg|

// as argument when the current AtExitManager goes out of scope. Then, returns

// the value stored in |state|. It is safe to have concurrent calls to this

// function with the same |state|.

template <typename CreatorFunc>

void* GetOrCreateLazyPointer(subtle::AtomicWord* state,

                             const CreatorFunc& creator_func,

                             void (*destructor)(void*),

                             void* destructor_arg) {

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

  // fully constructed.

  constexpr subtle::AtomicWord kLazyInstanceCreatedMask =

      ~internal::kLazyInstanceStateCreating;

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

  // Since a thread sees |state| == 0 or kLazyInstanceStateCreating at most

  // once, the load is taken out of NeedsLazyInstance() as a fast-path. The load

  // has acquire memory ordering as a thread which sees |state| > creating needs

  // to acquire visibility over the associated data. Pairing Release_Store is in

  // CompleteLazyInstance().

  subtle::AtomicWord value = subtle::Acquire_Load(state);

  if (!(value & kLazyInstanceCreatedMask) && NeedsLazyInstance(state)) {

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

    value = reinterpret_cast<subtle::AtomicWord>(creator_func());

    CompleteLazyInstance(state, value, destructor, destructor_arg);

  }

  return reinterpret_cast<void*>(subtle::NoBarrier_Load(state));

}

}  // namespace internal

template <

    typename Type,

    typename Traits =

        internal::ErrorMustSelectLazyOrDestructorAtExitForLazyInstance<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;

  typedef LazyInstance<Type, internal::DestructorAtExitLazyInstanceTraits<Type>>

      DestructorAtExit;

  Type& Get() {

    return *Pointer();

  }

  Type* Pointer() {

#if DCHECK_IS_ON()

    // Avoid making TLS lookup on release builds.

    if (!Traits::kAllowedToAccessOnNonjoinableThread)

      ThreadRestrictions::AssertSingletonAllowed();

#endif

    return static_cast<Type*>(internal::GetOrCreateLazyPointer(

        &private_instance_,

        [this]() { return Traits::New(private_buf_); },

        Traits::kRegisterOnExit ? OnExit : nullptr, this));

  }

  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_;

      default:

        return p == instance();

    }

  }

  // MSVC gives a warning that the alignment expands the size of the

  // LazyInstance struct to make the size a multiple of the alignment. This

  // is expected in this case.

#if defined(OS_WIN)

#pragma warning(push)

#pragma warning(disable: 4324)

#endif

  // 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.

  alignas(Type) char private_buf_[sizeof(Type)];

#if defined(OS_WIN)

#pragma warning(pop)

#endif

 private:

  Type* instance() {

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

  }

};

}  // namespace base

#endif  // BASE_LAZY_INSTANCE_H_