Move base to DEPS

base/memory/singleton.h

-// Copyright (c) 2011 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.
-
-// PLEASE READ: Do you really need a singleton?
-//
-// Singletons make it hard to determine the lifetime of an object, which can
-// lead to buggy code and spurious crashes.
-//
-// Instead of adding another singleton into the mix, try to identify either:
-//   a) An existing singleton that can manage your object's lifetime
-//   b) Locations where you can deterministically create the object and pass
-//      into other objects
-//
-// If you absolutely need a singleton, please keep them as trivial as possible
-// and ideally a leaf dependency. Singletons get problematic when they attempt
-// to do too much in their destructor or have circular dependencies.
-
-#ifndef BASE_MEMORY_SINGLETON_H_
-#define BASE_MEMORY_SINGLETON_H_
-
-#include "base/at_exit.h"
-#include "base/atomicops.h"
-#include "base/base_export.h"
-#include "base/memory/aligned_memory.h"
-#include "base/threading/thread_restrictions.h"
-
-namespace base {
-namespace internal {
-
-// Our AtomicWord doubles as a spinlock, where a value of
-// kBeingCreatedMarker means the spinlock is being held for creation.
-static const subtle::AtomicWord kBeingCreatedMarker = 1;
-
-// We pull out some of the functionality into a non-templated function, so that
-// we can implement the more complicated pieces out of line in the .cc file.
-BASE_EXPORT subtle::AtomicWord WaitForInstance(subtle::AtomicWord* instance);
-
-}  // namespace internal
-}  // namespace base
-
-// TODO(joth): Move more of this file into namespace base
-
-// Default traits for Singleton<Type>. Calls operator new and operator delete on
-// the object. Registers automatic deletion at process exit.
-// Overload if you need arguments or another memory allocation function.
-template<typename Type>
-struct DefaultSingletonTraits {
-  // Allocates the object.
-  static Type* New() {
-    // The parenthesis is very important here; it forces POD type
-    // initialization.
-    return new Type();
-  }
-
-  // Destroys the object.
-  static void Delete(Type* x) {
-    delete x;
-  }
-
-  // Set to true to automatically register deletion of the object on process
-  // exit. See below for the required call that makes this happen.
-  static const bool kRegisterAtExit = true;
-
-#ifndef NDEBUG
-  // Set to false to disallow access on a non-joinable thread.  This is
-  // different from kRegisterAtExit because StaticMemorySingletonTraits allows
-  // access on non-joinable threads, and gracefully handles this.
-  static const bool kAllowedToAccessOnNonjoinableThread = false;
-#endif
-};
-
-
-// Alternate traits for use with the Singleton<Type>.  Identical to
-// DefaultSingletonTraits except that the Singleton will not be cleaned up
-// at exit.
-template<typename Type>
-struct LeakySingletonTraits : public DefaultSingletonTraits<Type> {
-  static const bool kRegisterAtExit = false;
-#ifndef NDEBUG
-  static const bool kAllowedToAccessOnNonjoinableThread = true;
-#endif
-};
-
-
-// Alternate traits for use with the Singleton<Type>.  Allocates memory
-// for the singleton instance from a static buffer.  The singleton will
-// be cleaned up at exit, but can't be revived after destruction unless
-// the Resurrect() method is called.
-//
-// This is useful for a certain category of things, notably logging and
-// tracing, where the singleton instance is of a type carefully constructed to
-// be safe to access post-destruction.
-// In logging and tracing you'll typically get stray calls at odd times, like
-// during static destruction, thread teardown and the like, and there's a
-// termination race on the heap-based singleton - e.g. if one thread calls
-// get(), but then another thread initiates AtExit processing, the first thread
-// may call into an object residing in unallocated memory. If the instance is
-// allocated from the data segment, then this is survivable.
-//
-// The destructor is to deallocate system resources, in this case to unregister
-// a callback the system will invoke when logging levels change. Note that
-// this is also used in e.g. Chrome Frame, where you have to allow for the
-// possibility of loading briefly into someone else's process space, and
-// so leaking is not an option, as that would sabotage the state of your host
-// process once you've unloaded.
-template <typename Type>
-struct StaticMemorySingletonTraits {
-  // WARNING: User has to deal with get() in the singleton class
-  // this is traits for returning NULL.
-  static Type* New() {
-    // Only constructs once and returns pointer; otherwise returns NULL.
-    if (base::subtle::NoBarrier_AtomicExchange(&dead_, 1))
-      return NULL;
-
-    return new(buffer_.void_data()) Type();
-  }
-
-  static void Delete(Type* p) {
-    if (p != NULL)
-      p->Type::~Type();
-  }
-
-  static const bool kRegisterAtExit = true;
-  static const bool kAllowedToAccessOnNonjoinableThread = true;
-
-  // Exposed for unittesting.
-  static void Resurrect() {
-    base::subtle::NoBarrier_Store(&dead_, 0);
-  }
-
- private:
-  static base::AlignedMemory<sizeof(Type), ALIGNOF(Type)> buffer_;
-  // Signal the object was already deleted, so it is not revived.
-  static base::subtle::Atomic32 dead_;
-};
-
-template <typename Type> base::AlignedMemory<sizeof(Type), ALIGNOF(Type)>
-    StaticMemorySingletonTraits<Type>::buffer_;
-template <typename Type> base::subtle::Atomic32
-    StaticMemorySingletonTraits<Type>::dead_ = 0;
-
-// The Singleton<Type, Traits, DifferentiatingType> class manages a single
-// instance of Type which will be created on first use and will be destroyed at
-// normal process exit). The Trait::Delete function will not be called on
-// abnormal process exit.
-//
-// DifferentiatingType is used as a key to differentiate two different
-// singletons having the same memory allocation functions but serving a
-// different purpose. This is mainly used for Locks serving different purposes.
-//
-// Example usage:
-//
-// In your header:
-//   template <typename T> struct DefaultSingletonTraits;
-//   class FooClass {
-//    public:
-//     static FooClass* GetInstance();  <-- See comment below on this.
-//     void Bar() { ... }
-//    private:
-//     FooClass() { ... }
-//     friend struct DefaultSingletonTraits<FooClass>;
-//
-//     DISALLOW_COPY_AND_ASSIGN(FooClass);
-//   };
-//
-// In your source file:
-//  #include "base/memory/singleton.h"
-//  FooClass* FooClass::GetInstance() {
-//    return Singleton<FooClass>::get();
-//  }
-//
-// And to call methods on FooClass:
-//   FooClass::GetInstance()->Bar();
-//
-// NOTE: The method accessing Singleton<T>::get() has to be named as GetInstance
-// and it is important that FooClass::GetInstance() is not inlined in the
-// header. This makes sure that when source files from multiple targets include
-// this header they don't end up with different copies of the inlined code
-// creating multiple copies of the singleton.
-//
-// Singleton<> has no non-static members and doesn't need to actually be
-// instantiated.
-//
-// This class is itself thread-safe. The underlying Type must of course be
-// thread-safe if you want to use it concurrently. Two parameters may be tuned
-// depending on the user's requirements.
-//
-// Glossary:
-//   RAE = kRegisterAtExit
-//
-// On every platform, if Traits::RAE is true, the singleton will be destroyed at
-// process exit. More precisely it uses base::AtExitManager which requires an
-// object of this type to be instantiated. AtExitManager mimics the semantics
-// of atexit() such as LIFO order but under Windows is safer to call. For more
-// information see at_exit.h.
-//
-// If Traits::RAE is false, the singleton will not be freed at process exit,
-// thus the singleton will be leaked if it is ever accessed. Traits::RAE
-// shouldn't be false unless absolutely necessary. Remember that the heap where
-// the object is allocated may be destroyed by the CRT anyway.
-//
-// Caveats:
-// (a) Every call to get(), operator->() and operator*() incurs some overhead
-//     (16ns on my P4/2.8GHz) to check whether the object has already been
-//     initialized.  You may wish to cache the result of get(); it will not
-//     change.
-//
-// (b) Your factory function must never throw an exception. This class is not
-//     exception-safe.
-//
-template <typename Type,
-          typename Traits = DefaultSingletonTraits<Type>,
-          typename DifferentiatingType = Type>
-class Singleton {
- private:
-  // Classes using the Singleton<T> pattern should declare a GetInstance()
-  // method and call Singleton::get() from within that.
-  friend Type* Type::GetInstance();
-
-  // Allow TraceLog tests to test tracing after OnExit.
-  friend class DeleteTraceLogForTesting;
-
-  // This class is safe to be constructed and copy-constructed since it has no
-  // member.
-
-  // Return a pointer to the one true instance of the class.
-  static Type* get() {
-#ifndef NDEBUG
-    // Avoid making TLS lookup on release builds.
-    if (!Traits::kAllowedToAccessOnNonjoinableThread)
-      base::ThreadRestrictions::AssertSingletonAllowed();
-#endif
-
-    // The load has acquire memory ordering as the thread which reads the
-    // instance_ pointer must acquire visibility over the singleton data.
-    base::subtle::AtomicWord value = base::subtle::Acquire_Load(&instance_);
-    if (value != 0 && value != base::internal::kBeingCreatedMarker) {
-      return reinterpret_cast<Type*>(value);
-    }
-
-    // Object isn't created yet, maybe we will get to create it, let's try...
-    if (base::subtle::Acquire_CompareAndSwap(
-          &instance_, 0, base::internal::kBeingCreatedMarker) == 0) {
-      // instance_ was NULL and is now kBeingCreatedMarker.  Only one thread
-      // will ever get here.  Threads might be spinning on us, and they will
-      // stop right after we do this store.
-      Type* newval = Traits::New();
-
-      // Releases the visibility over instance_ to the readers.
-      base::subtle::Release_Store(
-          &instance_, reinterpret_cast<base::subtle::AtomicWord>(newval));
-
-      if (newval != NULL && Traits::kRegisterAtExit)
-        base::AtExitManager::RegisterCallback(OnExit, NULL);
-
-      return newval;
-    }
-
-    // We hit a race. Wait for the other thread to complete it.
-    value = base::internal::WaitForInstance(&instance_);
-
-    return reinterpret_cast<Type*>(value);
-  }
-
-  // Adapter function for use with AtExit().  This should be called single
-  // threaded, so don't use atomic operations.
-  // Calling OnExit while singleton is in use by other threads is a mistake.
-  static void OnExit(void* /*unused*/) {
-    // AtExit should only ever be register after the singleton instance was
-    // created.  We should only ever get here with a valid instance_ pointer.
-    Traits::Delete(
-        reinterpret_cast<Type*>(base::subtle::NoBarrier_Load(&instance_)));
-    instance_ = 0;
-  }
-  static base::subtle::AtomicWord instance_;
-};
-
-template <typename Type, typename Traits, typename DifferentiatingType>
-base::subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>::
-    instance_ = 0;
-
-#endif  // BASE_MEMORY_SINGLETON_H_