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Diffstat (limited to 'security/sandbox/chromium/base/memory/singleton.h')
| -rw-r--r-- | security/sandbox/chromium/base/memory/singleton.h | 284 |
1 files changed, 284 insertions, 0 deletions
diff --git a/security/sandbox/chromium/base/memory/singleton.h b/security/sandbox/chromium/base/memory/singleton.h new file mode 100644 index 000000000..79e4441a8 --- /dev/null +++ b/security/sandbox/chromium/base/memory/singleton.h @@ -0,0 +1,284 @@ +// 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/macros.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); + +class DeleteTraceLogForTesting; + +} // namespace internal + + +// 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 (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() { subtle::NoBarrier_Store(&dead_, 0); } + + private: + static AlignedMemory<sizeof(Type), ALIGNOF(Type)> buffer_; + // Signal the object was already deleted, so it is not revived. + static subtle::Atomic32 dead_; +}; + +template <typename Type> +AlignedMemory<sizeof(Type), ALIGNOF(Type)> + StaticMemorySingletonTraits<Type>::buffer_; +template <typename Type> +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 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 internal::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) + ThreadRestrictions::AssertSingletonAllowed(); +#endif + + // The load has acquire memory ordering as the thread which reads the + // instance_ pointer must acquire visibility over the singleton data. + subtle::AtomicWord value = subtle::Acquire_Load(&instance_); + if (value != 0 && value != internal::kBeingCreatedMarker) { + return reinterpret_cast<Type*>(value); + } + + // Object isn't created yet, maybe we will get to create it, let's try... + if (subtle::Acquire_CompareAndSwap(&instance_, 0, + 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. + subtle::Release_Store(&instance_, + reinterpret_cast<subtle::AtomicWord>(newval)); + + if (newval != NULL && Traits::kRegisterAtExit) + AtExitManager::RegisterCallback(OnExit, NULL); + + return newval; + } + + // We hit a race. Wait for the other thread to complete it. + value = 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*>(subtle::NoBarrier_Load(&instance_))); + instance_ = 0; + } + static subtle::AtomicWord instance_; +}; + +template <typename Type, typename Traits, typename DifferentiatingType> +subtle::AtomicWord Singleton<Type, Traits, DifferentiatingType>::instance_ = 0; + +} // namespace base + +#endif // BASE_MEMORY_SINGLETON_H_ |