The Tor Browser: security/sandbox/chromium/base/memory/singleton.h@141e0f1194b1 (annotated)

security/sandbox/chromium/base/memory/singleton.h@141e0f1194b1 (annotated)

security/sandbox/chromium/base/memory/singleton.h

Wed, 31 Dec 2014 07:16:47 +0100

author: Michael Schloh von Bennewitz <michael@schloh.com>
date: Wed, 31 Dec 2014 07:16:47 +0100
branch: TOR_BUG_9701
changeset 3: 141e0f1194b1
permissions: -rw-r--r--

Revert simplistic fix pending revisit of Mozilla integration attempt.

 // 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/third_party/dynamic_annotations/dynamic_annotations.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;
   // 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;
 };
 // 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;
   static const bool kAllowedToAccessOnNonjoinableThread = true;
 };
 // 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
     base::subtle::AtomicWord value = base::subtle::NoBarrier_Load(&instance_);
     if (value != 0 && value != base::internal::kBeingCreatedMarker) {
       // See the corresponding HAPPENS_BEFORE below.
       ANNOTATE_HAPPENS_AFTER(&instance_);
       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();
       // This annotation helps race detectors recognize correct lock-less
       // synchronization between different threads calling get().
       // See the corresponding HAPPENS_AFTER below and above.
       ANNOTATE_HAPPENS_BEFORE(&instance_);
       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_);
     // See the corresponding HAPPENS_BEFORE above.
     ANNOTATE_HAPPENS_AFTER(&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_

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security/sandbox/chromium/base/memory/singleton.h@141e0f1194b1 (annotated)

security/sandbox/chromium/base/memory/singleton.h