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// Copyright 2018 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.
#include <new>
#include <type_traits>
#include "base/bind.h"
#include "base/callback.h"
#include "base/compiler_specific.h"
#include "base/location.h"
#include "base/memory/aligned_memory.h"
#include "base/memory/ptr_util.h"
#include "base/sequenced_task_runner.h"
namespace base {
// SequenceBound facilitates owning objects that live on a specified sequence,
// which is potentially different than the owner's sequence. It encapsulates
// the work of posting tasks to the specified sequence to construct T, call
// methods on T, and destroy T.
// It does not provide explicit access to the underlying object directly, to
// prevent accidentally using it from the wrong sequence.
// Like std::unique_ptr<T>, a SequenceBound<T> may be moved between owners,
// and posted across threads. It may also be up-casted (only), to permit
// SequenceBound to be used with interfaces.
// Basic usage looks like this:
// // Some class that lives on |main_task_runner|.
// class MyClass {
// public:
// explicit MyClass(const char* widget_title) {}
// virtual ~MyClass() { ... }
// virtual void DoSomething(int arg) { ... }
// };
// // On any thread...
// scoped_refptr<SequencedTaskRunner> main_task_runner = ...;
// auto widget = SequenceBound<MyClass>(main_task_runner, "My Title");
// widget.Post(&MyObject::DoSomething, 1234);
// Note that |widget| is constructed asynchronously on |main_task_runner|,
// but calling Post() immediately is safe, since the actual call is posted
// to |main_task_runner| as well.
// |widget| will be deleted on |main_task_runner| asynchronously when it goes
// out of scope, or when Reset() is called.
// Here is a more complicated example that shows injection and upcasting:
// // Some unrelated class that uses a |MyClass| to do something.
// class SomeConsumer {
// public:
// // Note that ownership of |widget| is given to us!
// explicit SomeConsumer(SequenceBound<MyClass> widget)
// : widget_(std::move(widget)) { ... }
// ~SomeConsumer() {
// // |widget_| will be destroyed on the associated task runner.
// }
// SequenceBound<MyClass> widget_;
// };
// // Implementation of MyClass.
// class MyDerivedClass : public MyClass { ... };
// auto widget =
// SequenceBound<MyDerivedClass>(main_task_runner, ctor args);
// auto c = new SomeConsumer(std::move(widget)); // upcasts to MyClass
namespace internal {
// If we can't cast |Base*| into |Derived*|, then it's a virtual base if and
// only if |Base| is actually a base class of |Derived|. Otherwise (including
// unrelated types), it isn't. We default to Derived* so that the
// specialization below will apply when the cast to |Derived*| is valid.
template <typename Base, typename Derived, typename = Derived*>
struct is_virtual_base_of : public std::is_base_of<Base, Derived> {};
// If we can cast |Base*| into |Derived*|, then it's definitely not a virtual
// base. When this happens, we'll match the default third template argument.
template <typename Base, typename Derived>
struct is_virtual_base_of<Base,
static_cast<Base*>(nullptr)))> : std::false_type {
}; // namespace internal
template <typename T>
class SequenceBound {
// Allow explicit null.
SequenceBound() = default;
// Construct a new instance of |T| that will be accessed only on
// |task_runner|. One may post calls to it immediately upon return.
// This is marked as NO_SANITIZE because cfi doesn't like that we're casting
// uninitialized memory to a |T*|. However, it's safe since (a) the cast is
// defined (see for details), and (b) we
// don't use the resulting pointer in any way that requries it to be
// constructed, except by posting such a access to |impl_task_runner_| after
// posting construction there as well.
template <typename... Args>
SequenceBound(scoped_refptr<base::SequencedTaskRunner> task_runner,
Args&&... args) NO_SANITIZE("cfi-unrelated-cast")
: impl_task_runner_(std::move(task_runner)) {
// Allocate space for but do not construct an instance of |T|.
storage_ = AlignedAlloc(sizeof(T), alignof(T));
t_ = reinterpret_cast<T*>(storage_);
// Post construction to the impl thread.
base::BindOnce(&ConstructOwnerRecord<Args...>, base::Unretained(t_),
~SequenceBound() { Reset(); }
// Move construction from the same type can just take the pointer without
// adjusting anything. This is required in addition to the move conversion
// constructor below.
SequenceBound(SequenceBound&& other) { MoveRecordFrom(other); }
// Move construction is supported from any type that's compatible with |T|.
// This case handles |From| != |T|, so we must adjust the pointer offset.
template <typename From>
SequenceBound(SequenceBound<From>&& other) {
SequenceBound& operator=(SequenceBound&& other) {
// Clean up any object we currently own.
return *this;
template <typename From>
SequenceBound<T>& operator=(SequenceBound<From>&& other) {
// Clean up any object that we currently own.
return *this;
// Move everything from |other|, doing pointer adjustment as needed.
// This method is marked as NO_SANITIZE since (a) it might run before the
// posted ctor runs on |impl_task_runner_|, and (b) implicit conversions to
// non-virtual base classes are allowed before construction by the standard.
// See for more information.
template <typename From>
void MoveRecordFrom(From&& other) NO_SANITIZE("cfi-unrelated-cast") {
// |other| might be is_null(), but that's okay.
impl_task_runner_ = std::move(other.impl_task_runner_);
// Note that static_cast<> isn't, in general, safe, since |other| might not
// be constructed yet. Implicit conversion is supported, as long as it
// doesn't convert to a virtual base. Of course, it allows only upcasts.
t_ = other.t_;
// The original storage is kept unmodified, so we can free it later.
storage_ = other.storage_;
other.storage_ = nullptr;
other.t_ = nullptr;
// Post a call to |method| to |impl_task_runner_|.
template <typename... Args>
void Post(const base::Location& from_here,
void (T::*method)(Args...),
Args&&... args) const {
from_here, base::BindOnce(
[](void (T::*method)(Args...), T* t, Args... args) {
std::move(method), base::Unretained(t_),
// TODO(liberato): Add PostOrCall(), to support cases where synchronous calls
// are okay if it's the same task runner.
// TODO(liberato): Add PostAndReply()
// TODO(liberato): Allow creation of callbacks that bind to a weak pointer,
// and thread-hop to |impl_task_runner_| if needed.
// Post destruction of any object we own, and return to the null state.
void Reset() {
if (is_null())
// Destruct the object on the impl thread.
FROM_HERE, base::BindOnce(&DeleteOwnerRecord, base::Unretained(t_),
impl_task_runner_ = nullptr;
t_ = nullptr;
storage_ = nullptr;
// Return whether we own anything. Note that this does not guarantee that any
// previously owned object has been destroyed. In particular, it will return
// true immediately after a call to Reset(), though the underlying object
// might still be pending destruction on the impl thread.
bool is_null() const { return !t_; }
// Pointer to the object, Pointer may be modified on the owning thread.
T* t_ = nullptr;
// Original allocated storage for the object.
void* storage_ = nullptr;
// The task runner on which all access to |t_| should happen.
scoped_refptr<base::SequencedTaskRunner> impl_task_runner_;
// For move conversion.
template <typename U>
friend class SequenceBound;
// Run on impl thread to construct |t|'s storage.
template <typename... Args>
static void ConstructOwnerRecord(T* t, Args&&... args) {
new (t) T(std::forward<Args>(args)...);
// Destruct the object associated with |t|, and delete |storage|.
static void DeleteOwnerRecord(T* t, void* storage) {
// To preserve ownership semantics, we disallow copy construction / copy
// assignment. Move construction / assignment is fine.
} // namespace base