ladybird/AK/WeakPtr.h
Tom 75f61fe3d9 AK: Make RefPtr, NonnullRefPtr, WeakPtr thread safe
This makes most operations thread safe, especially so that they
can safely be used in the Kernel. This includes obtaining a strong
reference from a weak reference, which now requires an explicit
call to WeakPtr::strong_ref(). Another major change is that
Weakable::make_weak_ref() may require the explicit target type.
Previously we used reinterpret_cast in WeakPtr, assuming that it
can be properly converted. But WeakPtr does not necessarily have
the knowledge to be able to do this. Instead, we now ask the class
itself to deliver a WeakPtr to the type that we want.

Also, WeakLink is no longer specific to a target type. The reason
for this is that we want to be able to safely convert e.g. WeakPtr<T>
to WeakPtr<U>, and before this we just reinterpret_cast the internal
WeakLink<T> to WeakLink<U>, which is a bold assumption that it would
actually produce the correct code. Instead, WeakLink now operates
on just a raw pointer and we only make those constructors/operators
available if we can verify that it can be safely cast.

In order to guarantee thread safety, we now use the least significant
bit in the pointer for locking purposes. This also means that only
properly aligned pointers can be used.
2020-11-10 19:11:52 +01:00

243 lines
7.4 KiB
C++

/*
* Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* 1. Redistributions of source code must retain the above copyright notice, this
* list of conditions and the following disclaimer.
*
* 2. Redistributions in binary form must reproduce the above copyright notice,
* this list of conditions and the following disclaimer in the documentation
* and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
* DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
* SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
* CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
#pragma once
#include <AK/LogStream.h>
#include <AK/Weakable.h>
namespace AK {
template<typename T>
class WeakPtr {
template<typename U>
friend class Weakable;
public:
WeakPtr() { }
WeakPtr(std::nullptr_t) { }
template<typename U, typename EnableIf<IsBaseOf<T, U>::value>::Type* = nullptr>
WeakPtr(const WeakPtr<U>& other)
: m_link(other.m_link)
{
}
template<typename U, typename EnableIf<IsBaseOf<T, U>::value>::Type* = nullptr>
WeakPtr(WeakPtr<U>&& other)
: m_link(other.take_link())
{
}
template<typename U, typename EnableIf<IsBaseOf<T, U>::value>::Type* = nullptr>
WeakPtr& operator=(WeakPtr<U>&& other)
{
m_link = other.take_link();
return *this;
}
template<typename U, typename EnableIf<IsBaseOf<T, U>::value>::Type* = nullptr>
WeakPtr& operator=(const WeakPtr<U>& other)
{
if ((const void*)this != (const void*)&other)
m_link = other.m_link;
return *this;
}
WeakPtr& operator=(std::nullptr_t)
{
clear();
return *this;
}
template<typename U, typename EnableIf<IsBaseOf<T, U>::value>::Type* = nullptr>
WeakPtr(const U& object)
: m_link(object.template make_weak_ptr<U>().take_link())
{
}
template<typename U, typename EnableIf<IsBaseOf<T, U>::value>::Type* = nullptr>
WeakPtr(const U* object)
{
if (object)
m_link = object->template make_weak_ptr<U>().take_link();
}
template<typename U, typename EnableIf<IsBaseOf<T, U>::value>::Type* = nullptr>
WeakPtr(const RefPtr<U>& object)
{
object.do_while_locked([&](U* obj) {
if (obj)
obj->template make_weak_ptr<U>().take_link();
});
}
template<typename U, typename EnableIf<IsBaseOf<T, U>::value>::Type* = nullptr>
WeakPtr(const NonnullRefPtr<U>& object)
{
object.do_while_locked([&](U* obj) {
if (obj)
obj->template make_weak_ptr<U>().take_link();
});
}
template<typename U, typename EnableIf<IsBaseOf<T, U>::value>::Type* = nullptr>
WeakPtr& operator=(const U& object)
{
m_link = object.template make_weak_ptr<U>().take_link();
return *this;
}
template<typename U, typename EnableIf<IsBaseOf<T, U>::value>::Type* = nullptr>
WeakPtr& operator=(const U* object)
{
if (object)
m_link = object->template make_weak_ptr<U>().take_link();
else
m_link = nullptr;
return *this;
}
template<typename U, typename EnableIf<IsBaseOf<T, U>::value>::Type* = nullptr>
WeakPtr& operator=(const RefPtr<U>& object)
{
object.do_while_locked([&](U* obj) {
if (obj)
m_link = obj->template make_weak_ptr<U>().take_link();
else
m_link = nullptr;
});
return *this;
}
template<typename U, typename EnableIf<IsBaseOf<T, U>::value>::Type* = nullptr>
WeakPtr& operator=(const NonnullRefPtr<U>& object)
{
object.do_while_locked([&](U* obj) {
if (obj)
m_link = obj->template make_weak_ptr<U>().take_link();
else
m_link = nullptr;
});
return *this;
}
RefPtr<T> strong_ref() const
{
// This only works with RefCounted objects, but it is the only
// safe way to get a strong reference from a WeakPtr. Any code
// that uses objects not derived from RefCounted will have to
// use unsafe_ptr(), but as the name suggests, it is not safe...
RefPtr<T> ref;
// Using do_while_locked protects against a race with clear()!
m_link.do_while_locked([&](WeakLink* link) {
if (link)
ref = link->template strong_ref<T>();
});
return ref;
}
#ifndef KERNEL
// A lot of user mode code is single-threaded. But for kernel mode code
// this is generally not true as everything is multi-threaded. So make
// these shortcuts and aliases only available to non-kernel code.
T* ptr() const { return unsafe_ptr(); }
T* operator->() { return unsafe_ptr(); }
const T* operator->() const { return unsafe_ptr(); }
operator const T*() const { return unsafe_ptr(); }
operator T*() { return unsafe_ptr(); }
#endif
T* unsafe_ptr() const
{
T* ptr = nullptr;
m_link.do_while_locked([&](WeakLink* link) {
if (link)
ptr = link->unsafe_ptr<T>();
});
return ptr;
}
operator bool() const { return m_link ? !m_link->is_null() : false; }
bool is_null() const { return !m_link || m_link->is_null(); }
void clear() { m_link = nullptr; }
RefPtr<WeakLink> take_link() { return move(m_link); }
private:
WeakPtr(const RefPtr<WeakLink>& link)
: m_link(link)
{
}
RefPtr<WeakLink> m_link;
};
template<typename T>
template<typename U>
inline WeakPtr<U> Weakable<T>::make_weak_ptr() const
{
#ifdef DEBUG
ASSERT(!m_being_destroyed);
#endif
if (!m_link) {
// There is a small chance that we create a new WeakLink and throw
// it away because another thread beat us to it. But the window is
// pretty small and the overhead isn't terrible.
m_link.assign_if_null(adopt(*new WeakLink(const_cast<T&>(static_cast<const T&>(*this)))));
}
return WeakPtr<U>(m_link);
}
template<typename T>
inline const LogStream& operator<<(const LogStream& stream, const WeakPtr<T>& value)
{
#ifdef KERNEL
auto ref = value.strong_ref();
return stream << ref.ptr();
#else
return stream << value.ptr();
#endif
}
template<typename T>
struct Formatter<WeakPtr<T>> : Formatter<const T*> {
void format(TypeErasedFormatParams& params, FormatBuilder& builder, const WeakPtr<T>& value)
{
#ifdef KERNEL
auto ref = value.strong_ref();
Formatter<const T*>::format(params, builder, ref.ptr());
#else
Formatter<const T*>::format(params, builder, value.ptr());
#endif
}
};
}
using AK::WeakPtr;