serenity/AK/NonnullRefPtr.h

/*
 * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#pragma once

#include <AK/Assertions.h>
#include <AK/Atomic.h>
#include <AK/Format.h>
#include <AK/Traits.h>
#include <AK/Types.h>
#ifdef KERNEL
#    include <Kernel/Arch/x86/Processor.h>
#    include <Kernel/Arch/x86/ScopedCritical.h>
#endif

namespace AK {

template<typename T>
class OwnPtr;
template<typename T, typename PtrTraits>
class RefPtr;

template<typename T>
ALWAYS_INLINE void ref_if_not_null(T* ptr)
{
    if (ptr)
        ptr->ref();
}

template<typename T>
ALWAYS_INLINE void unref_if_not_null(T* ptr)
{
    if (ptr)
        ptr->unref();
}

template<typename T>
class NonnullRefPtr {
    template<typename U, typename P>
    friend class RefPtr;
    template<typename U>
    friend class NonnullRefPtr;
    template<typename U>
    friend class WeakPtr;

public:
    using ElementType = T;

    enum AdoptTag { Adopt };

    ALWAYS_INLINE NonnullRefPtr(const T& object)
        : m_bits((FlatPtr)&object)
    {
        VERIFY(!(m_bits & 1));
        const_cast<T&>(object).ref();
    }
    template<typename U>
    ALWAYS_INLINE NonnullRefPtr(const U& object) requires(IsConvertible<U*, T*>)
        : m_bits((FlatPtr) static_cast<const T*>(&object))
    {
        VERIFY(!(m_bits & 1));
        const_cast<T&>(static_cast<const T&>(object)).ref();
    }
    ALWAYS_INLINE NonnullRefPtr(AdoptTag, T& object)
        : m_bits((FlatPtr)&object)
    {
        VERIFY(!(m_bits & 1));
    }
    ALWAYS_INLINE NonnullRefPtr(NonnullRefPtr&& other)
        : m_bits((FlatPtr)&other.leak_ref())
    {
        VERIFY(!(m_bits & 1));
    }
    template<typename U>
    ALWAYS_INLINE NonnullRefPtr(NonnullRefPtr<U>&& other) requires(IsConvertible<U*, T*>)
        : m_bits((FlatPtr)&other.leak_ref())
    {
        VERIFY(!(m_bits & 1));
    }
    ALWAYS_INLINE NonnullRefPtr(const NonnullRefPtr& other)
        : m_bits((FlatPtr)other.add_ref())
    {
        VERIFY(!(m_bits & 1));
    }
    template<typename U>
    ALWAYS_INLINE NonnullRefPtr(const NonnullRefPtr<U>& other) requires(IsConvertible<U*, T*>)
        : m_bits((FlatPtr)other.add_ref())
    {
        VERIFY(!(m_bits & 1));
    }
    ALWAYS_INLINE ~NonnullRefPtr()
    {
        assign(nullptr);
#ifdef SANITIZE_PTRS
        m_bits.store(explode_byte(0xb0), AK::MemoryOrder::memory_order_relaxed);
#endif
    }

    template<typename U>
    NonnullRefPtr(const OwnPtr<U>&) = delete;
    template<typename U>
    NonnullRefPtr& operator=(const OwnPtr<U>&) = delete;

    template<typename U>
    NonnullRefPtr(const RefPtr<U>&) = delete;
    template<typename U>
    NonnullRefPtr& operator=(const RefPtr<U>&) = delete;
    NonnullRefPtr(const RefPtr<T>&) = delete;
    NonnullRefPtr& operator=(const RefPtr<T>&) = delete;

    NonnullRefPtr& operator=(const NonnullRefPtr& other)
    {
        if (this != &other)
            assign(other.add_ref());
        return *this;
    }

    template<typename U>
    NonnullRefPtr& operator=(const NonnullRefPtr<U>& other) requires(IsConvertible<U*, T*>)
    {
        assign(other.add_ref());
        return *this;
    }

    ALWAYS_INLINE NonnullRefPtr& operator=(NonnullRefPtr&& other)
    {
        if (this != &other)
            assign(&other.leak_ref());
        return *this;
    }

    template<typename U>
    NonnullRefPtr& operator=(NonnullRefPtr<U>&& other) requires(IsConvertible<U*, T*>)
    {
        assign(&other.leak_ref());
        return *this;
    }

    NonnullRefPtr& operator=(const T& object)
    {
        const_cast<T&>(object).ref();
        assign(const_cast<T*>(&object));
        return *this;
    }

    [[nodiscard]] ALWAYS_INLINE T& leak_ref()
    {
        T* ptr = exchange(nullptr);
        VERIFY(ptr);
        return *ptr;
    }

    ALWAYS_INLINE RETURNS_NONNULL T* ptr()
    {
        return as_nonnull_ptr();
    }
    ALWAYS_INLINE RETURNS_NONNULL const T* ptr() const
    {
        return as_nonnull_ptr();
    }

    ALWAYS_INLINE RETURNS_NONNULL T* operator->()
    {
        return as_nonnull_ptr();
    }
    ALWAYS_INLINE RETURNS_NONNULL const T* operator->() const
    {
        return as_nonnull_ptr();
    }

    ALWAYS_INLINE T& operator*()
    {
        return *as_nonnull_ptr();
    }
    ALWAYS_INLINE const T& operator*() const
    {
        return *as_nonnull_ptr();
    }

    ALWAYS_INLINE RETURNS_NONNULL operator T*()
    {
        return as_nonnull_ptr();
    }
    ALWAYS_INLINE RETURNS_NONNULL operator const T*() const
    {
        return as_nonnull_ptr();
    }

    ALWAYS_INLINE operator T&()
    {
        return *as_nonnull_ptr();
    }
    ALWAYS_INLINE operator const T&() const
    {
        return *as_nonnull_ptr();
    }

    operator bool() const = delete;
    bool operator!() const = delete;

    void swap(NonnullRefPtr& other)
    {
        if (this == &other)
            return;

        // NOTE: swap is not atomic!
        T* other_ptr = other.exchange(nullptr);
        T* ptr = exchange(other_ptr);
        other.exchange(ptr);
    }

    template<typename U>
    void swap(NonnullRefPtr<U>& other) requires(IsConvertible<U*, T*>)
    {
        // NOTE: swap is not atomic!
        U* other_ptr = other.exchange(nullptr);
        T* ptr = exchange(other_ptr);
        other.exchange(ptr);
    }

private:
    NonnullRefPtr() = delete;

    ALWAYS_INLINE T* as_ptr() const
    {
        return (T*)(m_bits.load(AK::MemoryOrder::memory_order_relaxed) & ~(FlatPtr)1);
    }

    ALWAYS_INLINE RETURNS_NONNULL T* as_nonnull_ptr() const
    {
        T* ptr = (T*)(m_bits.load(AK::MemoryOrder::memory_order_relaxed) & ~(FlatPtr)1);
        VERIFY(ptr);
        return ptr;
    }

    template<typename F>
    void do_while_locked(F f) const
    {
#ifdef KERNEL
        // We don't want to be pre-empted while we have the lock bit set
        Kernel::ScopedCritical critical;
#endif
        FlatPtr bits;
        for (;;) {
            bits = m_bits.fetch_or(1, AK::MemoryOrder::memory_order_acq_rel);
            if (!(bits & 1))
                break;
#ifdef KERNEL
            Kernel::Processor::wait_check();
#endif
        }
        VERIFY(!(bits & 1));
        f((T*)bits);
        m_bits.store(bits, AK::MemoryOrder::memory_order_release);
    }

    ALWAYS_INLINE void assign(T* new_ptr)
    {
        T* prev_ptr = exchange(new_ptr);
        unref_if_not_null(prev_ptr);
    }

    ALWAYS_INLINE T* exchange(T* new_ptr)
    {
        VERIFY(!((FlatPtr)new_ptr & 1));
#ifdef KERNEL
        // We don't want to be pre-empted while we have the lock bit set
        Kernel::ScopedCritical critical;
#endif
        // Only exchange while not locked
        FlatPtr expected = m_bits.load(AK::MemoryOrder::memory_order_relaxed);
        for (;;) {
            expected &= ~(FlatPtr)1; // only if lock bit is not set
            if (m_bits.compare_exchange_strong(expected, (FlatPtr)new_ptr, AK::MemoryOrder::memory_order_acq_rel))
                break;
#ifdef KERNEL
            Kernel::Processor::wait_check();
#endif
        }
        VERIFY(!(expected & 1));
        return (T*)expected;
    }

    T* add_ref() const
    {
#ifdef KERNEL
        // We don't want to be pre-empted while we have the lock bit set
        Kernel::ScopedCritical critical;
#endif
        // Lock the pointer
        FlatPtr expected = m_bits.load(AK::MemoryOrder::memory_order_relaxed);
        for (;;) {
            expected &= ~(FlatPtr)1; // only if lock bit is not set
            if (m_bits.compare_exchange_strong(expected, expected | 1, AK::MemoryOrder::memory_order_acq_rel))
                break;
#ifdef KERNEL
            Kernel::Processor::wait_check();
#endif
        }

        // Add a reference now that we locked the pointer
        ref_if_not_null((T*)expected);

        // Unlock the pointer again
        m_bits.store(expected, AK::MemoryOrder::memory_order_release);
        return (T*)expected;
    }

    mutable Atomic<FlatPtr> m_bits { 0 };
};

template<typename T>
inline NonnullRefPtr<T> adopt_ref(T& object)
{
    return NonnullRefPtr<T>(NonnullRefPtr<T>::Adopt, object);
}

template<typename T>
struct Formatter<NonnullRefPtr<T>> : Formatter<const T*> {
    void format(FormatBuilder& builder, const NonnullRefPtr<T>& value)
    {
        Formatter<const T*>::format(builder, value.ptr());
    }
};

template<typename T, typename U>
inline void swap(NonnullRefPtr<T>& a, NonnullRefPtr<U>& b) requires(IsConvertible<U*, T*>)
{
    a.swap(b);
}

template<typename T, class... Args>
requires(IsConstructible<T, Args...>) inline NonnullRefPtr<T> make_ref_counted(Args&&... args)
{
    return NonnullRefPtr<T>(NonnullRefPtr<T>::Adopt, *new T(forward<Args>(args)...));
}

// FIXME: Remove once P0960R3 is available in Clang.
template<typename T, class... Args>
inline NonnullRefPtr<T> make_ref_counted(Args&&... args)
{
    return NonnullRefPtr<T>(NonnullRefPtr<T>::Adopt, *new T { forward<Args>(args)... });
}
}

template<typename T>
struct Traits<NonnullRefPtr<T>> : public GenericTraits<NonnullRefPtr<T>> {
    using PeekType = T*;
    using ConstPeekType = const T*;
    static unsigned hash(const NonnullRefPtr<T>& p) { return ptr_hash(p.ptr()); }
    static bool equals(const NonnullRefPtr<T>& a, const NonnullRefPtr<T>& b) { return a.ptr() == b.ptr(); }
};

using AK::adopt_ref;
using AK::make_ref_counted;
using AK::NonnullRefPtr;