// SPDX-License-Identifier: GPL-2.0
// Copyright (C) 2024 Google LLC.
//! A wrapper around `Arc` for linked lists.
use crate::alloc::{AllocError, Flags};
use crate::prelude::*;
use crate::sync::{Arc, ArcBorrow, UniqueArc};
use core::marker::{PhantomPinned, Unsize};
use core::ops::Deref;
use core::pin::Pin;
use core::sync::atomic::{AtomicBool, Ordering};
/// Declares that this type has some way to ensure that there is exactly one `ListArc` instance for
/// this id.
///
/// Types that implement this trait should include some kind of logic for keeping track of whether
/// a [`ListArc`] exists or not. We refer to this logic as "the tracking inside `T`".
///
/// We allow the case where the tracking inside `T` thinks that a [`ListArc`] exists, but actually,
/// there isn't a [`ListArc`]. However, we do not allow the opposite situation where a [`ListArc`]
/// exists, but the tracking thinks it doesn't. This is because the former can at most result in us
/// failing to create a [`ListArc`] when the operation could succeed, whereas the latter can result
/// in the creation of two [`ListArc`] references. Only the latter situation can lead to memory
/// safety issues.
///
/// A consequence of the above is that you may implement the tracking inside `T` by not actually
/// keeping track of anything. To do this, you always claim that a [`ListArc`] exists, even if
/// there isn't one. This implementation is allowed by the above rule, but it means that
/// [`ListArc`] references can only be created if you have ownership of *all* references to the
/// refcounted object, as you otherwise have no way of knowing whether a [`ListArc`] exists.
pub trait ListArcSafe<const ID: u64 = 0> {
/// Informs the tracking inside this type that it now has a [`ListArc`] reference.
///
/// This method may be called even if the tracking inside this type thinks that a `ListArc`
/// reference exists. (But only if that's not actually the case.)
///
/// # Safety
///
/// Must not be called if a [`ListArc`] already exist for this value.
unsafe fn on_create_list_arc_from_unique(self: Pin<&mut Self>);
/// Informs the tracking inside this type that there is no [`ListArc`] reference anymore.
///
/// # Safety
///
/// Must only be called if there is no [`ListArc`] reference, but the tracking thinks there is.
unsafe fn on_drop_list_arc(&self);
}
/// Declares that this type is able to safely attempt to create `ListArc`s at any time.
///
/// # Safety
///
/// The guarantees of `try_new_list_arc` must be upheld.
pub unsafe trait TryNewListArc<const ID: u64 = 0>: ListArcSafe<ID> {
/// Attempts to convert an `Arc<Self>` into an `ListArc<Self>`. Returns `true` if the
/// conversion was successful.
///
/// This method should not be called directly. Use [`ListArc::try_from_arc`] instead.
///
/// # Guarantees
///
/// If this call returns `true`, then there is no [`ListArc`] pointing to this value.
/// Additionally, this call will have transitioned the tracking inside `Self` from not thinking
/// that a [`ListArc`] exists, to thinking that a [`ListArc`] exists.
fn try_new_list_arc(&self) -> bool;
}
/// Declares that this type supports [`ListArc`].
///
/// This macro supports a few different strategies for implementing the tracking inside the type:
///
/// * The `untracked` strategy does not actually keep track of whether a [`ListArc`] exists. When
/// using this strategy, the only way to create a [`ListArc`] is using a [`UniqueArc`].
/// * The `tracked_by` strategy defers the tracking to a field of the struct. The user much specify
/// which field to defer the tracking to. The field must implement [`ListArcSafe`]. If the field
/// implements [`TryNewListArc`], then the type will also implement [`TryNewListArc`].
///
/// The `tracked_by` strategy is usually used by deferring to a field of type
/// [`AtomicTracker`]. However, it is also possible to defer the tracking to another struct
/// using also using this macro.
#[macro_export]
macro_rules! impl_list_arc_safe {
(impl$({$($generics:tt)*})? ListArcSafe<$num:tt> for $t:ty { untracked; } $($rest:tt)*) => {
impl$(<$($generics)*>)? $crate::list::ListArcSafe<$num> for $t {
unsafe fn on_create_list_arc_from_unique(self: ::core::pin::Pin<&mut Self>) {}
unsafe fn on_drop_list_arc(&self) {}
}
$crate::list::impl_list_arc_safe! { $($rest)* }
};
(impl$({$($generics:tt)*})? ListArcSafe<$num:tt> for $t:ty {
tracked_by $field:ident : $fty:ty;
} $($rest:tt)*) => {
impl$(<$($generics)*>)? $crate::list::ListArcSafe<$num> for $t {
unsafe fn on_create_list_arc_from_unique(self: ::core::pin::Pin<&mut Self>) {
$crate::assert_pinned!($t, $field, $fty, inline);
// SAFETY: This field is structurally pinned as per the above assertion.
let field = unsafe {
::core::pin::Pin::map_unchecked_mut(self, |me| &mut me.$field)
};
// SAFETY: The caller promises that there is no `ListArc`.
unsafe {
<$fty as $crate::list::ListArcSafe<$num>>::on_create_list_arc_from_unique(field)
};
}
unsafe fn on_drop_list_arc(&self) {
// SAFETY: The caller promises that there is no `ListArc` reference, and also
// promises that the tracking thinks there is a `ListArc` reference.
unsafe { <$fty as $crate::list::ListArcSafe<$num>>::on_drop_list_arc(&self.$field) };
}
}
unsafe impl$(<$($generics)*>)? $crate::list::TryNewListArc<$num> for $t
where
$fty: TryNewListArc<$num>,
{
fn try_new_list_arc(&self) -> bool {
<$fty as $crate::list::TryNewListArc<$num>>::try_new_list_arc(&self.$field)
}
}
$crate::list::impl_list_arc_safe! { $($rest)* }
};
() => {};
}
pub use impl_list_arc_safe;
/// A wrapper around [`Arc`] that's guaranteed unique for the given id.
///
/// The `ListArc` type can be thought of as a special reference to a refcounted object that owns the
/// permission to manipulate the `next`/`prev` pointers stored in the refcounted object. By ensuring
/// that each object has only one `ListArc` reference, the owner of that reference is assured
/// exclusive access to the `next`/`prev` pointers. When a `ListArc` is inserted into a [`List`],
/// the [`List`] takes ownership of the `ListArc` reference.
///
/// There are various strategies to ensuring that a value has only one `ListArc` reference. The
/// simplest is to convert a [`UniqueArc`] into a `ListArc`. However, the refcounted object could
/// also keep track of whether a `ListArc` exists using a boolean, which could allow for the
/// creation of new `ListArc` references from an [`Arc`] reference. Whatever strategy is used, the
/// relevant tracking is referred to as "the tracking inside `T`", and the [`ListArcSafe`] trait
/// (and its subtraits) are used to update the tracking when a `ListArc` is created or destroyed.
///
/// Note that we allow the case where the tracking inside `T` thinks that a `ListArc` exists, but
/// actually, there isn't a `ListArc`. However, we do not allow the opposite situation where a
/// `ListArc` exists, but the tracking thinks it doesn't. This is because the former can at most
/// result in us failing to create a `ListArc` when the operation could succeed, whereas the latter
/// can result in the creation of two `ListArc` references.
///
/// While this `ListArc` is unique for the given id, there still might exist normal `Arc`
/// references to the object.
///
/// # Invariants
///
/// * Each reference counted object has at most one `ListArc` for each value of `ID`.
/// * The tracking inside `T` is aware that a `ListArc` reference exists.
///
/// [`List`]: crate::list::List
#[repr(transparent)]
pub struct ListArc<T, const ID: u64 = 0>
where
T: ListArcSafe<ID> + ?Sized,
{
arc: Arc<T>,
}
impl<T: ListArcSafe<ID>, const ID: u64> ListArc<T, ID> {
/// Constructs a new reference counted instance of `T`.
#[inline]
pub fn new(contents: T, flags: Flags) -> Result<Self, AllocError> {
Ok(Self::from(UniqueArc::new(contents, flags)?))
}
/// Use the given initializer to in-place initialize a `T`.
///
/// If `T: !Unpin` it will not be able to move afterwards.
// We don't implement `InPlaceInit` because `ListArc` is implicitly pinned. This is similar to
// what we do for `Arc`.
#[inline]
pub fn pin_init<E>(init: impl PinInit<T, E>, flags: Flags) -> Result<Self, E>
where
E: From<AllocError>,
{
Ok(Self::from(UniqueArc::try_pin_init(init, flags)?))
}
/// Use the given initializer to in-place initialize a `T`.
///
/// This is equivalent to [`ListArc<T>::pin_init`], since a [`ListArc`] is always pinned.
#[inline]
pub fn init<E>(init: impl Init<T, E>, flags: Flags) -> Result<Self, E>
where
E: From<AllocError>,
{
Ok(Self::from(UniqueArc::try_init(init, flags)?))
}
}
impl<T, const ID: u64> From<UniqueArc<T>> for ListArc<T, ID>
where
T: ListArcSafe<ID> + ?Sized,
{
/// Convert a [`UniqueArc`] into a [`ListArc`].
#[inline]
fn from(unique: UniqueArc<T>) -> Self {
Self::from(Pin::from(unique))
}
}
impl<T, const ID: u64> From<Pin<UniqueArc<T>>> for ListArc<T, ID>
where
T: ListArcSafe<ID> + ?Sized,
{
/// Convert a pinned [`UniqueArc`] into a [`ListArc`].
#[inline]
fn from(mut unique: Pin<UniqueArc<T>>) -> Self {
// SAFETY: We have a `UniqueArc`, so there is no `ListArc`.
unsafe { T::on_create_list_arc_from_unique(unique.as_mut()) };
let arc = Arc::from(unique);
// SAFETY: We just called `on_create_list_arc_from_unique` on an arc without a `ListArc`,
// so we can create a `ListArc`.
unsafe { Self::transmute_from_arc(arc) }
}
}
impl<T, const ID: u64> ListArc<T, ID>
where
T: ListArcSafe<ID> + ?Sized,
{
/// Creates two `ListArc`s from a [`UniqueArc`].
///
/// The two ids must be different.
#[inline]
pub fn pair_from_unique<const ID2: u64>(unique: UniqueArc<T>) -> (Self, ListArc<T, ID2>)
where
T: ListArcSafe<ID2>,
{
Self::pair_from_pin_unique(Pin::from(unique))
}
/// Creates two `ListArc`s from a pinned [`UniqueArc`].
///
/// The two ids must be different.
#[inline]
pub fn pair_from_pin_unique<const ID2: u64>(
mut unique: Pin<UniqueArc<T>>,
) -> (Self, ListArc<T, ID2>)
where
T: ListArcSafe<ID2>,
{
build_assert!(ID != ID2);
// SAFETY: We have a `UniqueArc`, so there is no `ListArc`.
unsafe { <T as ListArcSafe<ID>>::on_create_list_arc_from_unique(unique.as_mut()) };
// SAFETY: We have a `UniqueArc`, so there is no `ListArc`.
unsafe { <T as ListArcSafe<ID2>>::on_create_list_arc_from_unique(unique.as_mut()) };
let arc1 = Arc::from(unique);
let arc2 = Arc::clone(&arc1);
// SAFETY: We just called `on_create_list_arc_from_unique` on an arc without a `ListArc`
// for both IDs (which are different), so we can create two `ListArc`s.
unsafe {
(
Self::transmute_from_arc(arc1),
ListArc::transmute_from_arc(arc2),
)
}
}
/// Try to create a new `ListArc`.
///
/// This fails if this value already has a `ListArc`.
pub fn try_from_arc(arc: Arc<T>) -> Result<Self, Arc<T>>
where
T: TryNewListArc<ID>,
{
if arc.try_new_list_arc() {
// SAFETY: The `try_new_list_arc` method returned true, so we made the tracking think
// that a `ListArc` exists. This lets us create a `ListArc`.
Ok(unsafe { Self::transmute_from_arc(arc) })
} else {
Err(arc)
}
}
/// Try to create a new `ListArc`.
///
/// This fails if this value already has a `ListArc`.
pub fn try_from_arc_borrow(arc: ArcBorrow<'_, T>) -> Option<Self>
where
T: TryNewListArc<ID>,
{
if arc.try_new_list_arc() {
// SAFETY: The `try_new_list_arc` method returned true, so we made the tracking think
// that a `ListArc` exists. This lets us create a `ListArc`.
Some(unsafe { Self::transmute_from_arc(Arc::from(arc)) })
} else {
None
}
}
/// Try to create a new `ListArc`.
///
/// If it's not possible to create a new `ListArc`, then the `Arc` is dropped. This will never
/// run the destructor of the value.
pub fn try_from_arc_or_drop(arc: Arc<T>) -> Option<Self>
where
T: TryNewListArc<ID>,
{
match Self::try_from_arc(arc) {
Ok(list_arc) => Some(list_arc),
Err(arc) => Arc::into_unique_or_drop(arc).map(Self::from),
}
}
/// Transmutes an [`Arc`] into a `ListArc` without updating the tracking inside `T`.
///
/// # Safety
///
/// * The value must not already have a `ListArc` reference.
/// * The tracking inside `T` must think that there is a `ListArc` reference.
#[inline]
unsafe fn transmute_from_arc(arc: Arc<T>) -> Self {
// INVARIANT: By the safety requirements, the invariants on `ListArc` are satisfied.
Self { arc }
}
/// Transmutes a `ListArc` into an [`Arc`] without updating the tracking inside `T`.
///
/// After this call, the tracking inside `T` will still think that there is a `ListArc`
/// reference.
#[inline]
fn transmute_to_arc(self) -> Arc<T> {
// Use a transmute to skip destructor.
//
// SAFETY: ListArc is repr(transparent).
unsafe { core::mem::transmute(self) }
}
/// Convert ownership of this `ListArc` into a raw pointer.
///
/// The returned pointer is indistinguishable from pointers returned by [`Arc::into_raw`]. The
/// tracking inside `T` will still think that a `ListArc` exists after this call.
#[inline]
pub fn into_raw(self) -> *const T {
Arc::into_raw(Self::transmute_to_arc(self))
}
/// Take ownership of the `ListArc` from a raw pointer.
///
/// # Safety
///
/// * `ptr` must satisfy the safety requirements of [`Arc::from_raw`].
/// * The value must not already have a `ListArc` reference.
/// * The tracking inside `T` must think that there is a `ListArc` reference.
#[inline]
pub unsafe fn from_raw(ptr: *const T) -> Self {
// SAFETY: The pointer satisfies the safety requirements for `Arc::from_raw`.
let arc = unsafe { Arc::from_raw(ptr) };
// SAFETY: The value doesn't already have a `ListArc` reference, but the tracking thinks it
// does.
unsafe { Self::transmute_from_arc(arc) }
}
/// Converts the `ListArc` into an [`Arc`].
#[inline]
pub fn into_arc(self) -> Arc<T> {
let arc = Self::transmute_to_arc(self);
// SAFETY: There is no longer a `ListArc`, but the tracking thinks there is.
unsafe { T::on_drop_list_arc(&arc) };
arc
}
/// Clone a `ListArc` into an [`Arc`].
#[inline]
pub fn clone_arc(&self) -> Arc<T> {
self.arc.clone()
}
/// Returns a reference to an [`Arc`] from the given [`ListArc`].
///
/// This is useful when the argument of a function call is an [`&Arc`] (e.g., in a method
/// receiver), but we have a [`ListArc`] instead.
///
/// [`&Arc`]: Arc
#[inline]
pub fn as_arc(&self) -> &Arc<T> {
&self.arc
}
/// Returns an [`ArcBorrow`] from the given [`ListArc`].
///
/// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method
/// receiver), but we have an [`Arc`] instead. Getting an [`ArcBorrow`] is free when optimised.
#[inline]
pub fn as_arc_borrow(&self) -> ArcBorrow<'_, T> {
self.arc.as_arc_borrow()
}
/// Compare whether two [`ListArc`] pointers reference the same underlying object.
#[inline]
pub fn ptr_eq(this: &Self, other: &Self) -> bool {
Arc::ptr_eq(&this.arc, &other.arc)
}
}
impl<T, const ID: u64> Deref for ListArc<T, ID>
where
T: ListArcSafe<ID> + ?Sized,
{
type Target = T;
#[inline]
fn deref(&self) -> &Self::Target {
self.arc.deref()
}
}
impl<T, const ID: u64> Drop for ListArc<T, ID>
where
T: ListArcSafe<ID> + ?Sized,
{
#[inline]
fn drop(&mut self) {
// SAFETY: There is no longer a `ListArc`, but the tracking thinks there is by the type
// invariants on `Self`.
unsafe { T::on_drop_list_arc(&self.arc) };
}
}
impl<T, const ID: u64> AsRef<Arc<T>> for ListArc<T, ID>
where
T: ListArcSafe<ID> + ?Sized,
{
#[inline]
fn as_ref(&self) -> &Arc<T> {
self.as_arc()
}
}
// This is to allow [`ListArc`] (and variants) to be used as the type of `self`.
impl<T, const ID: u64> core::ops::Receiver for ListArc<T, ID> where T: ListArcSafe<ID> + ?Sized {}
// This is to allow coercion from `ListArc<T>` to `ListArc<U>` if `T` can be converted to the
// dynamically-sized type (DST) `U`.
impl<T, U, const ID: u64> core::ops::CoerceUnsized<ListArc<U, ID>> for ListArc<T, ID>
where
T: ListArcSafe<ID> + Unsize<U> + ?Sized,
U: ListArcSafe<ID> + ?Sized,
{
}
// This is to allow `ListArc<U>` to be dispatched on when `ListArc<T>` can be coerced into
// `ListArc<U>`.
impl<T, U, const ID: u64> core::ops::DispatchFromDyn<ListArc<U, ID>> for ListArc<T, ID>
where
T: ListArcSafe<ID> + Unsize<U> + ?Sized,
U: ListArcSafe<ID> + ?Sized,
{
}
/// A utility for tracking whether a [`ListArc`] exists using an atomic.
///
/// # Invariant
///
/// If the boolean is `false`, then there is no [`ListArc`] for this value.
#[repr(transparent)]
pub struct AtomicTracker<const ID: u64 = 0> {
inner: AtomicBool,
// This value needs to be pinned to justify the INVARIANT: comment in `AtomicTracker::new`.
_pin: PhantomPinned,
}
impl<const ID: u64> AtomicTracker<ID> {
/// Creates a new initializer for this type.
pub fn new() -> impl PinInit<Self> {
// INVARIANT: Pin-init initializers can't be used on an existing `Arc`, so this value will
// not be constructed in an `Arc` that already has a `ListArc`.
Self {
inner: AtomicBool::new(false),
_pin: PhantomPinned,
}
}
fn project_inner(self: Pin<&mut Self>) -> &mut AtomicBool {
// SAFETY: The `inner` field is not structurally pinned, so we may obtain a mutable
// reference to it even if we only have a pinned reference to `self`.
unsafe { &mut Pin::into_inner_unchecked(self).inner }
}
}
impl<const ID: u64> ListArcSafe<ID> for AtomicTracker<ID> {
unsafe fn on_create_list_arc_from_unique(self: Pin<&mut Self>) {
// INVARIANT: We just created a ListArc, so the boolean should be true.
*self.project_inner().get_mut() = true;
}
unsafe fn on_drop_list_arc(&self) {
// INVARIANT: We just dropped a ListArc, so the boolean should be false.
self.inner.store(false, Ordering::Release);
}
}
// SAFETY: If this method returns `true`, then by the type invariant there is no `ListArc` before
// this call, so it is okay to create a new `ListArc`.
//
// The acquire ordering will synchronize with the release store from the destruction of any
// previous `ListArc`, so if there was a previous `ListArc`, then the destruction of the previous
// `ListArc` happens-before the creation of the new `ListArc`.
unsafe impl<const ID: u64> TryNewListArc<ID> for AtomicTracker<ID> {
fn try_new_list_arc(&self) -> bool {
// INVARIANT: If this method returns true, then the boolean used to be false, and is no
// longer false, so it is okay for the caller to create a new [`ListArc`].
self.inner
.compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed)
.is_ok()
}
}
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