// SPDX-License-Identifier: GPL-2.0 // Copyright (C) 2024 Google LLC. //! A linked list implementation. use crate::init::PinInit; use crate::sync::ArcBorrow; use crate::types::Opaque; use core::iter::{DoubleEndedIterator, FusedIterator}; use core::marker::PhantomData; use core::ptr; mod impl_list_item_mod; pub use self::impl_list_item_mod::{ impl_has_list_links, impl_has_list_links_self_ptr, impl_list_item, HasListLinks, HasSelfPtr, }; mod arc; pub use self::arc::{impl_list_arc_safe, AtomicTracker, ListArc, ListArcSafe, TryNewListArc}; mod arc_field; pub use self::arc_field::{define_list_arc_field_getter, ListArcField}; /// A linked list. /// /// All elements in this linked list will be [`ListArc`] references to the value. Since a value can /// only have one `ListArc` (for each pair of prev/next pointers), this ensures that the same /// prev/next pointers are not used for several linked lists. /// /// # Invariants /// /// * If the list is empty, then `first` is null. Otherwise, `first` points at the `ListLinks` /// field of the first element in the list. /// * All prev/next pointers in `ListLinks` fields of items in the list are valid and form a cycle. /// * For every item in the list, the list owns the associated [`ListArc`] reference and has /// exclusive access to the `ListLinks` field. pub struct List, const ID: u64 = 0> { first: *mut ListLinksFields, _ty: PhantomData>, } // SAFETY: This is a container of `ListArc`, and access to the container allows the same // type of access to the `ListArc` elements. unsafe impl Send for List where ListArc: Send, T: ?Sized + ListItem, { } // SAFETY: This is a container of `ListArc`, and access to the container allows the same // type of access to the `ListArc` elements. unsafe impl Sync for List where ListArc: Sync, T: ?Sized + ListItem, { } /// Implemented by types where a [`ListArc`] can be inserted into a [`List`]. /// /// # Safety /// /// Implementers must ensure that they provide the guarantees documented on methods provided by /// this trait. /// /// [`ListArc`]: ListArc pub unsafe trait ListItem: ListArcSafe { /// Views the [`ListLinks`] for this value. /// /// # Guarantees /// /// If there is a previous call to `prepare_to_insert` and there is no call to `post_remove` /// since the most recent such call, then this returns the same pointer as the one returned by /// the most recent call to `prepare_to_insert`. /// /// Otherwise, the returned pointer points at a read-only [`ListLinks`] with two null pointers. /// /// # Safety /// /// The provided pointer must point at a valid value. (It need not be in an `Arc`.) unsafe fn view_links(me: *const Self) -> *mut ListLinks; /// View the full value given its [`ListLinks`] field. /// /// Can only be used when the value is in a list. /// /// # Guarantees /// /// * Returns the same pointer as the one passed to the most recent call to `prepare_to_insert`. /// * The returned pointer is valid until the next call to `post_remove`. /// /// # Safety /// /// * The provided pointer must originate from the most recent call to `prepare_to_insert`, or /// from a call to `view_links` that happened after the most recent call to /// `prepare_to_insert`. /// * Since the most recent call to `prepare_to_insert`, the `post_remove` method must not have /// been called. unsafe fn view_value(me: *mut ListLinks) -> *const Self; /// This is called when an item is inserted into a [`List`]. /// /// # Guarantees /// /// The caller is granted exclusive access to the returned [`ListLinks`] until `post_remove` is /// called. /// /// # Safety /// /// * The provided pointer must point at a valid value in an [`Arc`]. /// * Calls to `prepare_to_insert` and `post_remove` on the same value must alternate. /// * The caller must own the [`ListArc`] for this value. /// * The caller must not give up ownership of the [`ListArc`] unless `post_remove` has been /// called after this call to `prepare_to_insert`. /// /// [`Arc`]: crate::sync::Arc unsafe fn prepare_to_insert(me: *const Self) -> *mut ListLinks; /// This undoes a previous call to `prepare_to_insert`. /// /// # Guarantees /// /// The returned pointer is the pointer that was originally passed to `prepare_to_insert`. /// /// # Safety /// /// The provided pointer must be the pointer returned by the most recent call to /// `prepare_to_insert`. unsafe fn post_remove(me: *mut ListLinks) -> *const Self; } #[repr(C)] #[derive(Copy, Clone)] struct ListLinksFields { next: *mut ListLinksFields, prev: *mut ListLinksFields, } /// The prev/next pointers for an item in a linked list. /// /// # Invariants /// /// The fields are null if and only if this item is not in a list. #[repr(transparent)] pub struct ListLinks { // This type is `!Unpin` for aliasing reasons as the pointers are part of an intrusive linked // list. inner: Opaque, } // SAFETY: The only way to access/modify the pointers inside of `ListLinks` is via holding the // associated `ListArc`. Since that type correctly implements `Send`, it is impossible to // move this an instance of this type to a different thread if the pointees are `!Send`. unsafe impl Send for ListLinks {} // SAFETY: The type is opaque so immutable references to a ListLinks are useless. Therefore, it's // okay to have immutable access to a ListLinks from several threads at once. unsafe impl Sync for ListLinks {} impl ListLinks { /// Creates a new initializer for this type. pub fn new() -> impl PinInit { // 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`. ListLinks { inner: Opaque::new(ListLinksFields { prev: ptr::null_mut(), next: ptr::null_mut(), }), } } /// # Safety /// /// `me` must be dereferenceable. #[inline] unsafe fn fields(me: *mut Self) -> *mut ListLinksFields { // SAFETY: The caller promises that the pointer is valid. unsafe { Opaque::raw_get(ptr::addr_of!((*me).inner)) } } /// # Safety /// /// `me` must be dereferenceable. #[inline] unsafe fn from_fields(me: *mut ListLinksFields) -> *mut Self { me.cast() } } /// Similar to [`ListLinks`], but also contains a pointer to the full value. /// /// This type can be used instead of [`ListLinks`] to support lists with trait objects. #[repr(C)] pub struct ListLinksSelfPtr { /// The `ListLinks` field inside this value. /// /// This is public so that it can be used with `impl_has_list_links!`. pub inner: ListLinks, // UnsafeCell is not enough here because we use `Opaque::uninit` as a dummy value, and // `ptr::null()` doesn't work for `T: ?Sized`. self_ptr: Opaque<*const T>, } // SAFETY: The fields of a ListLinksSelfPtr can be moved across thread boundaries. unsafe impl Send for ListLinksSelfPtr {} // SAFETY: The type is opaque so immutable references to a ListLinksSelfPtr are useless. Therefore, // it's okay to have immutable access to a ListLinks from several threads at once. // // Note that `inner` being a public field does not prevent this type from being opaque, since // `inner` is a opaque type. unsafe impl Sync for ListLinksSelfPtr {} impl ListLinksSelfPtr { /// The offset from the [`ListLinks`] to the self pointer field. pub const LIST_LINKS_SELF_PTR_OFFSET: usize = core::mem::offset_of!(Self, self_ptr); /// Creates a new initializer for this type. pub fn new() -> impl PinInit { // 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: ListLinks { inner: Opaque::new(ListLinksFields { prev: ptr::null_mut(), next: ptr::null_mut(), }), }, self_ptr: Opaque::uninit(), } } } impl, const ID: u64> List { /// Creates a new empty list. pub const fn new() -> Self { Self { first: ptr::null_mut(), _ty: PhantomData, } } /// Returns whether this list is empty. pub fn is_empty(&self) -> bool { self.first.is_null() } /// Add the provided item to the back of the list. pub fn push_back(&mut self, item: ListArc) { let raw_item = ListArc::into_raw(item); // SAFETY: // * We just got `raw_item` from a `ListArc`, so it's in an `Arc`. // * Since we have ownership of the `ListArc`, `post_remove` must have been called after // the most recent call to `prepare_to_insert`, if any. // * We own the `ListArc`. // * Removing items from this list is always done using `remove_internal_inner`, which // calls `post_remove` before giving up ownership. let list_links = unsafe { T::prepare_to_insert(raw_item) }; // SAFETY: We have not yet called `post_remove`, so `list_links` is still valid. let item = unsafe { ListLinks::fields(list_links) }; if self.first.is_null() { self.first = item; // SAFETY: The caller just gave us ownership of these fields. // INVARIANT: A linked list with one item should be cyclic. unsafe { (*item).next = item; (*item).prev = item; } } else { let next = self.first; // SAFETY: By the type invariant, this pointer is valid or null. We just checked that // it's not null, so it must be valid. let prev = unsafe { (*next).prev }; // SAFETY: Pointers in a linked list are never dangling, and the caller just gave us // ownership of the fields on `item`. // INVARIANT: This correctly inserts `item` between `prev` and `next`. unsafe { (*item).next = next; (*item).prev = prev; (*prev).next = item; (*next).prev = item; } } } /// Add the provided item to the front of the list. pub fn push_front(&mut self, item: ListArc) { let raw_item = ListArc::into_raw(item); // SAFETY: // * We just got `raw_item` from a `ListArc`, so it's in an `Arc`. // * If this requirement is violated, then the previous caller of `prepare_to_insert` // violated the safety requirement that they can't give up ownership of the `ListArc` // until they call `post_remove`. // * We own the `ListArc`. // * Removing items] from this list is always done using `remove_internal_inner`, which // calls `post_remove` before giving up ownership. let list_links = unsafe { T::prepare_to_insert(raw_item) }; // SAFETY: We have not yet called `post_remove`, so `list_links` is still valid. let item = unsafe { ListLinks::fields(list_links) }; if self.first.is_null() { // SAFETY: The caller just gave us ownership of these fields. // INVARIANT: A linked list with one item should be cyclic. unsafe { (*item).next = item; (*item).prev = item; } } else { let next = self.first; // SAFETY: We just checked that `next` is non-null. let prev = unsafe { (*next).prev }; // SAFETY: Pointers in a linked list are never dangling, and the caller just gave us // ownership of the fields on `item`. // INVARIANT: This correctly inserts `item` between `prev` and `next`. unsafe { (*item).next = next; (*item).prev = prev; (*prev).next = item; (*next).prev = item; } } self.first = item; } /// Removes the last item from this list. pub fn pop_back(&mut self) -> Option> { if self.first.is_null() { return None; } // SAFETY: We just checked that the list is not empty. let last = unsafe { (*self.first).prev }; // SAFETY: The last item of this list is in this list. Some(unsafe { self.remove_internal(last) }) } /// Removes the first item from this list. pub fn pop_front(&mut self) -> Option> { if self.first.is_null() { return None; } // SAFETY: The first item of this list is in this list. Some(unsafe { self.remove_internal(self.first) }) } /// Removes the provided item from this list and returns it. /// /// This returns `None` if the item is not in the list. (Note that by the safety requirements, /// this means that the item is not in any list.) /// /// # Safety /// /// `item` must not be in a different linked list (with the same id). pub unsafe fn remove(&mut self, item: &T) -> Option> { // SAFETY: TODO. let mut item = unsafe { ListLinks::fields(T::view_links(item)) }; // SAFETY: The user provided a reference, and reference are never dangling. // // As for why this is not a data race, there are two cases: // // * If `item` is not in any list, then these fields are read-only and null. // * If `item` is in this list, then we have exclusive access to these fields since we // have a mutable reference to the list. // // In either case, there's no race. let ListLinksFields { next, prev } = unsafe { *item }; debug_assert_eq!(next.is_null(), prev.is_null()); if !next.is_null() { // This is really a no-op, but this ensures that `item` is a raw pointer that was // obtained without going through a pointer->reference->pointer conversion roundtrip. // This ensures that the list is valid under the more restrictive strict provenance // ruleset. // // SAFETY: We just checked that `next` is not null, and it's not dangling by the // list invariants. unsafe { debug_assert_eq!(item, (*next).prev); item = (*next).prev; } // SAFETY: We just checked that `item` is in a list, so the caller guarantees that it // is in this list. The pointers are in the right order. Some(unsafe { self.remove_internal_inner(item, next, prev) }) } else { None } } /// Removes the provided item from the list. /// /// # Safety /// /// `item` must point at an item in this list. unsafe fn remove_internal(&mut self, item: *mut ListLinksFields) -> ListArc { // SAFETY: The caller promises that this pointer is not dangling, and there's no data race // since we have a mutable reference to the list containing `item`. let ListLinksFields { next, prev } = unsafe { *item }; // SAFETY: The pointers are ok and in the right order. unsafe { self.remove_internal_inner(item, next, prev) } } /// Removes the provided item from the list. /// /// # Safety /// /// The `item` pointer must point at an item in this list, and we must have `(*item).next == /// next` and `(*item).prev == prev`. unsafe fn remove_internal_inner( &mut self, item: *mut ListLinksFields, next: *mut ListLinksFields, prev: *mut ListLinksFields, ) -> ListArc { // SAFETY: We have exclusive access to the pointers of items in the list, and the prev/next // pointers are always valid for items in a list. // // INVARIANT: There are three cases: // * If the list has at least three items, then after removing the item, `prev` and `next` // will be next to each other. // * If the list has two items, then the remaining item will point at itself. // * If the list has one item, then `next == prev == item`, so these writes have no // effect. The list remains unchanged and `item` is still in the list for now. unsafe { (*next).prev = prev; (*prev).next = next; } // SAFETY: We have exclusive access to items in the list. // INVARIANT: `item` is being removed, so the pointers should be null. unsafe { (*item).prev = ptr::null_mut(); (*item).next = ptr::null_mut(); } // INVARIANT: There are three cases: // * If `item` was not the first item, then `self.first` should remain unchanged. // * If `item` was the first item and there is another item, then we just updated // `prev->next` to `next`, which is the new first item, and setting `item->next` to null // did not modify `prev->next`. // * If `item` was the only item in the list, then `prev == item`, and we just set // `item->next` to null, so this correctly sets `first` to null now that the list is // empty. if self.first == item { // SAFETY: The `prev` pointer is the value that `item->prev` had when it was in this // list, so it must be valid. There is no race since `prev` is still in the list and we // still have exclusive access to the list. self.first = unsafe { (*prev).next }; } // SAFETY: `item` used to be in the list, so it is dereferenceable by the type invariants // of `List`. let list_links = unsafe { ListLinks::from_fields(item) }; // SAFETY: Any pointer in the list originates from a `prepare_to_insert` call. let raw_item = unsafe { T::post_remove(list_links) }; // SAFETY: The above call to `post_remove` guarantees that we can recreate the `ListArc`. unsafe { ListArc::from_raw(raw_item) } } /// Moves all items from `other` into `self`. /// /// The items of `other` are added to the back of `self`, so the last item of `other` becomes /// the last item of `self`. pub fn push_all_back(&mut self, other: &mut List) { // First, we insert the elements into `self`. At the end, we make `other` empty. if self.is_empty() { // INVARIANT: All of the elements in `other` become elements of `self`. self.first = other.first; } else if !other.is_empty() { let other_first = other.first; // SAFETY: The other list is not empty, so this pointer is valid. let other_last = unsafe { (*other_first).prev }; let self_first = self.first; // SAFETY: The self list is not empty, so this pointer is valid. let self_last = unsafe { (*self_first).prev }; // SAFETY: We have exclusive access to both lists, so we can update the pointers. // INVARIANT: This correctly sets the pointers to merge both lists. We do not need to // update `self.first` because the first element of `self` does not change. unsafe { (*self_first).prev = other_last; (*other_last).next = self_first; (*self_last).next = other_first; (*other_first).prev = self_last; } } // INVARIANT: The other list is now empty, so update its pointer. other.first = ptr::null_mut(); } /// Returns a cursor to the first element of the list. /// /// If the list is empty, this returns `None`. pub fn cursor_front(&mut self) -> Option> { if self.first.is_null() { None } else { Some(Cursor { current: self.first, list: self, }) } } /// Creates an iterator over the list. pub fn iter(&self) -> Iter<'_, T, ID> { // INVARIANT: If the list is empty, both pointers are null. Otherwise, both pointers point // at the first element of the same list. Iter { current: self.first, stop: self.first, _ty: PhantomData, } } } impl, const ID: u64> Default for List { fn default() -> Self { List::new() } } impl, const ID: u64> Drop for List { fn drop(&mut self) { while let Some(item) = self.pop_front() { drop(item); } } } /// An iterator over a [`List`]. /// /// # Invariants /// /// * There must be a [`List`] that is immutably borrowed for the duration of `'a`. /// * The `current` pointer is null or points at a value in that [`List`]. /// * The `stop` pointer is equal to the `first` field of that [`List`]. #[derive(Clone)] pub struct Iter<'a, T: ?Sized + ListItem, const ID: u64 = 0> { current: *mut ListLinksFields, stop: *mut ListLinksFields, _ty: PhantomData<&'a ListArc>, } impl<'a, T: ?Sized + ListItem, const ID: u64> Iterator for Iter<'a, T, ID> { type Item = ArcBorrow<'a, T>; fn next(&mut self) -> Option> { if self.current.is_null() { return None; } let current = self.current; // SAFETY: We just checked that `current` is not null, so it is in a list, and hence not // dangling. There's no race because the iterator holds an immutable borrow to the list. let next = unsafe { (*current).next }; // INVARIANT: If `current` was the last element of the list, then this updates it to null. // Otherwise, we update it to the next element. self.current = if next != self.stop { next } else { ptr::null_mut() }; // SAFETY: The `current` pointer points at a value in the list. let item = unsafe { T::view_value(ListLinks::from_fields(current)) }; // SAFETY: // * All values in a list are stored in an `Arc`. // * The value cannot be removed from the list for the duration of the lifetime annotated // on the returned `ArcBorrow`, because removing it from the list would require mutable // access to the list. However, the `ArcBorrow` is annotated with the iterator's // lifetime, and the list is immutably borrowed for that lifetime. // * Values in a list never have a `UniqueArc` reference. Some(unsafe { ArcBorrow::from_raw(item) }) } } /// A cursor into a [`List`]. /// /// # Invariants /// /// The `current` pointer points a value in `list`. pub struct Cursor<'a, T: ?Sized + ListItem, const ID: u64 = 0> { current: *mut ListLinksFields, list: &'a mut List, } impl<'a, T: ?Sized + ListItem, const ID: u64> Cursor<'a, T, ID> { /// Access the current element of this cursor. pub fn current(&self) -> ArcBorrow<'_, T> { // SAFETY: The `current` pointer points a value in the list. let me = unsafe { T::view_value(ListLinks::from_fields(self.current)) }; // SAFETY: // * All values in a list are stored in an `Arc`. // * The value cannot be removed from the list for the duration of the lifetime annotated // on the returned `ArcBorrow`, because removing it from the list would require mutable // access to the cursor or the list. However, the `ArcBorrow` holds an immutable borrow // on the cursor, which in turn holds a mutable borrow on the list, so any such // mutable access requires first releasing the immutable borrow on the cursor. // * Values in a list never have a `UniqueArc` reference, because the list has a `ListArc` // reference, and `UniqueArc` references must be unique. unsafe { ArcBorrow::from_raw(me) } } /// Move the cursor to the next element. pub fn next(self) -> Option> { // SAFETY: The `current` field is always in a list. let next = unsafe { (*self.current).next }; if next == self.list.first { None } else { // INVARIANT: Since `self.current` is in the `list`, its `next` pointer is also in the // `list`. Some(Cursor { current: next, list: self.list, }) } } /// Move the cursor to the previous element. pub fn prev(self) -> Option> { // SAFETY: The `current` field is always in a list. let prev = unsafe { (*self.current).prev }; if self.current == self.list.first { None } else { // INVARIANT: Since `self.current` is in the `list`, its `prev` pointer is also in the // `list`. Some(Cursor { current: prev, list: self.list, }) } } /// Remove the current element from the list. pub fn remove(self) -> ListArc { // SAFETY: The `current` pointer always points at a member of the list. unsafe { self.list.remove_internal(self.current) } } } impl<'a, T: ?Sized + ListItem, const ID: u64> FusedIterator for Iter<'a, T, ID> {} impl<'a, T: ?Sized + ListItem, const ID: u64> IntoIterator for &'a List { type IntoIter = Iter<'a, T, ID>; type Item = ArcBorrow<'a, T>; fn into_iter(self) -> Iter<'a, T, ID> { self.iter() } } /// An owning iterator into a [`List`]. pub struct IntoIter, const ID: u64 = 0> { list: List, } impl, const ID: u64> Iterator for IntoIter { type Item = ListArc; fn next(&mut self) -> Option> { self.list.pop_front() } } impl, const ID: u64> FusedIterator for IntoIter {} impl, const ID: u64> DoubleEndedIterator for IntoIter { fn next_back(&mut self) -> Option> { self.list.pop_back() } } impl, const ID: u64> IntoIterator for List { type IntoIter = IntoIter; type Item = ListArc; fn into_iter(self) -> IntoIter { IntoIter { list: self } } }