Re: [PATCH v3 06/10] rust: list: add List

[Benno Lossin <benno.lossin@proton.me>]

On 23.07.24 10:22, Alice Ryhl wrote:
> +    /// Add the provided item to the back of the list.
> +    pub fn push_back(&mut self, item: ListArc<T, ID>) {
> +        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`.

I don't like this negative phrasing, what about "Since we have ownership
of the `ListArc`, `post_remove` must have been called after each
previous call to `prepare_to_insert`."?

> +        // * 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;
> +            }

You have this pattern several times, maybe make a function for this?

> +        }
> +    }
> +
> +    /// Add the provided item to the front of the list.
> +    pub fn push_front(&mut self, item: ListArc<T, ID>) {
> +        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

Typo: "]".

> +        //   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<ListArc<T, ID>> {
> +        if self.first.is_null() {
> +            return None;
> +        }
> +
> +        // SAFETY: We just checked that the list is not empty.

Additionally you need the type invariant that pointers in linked lists
are valid... This is a bit annoying, maybe in the future, we can have a
`ValidPtr` type that we could use here instead to avoid these
comments...

> +        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<ListArc<T, ID>> {
> +        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<ListArc<T, ID>> {
> +        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 rountrip.
> +            // 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;
> +            }

How bad do you reckon is this for performance?

---
Cheers,
Benno

> +
> +            // 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
> +        }
> +    }