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

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

On 01.08.24 11:40, Alice Ryhl wrote:
> On Thu, Aug 1, 2024 at 11:11 AM Benno Lossin <benno.lossin@proton.me> wrote:
>>
>> 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`."?
> 
> I think we just need to argue about the most recent call to
> prepare_to_insert but ok.

I would argue that's exactly what my version does. Maybe "Since we have
ownership of the `ListArc`, the most recent call to `prepare_to_insert`
must have had a matching `post_remove` call afterwards."
But I liked the above version more.

>>> +        // * 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?
> 
> It's just two times. I think it's fine.

Sure, it seemed more in my mind.

>>> +        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?
> 
> I don't think it's a problem at all.

Sounds good.

---
Cheers,
Benno