Re: [PATCH v15 2/3] rust: xarray: Add an abstraction for XArray

[Tamir Duberstein]

Hi Boqun,

On Thu, Feb 6, 2025 at 12:18 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
>
> Hi Tamir,
>
> This looks good to me overall, a few comments below:
>
> On Thu, Feb 06, 2025 at 11:24:44AM -0500, Tamir Duberstein wrote:
> [...]
> > +impl<'a, T: ForeignOwnable> Guard<'a, T> {
> [...]
> > +    /// Loads an entry from the array.
> > +    ///
> > +    /// Returns the entry at the given index.
> > +    pub fn get(&self, index: usize) -> Option<T::Borrowed<'_>> {
> > +        self.load(index, |ptr| {
> > +            // SAFETY: `ptr` came from `T::into_foreign`.
> > +            unsafe { T::borrow(ptr.as_ptr()) }
> > +        })
> > +    }
> > +
> > +    /// Loads an entry from the array.
>
> Nit: firstly, this function has the same description of `get()`, also
> I would prefer something like "Returns a [`T::Borrowed`] of the object
> at `index`" rather then "Loads an entry from the array", thoughts?

I was trying to avoid repeating the signature in the comment. In other
words I was trying to write a comment that wouldn't have to change if
the signature (but not the semantics) of the function changed. Since
the difference between `get` and `get_mut` is completely described in
the type system, the two functions got the same comment. Shall I
change it?

> > +    ///
> > +    /// Returns the entry at the given index.
> > +    pub fn get_mut(&mut self, index: usize) -> Option<T::BorrowedMut<'_>> {
> > +        self.load(index, |ptr| {
> > +            // SAFETY: `ptr` came from `T::into_foreign`.
> > +            unsafe { T::borrow_mut(ptr.as_ptr()) }
> > +        })
> > +    }
> > +
> > +    /// Erases an entry from the array.
>
> Nit: s/Erases/Removes?

Will change. I used "erase" because that's the verb used in the C
function name but named it "remove" because that's the verb used in
the Rust standard library. The result is neither here nor there :)

>
> > +    ///
> > +    /// Returns the entry which was previously at the given index.
> > +    pub fn remove(&mut self, index: usize) -> Option<T> {
> > +        // SAFETY: `self.xa.xa` is always valid by the type invariant.
> > +        //
> > +        // SAFETY: The caller holds the lock.
> > +        let ptr = unsafe { bindings::__xa_erase(self.xa.xa.get(), index) }.cast();
> > +        // SAFETY: `ptr` is either NULL or came from `T::into_foreign`.
>
> SAFETY comment here needs to mention why there is no alive `T::Borrowed`
> or `T::BorrowedMut` out there per the safety requirement.

Will do.

> Regards,
> Boqun
>
> > +        unsafe { T::try_from_foreign(ptr) }
> > +    }
> > +
> [...]

Thanks for the review!