Re: [PATCH v3 09/10] rust: list: support heterogeneous lists

Alice Ryhl
2024-08-01T09:39:04Z

On Thu, Aug 1, 2024 at 11:24 AM Benno Lossin <benno.lossin@proton.me> wrote:
>
> On 23.07.24 10:22, Alice Ryhl wrote:
> > @@ -181,6 +185,47 @@ unsafe fn from_fields(me: *mut ListLinksFields) -> *mut Self {
> > }
> > }
> >
> > +/// 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<T: ?Sized, const ID: u64 = 0> {
> > + /// The `ListLinks` field inside this value.
> > + ///
> > + /// This is public so that it can be used with `impl_has_list_links!`.
> > + pub inner: ListLinks<ID>,
> > + self_ptr: UnsafeCell<MaybeUninit<*const T>>,
>
> Why do you need `MaybeUninit`?

Right now the constructor initializes it to MaybeUninit::zeroed().
What would you initialize it to without MaybeUninit? Remember that the
vtable pointer in a fat pointer has strict validity requirements.

> > +}
> > +
> > +// SAFETY: The fields of a ListLinksSelfPtr can be moved across thread boundaries.
> > +unsafe impl<T: ?Sized + Send, const ID: u64> Send for ListLinksSelfPtr<T, ID> {}
> > +// 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<T: ?Sized + Sync, const ID: u64> Sync for ListLinksSelfPtr<T, ID> {}
>
> [...]
>
> > @@ -135,5 +178,91 @@ unsafe fn post_remove(me: *mut $crate::list::ListLinks<$num>) -> *const Self {
> > }
> > }
> > };
> > +
> > + (
> > + impl$({$($generics:tt)*})? ListItem<$num:tt> for $t:ty {
> > + using ListLinksSelfPtr;
> > + } $($rest:tt)*
> > + ) => {
> > + // SAFETY: See GUARANTEES comment on each method.
> > + unsafe impl$(<$($generics)*>)? $crate::list::ListItem<$num> for $t {
> > + // GUARANTEES:
> > + // This implementation of `ListItem` will not give out exclusive access to the same
> > + // `ListLinks` several times because calls to `prepare_to_insert` and `post_remove`
> > + // must alternate and exclusive access is given up when `post_remove` is called.
> > + //
> > + // Other invocations of `impl_list_item!` also cannot give out exclusive access to the
> > + // same `ListLinks` because you can only implement `ListItem` once for each value of
> > + // `ID`, and the `ListLinks` fields only work with the specified `ID`.
> > + unsafe fn prepare_to_insert(me: *const Self) -> *mut $crate::list::ListLinks<$num> {
> > + // SAFETY: The caller promises that `me` points at a valid value of type `Self`.
> > + let links_field = unsafe { <Self as $crate::list::ListItem<$num>>::view_links(me) };
> > +
> > + let spoff = $crate::list::ListLinksSelfPtr::<Self, $num>::LIST_LINKS_SELF_PTR_OFFSET;
> > + // SAFETY: The constant is equal to `offset_of!(ListLinksSelfPtr, self_ptr)`, so
> > + // the pointer stays in bounds of the allocation.
> > + let self_ptr = unsafe { (links_field as *const u8).add(spoff) }
> > + as *const ::core::cell::UnsafeCell<*const Self>;
>
> A bit confused why you need to do it this way, can't you just do this?:
>
> let links_self_field = links_field.cast::<$crate::list::ListLinksSelfPtr>();
> // SAFETY: ...
> let self_ptr = unsafe { ::core::ptr::addr_of_mut!((*links_self_field).self_ptr) };

If nothing else, the field is not public. I can't remember if there
was another reason.

Alice