Re: [PATCH] rust: implement ForeignOwnable for Pin<Box<T>>

[Alice Ryhl <aliceryhl@google.com>]

On Tue, Jul 30, 2024 at 7:14 PM Benno Lossin <benno.lossin@proton.me> wrote:
>
> On 30.07.24 15:06, Alice Ryhl wrote:
> > @@ -89,6 +90,32 @@ unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
> >      }
> >  }
> >
> > +impl<T: 'static> ForeignOwnable for Pin<Box<T>> {
> > +    type Borrowed<'a> = Pin<&'a T>;
> > +
> > +    fn into_foreign(self) -> *const core::ffi::c_void {
> > +        // SAFETY: We are still treating the box as pinned.
>
> I don't think that we have the guarantee that the pointee at the pointer
> that is returned by `into_foreign` is not moved.

That doesn't seem like the kind of thing we need a guarantee for.
Rather, unless we give anyone a guarantee that dereferencing it is
okay, it seems reasonable to assume that it won't be dereferenced.
Right now, it's just an opaque pointer, so C really shouldn't be
touching it. We already implement the trait for Arc which is also
pinned and also doesn't even point at a value of type T.

> AFAIU `ForeignOwnable` is used to store these pointers in C structures
> and never to actually access the value behind the returned pointer. So
> we could add the requirement to `into_foreign` (thus making it `unsafe`)
> that the pointer should not be dereferenced/used aside from `borrow` and
> `from_foreign`. Otherwise I don't see how the call below can be OK.
> What do you think?

Dereferencing the void pointer is unsafe in itself, so I don't see why
`into_foreign` has to be unsafe too.

Alice