Re: [PATCH 4/7] rust: file: add `FileDescriptorReservation`

[Benno Lossin]

On 11/29/23 14:11, Alice Ryhl wrote:
> +impl FileDescriptorReservation {
> +    /// Creates a new file descriptor reservation.
> +    pub fn new(flags: u32) -> Result<Self> {
> +        // SAFETY: FFI call, there are no safety requirements on `flags`.
> +        let fd: i32 = unsafe { bindings::get_unused_fd_flags(flags) };
> +        if fd < 0 {
> +            return Err(Error::from_errno(fd));
> +        }

I think here we could also use the modified `to_result` function that
returns a `u32` if the value is non-negative.

> +        Ok(Self {
> +            fd: fd as _,
> +            _not_send_sync: PhantomData,
> +        })
> +    }
> +
> +    /// Returns the file descriptor number that was reserved.
> +    pub fn reserved_fd(&self) -> u32 {
> +        self.fd
> +    }
> +
> +    /// Commits the reservation.
> +    ///
> +    /// The previously reserved file descriptor is bound to `file`. This method consumes the
> +    /// [`FileDescriptorReservation`], so it will not be usable after this call.
> +    pub fn commit(self, file: ARef<File>) {
> +        // SAFETY: `self.fd` was previously returned by `get_unused_fd_flags`, and `file.ptr` is
> +        // guaranteed to have an owned ref count by its type invariants.
> +        unsafe { bindings::fd_install(self.fd, file.0.get()) };
> +
> +        // `fd_install` consumes both the file descriptor and the file reference, so we cannot run
> +        // the destructors.
> +        core::mem::forget(self);
> +        core::mem::forget(file);

Would be useful to have an `ARef::into_raw` function that would do
the `forget` for us.

-- 
Cheers,
Benno