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

[Alice Ryhl]

Benno Lossin <benno.lossin@xxxxxxxxx> writes:
> 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.

I'll look into that for the next version.

>> +    /// 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.

That makes sense to me, but I don't think it needs to happen in this patchset.

Alice