Re: [PATCH v2 1/7] rust: file: add Rust abstraction for `struct file`

[Benno Lossin]

On 12/6/23 12:59, Alice Ryhl wrote:
> +impl File {
> +    /// Constructs a new `struct file` wrapper from a file descriptor.
> +    ///
> +    /// The file descriptor belongs to the current process.
> +    pub fn fget(fd: u32) -> Result<ARef<Self>, BadFdError> {
> +        // SAFETY: FFI call, there are no requirements on `fd`.
> +        let ptr = ptr::NonNull::new(unsafe { bindings::fget(fd) }).ok_or(BadFdError)?;
> +
> +        // SAFETY: `fget` either returns null or a valid pointer to a file, and we checked for null
> +        // above.

Since now both the Rust and C functions are called `fget`, I think you
should refer to `bindings::fget`.

> +        //
> +        // INVARIANT: `fget` increments the refcount before returning.
> +        Ok(unsafe { ARef::from_raw(ptr.cast()) })
> +    }

[...]

> +// SAFETY: The type invariants guarantee that `File` is always ref-counted.
> +unsafe impl AlwaysRefCounted for File {
> +    fn inc_ref(&self) {
> +        // SAFETY: The existence of a shared reference means that the refcount is nonzero.
> +        unsafe { bindings::get_file(self.as_ptr()) };
> +    }
> +
> +    unsafe fn dec_ref(obj: ptr::NonNull<Self>) {
> +        // SAFETY: The safety requirements guarantee that the refcount is nonzero.
> +        unsafe { bindings::fput(obj.cast().as_ptr()) }

The comment should also justify the cast.

-- 
Cheers,
Benno

> +    }
> +}