Re: [RFC PATCH v1 1/5] rust: file: add bindings for `struct file`

[Martin Rodriguez Reboredo <yakoyoku@gmail.com>]

On 7/20/23 12:28, Alice Ryhl wrote:
> From: Wedson Almeida Filho <walmeida@microsoft.com>
> 
> Using these bindings it becomes possible to access files from drivers
> written in Rust. This patch only adds support for accessing the flags,
> and for managing the refcount of the file.
> 
> Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
> Co-Developed-by: Daniel Xu <dxu@dxuuu.xyz>
> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> Co-Developed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
> In this patch, I am defining an error type called `BadFdError`. I'd like
> your thoughts on doing it this way vs just using the normal `Error`
> type.
> 
> Pros:
>   * The type system makes it clear that the function can only fail with
>     EBADF, and that no other errors are possible.
>   * Since the compiler knows that `ARef<Self>` cannot be null and that
>     `BadFdError` has only one possible value, the return type of
>     `File::from_fd` is represented as a pointer with null being an error.
> 
> Cons:
>   * Defining additional error types involves boilerplate.
>   * The return type becomes a tagged union, making it larger than a
>     pointer.

These two are kinda passable, as a `impl_null_ptr_err` macro can be opted
to implement error types from nulls. Also if we consider that
`File::from_fd` isn't going to be called a gorillion times a second or a
recursive call is done, then I'd say that the tagged union won't bring
any other problems, except that a compiler optim is skipped.

>   * The question mark operator will only utilize the `From` trait once,
>     which prevents you from using the question mark operator on
>     `BadFdError` in methods that return some third error type that the
>     kernel `Error` is convertible into.

Although, I want this to be mentioned in the doc of `BadFdError` as this
cannot be overlooked when said usage of `?` is done.

> 
>   rust/bindings/bindings_helper.h |   2 +
>   rust/helpers.c                  |   7 ++
>   rust/kernel/file.rs             | 176 ++++++++++++++++++++++++++++++++
>   rust/kernel/lib.rs              |   1 +
>   4 files changed, 186 insertions(+)
>   create mode 100644 rust/kernel/file.rs
> 
> [...]
> diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
> [...]
> diff --git a/rust/helpers.c b/rust/helpers.c
> [...]

This one is making my head spin, shouldn't they go first?