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

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