Re: [RFC PATCH 2/6] repository: add initialize_repo wrapper without pointer

On Wed, Aug 07, 2024 at 04:23:05PM -0700, Josh Steadmon wrote:

On 2024.08.08 07:52, Mike Hommey wrote:

quoted

On Wed, Aug 07, 2024 at 11:21:27AM -0700, Josh Steadmon wrote:

quoted

Non-C external consumers of libgit.a have to redefine the `repository`
object in their own language if they want to call
initialize_repository() to ensure memory for the object is allocated
correctly. This is not ideal for external consumers that have no need
for the entire `the_repository` object but need to call other functions
from an initialized repository. Therefore, add a friendly
initialize_repository() wrapper without a `the_repository` pointer.

Technically speaking, you don't really need this.

You can define `repository` as an opaque type in Rust:

#[allow(non_camel_case_types)]
#[repr(C)]
pub struct repository([u8; 0]);

And define `the_repository` as an extern symbol:

extern "C" {
    pub static mut the_repository: *mut repository;
}

Mike

I've actually already done a refactor for V2 that will avoid using this
patch entirely, but thank you for the pointer. We do something similar
to opaquely wrap configset pointers in a later patch (we use an empty
enum there, I'm not sure whether that approach or a zero-size array is
preferred).

An empty enum is a never type, I wouldn't recommend using it as an opaque
wrapper. It will likely lead to the compiler doing bad things.
https://rust-lang.github.io/never-type-initiative/RFC.html

`#[repr(C)]` and `[u8; 0]` are recommended by the nomicon.
https://doc.rust-lang.org/nomicon/ffi.html#representing-opaque-structs

(the PhantomPinned wasn't there last time I saw that page)

Mike