Re: [PATCH 15/17] odb/source: make `read_alternates()` function pluggable

> Introduce a new callback function in `struct odb_source` to make the
> function pluggable.
> 
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---

> diff --git a/odb/source.h b/odb/source.h
> index ddce43eb20..14f5d56f68 100644
> --- a/odb/source.h
> +++ b/odb/source.h
> @@ -55,6 +55,7 @@ struct object_id;
>  struct object_info;
>  struct odb_read_stream;
>  struct odb_write_stream;
> +struct strvec;
>  
>  /*
>   * A callback function that can be used to iterate through objects. If given,
> @@ -229,6 +230,20 @@ struct odb_source {
>  	int (*write_object_stream)(struct odb_source *source,
>  				   struct odb_write_stream *stream, size_t len,
>  				   struct object_id *oid);
> +
> +	/*
> +	 * This callback is expected to read the list of alternate object
> +	 * database sources connected to it and write them into the `strvec`.
> +	 *
> +	 * The format is expected to follow the "objectStorage" extension
> +	 * format with `(backend://)?payload` syntax. If the payload contains
> +	 * paths, these paths must be resolved to absolute paths.