Re: [PATCH 00/12] Remove more index compatibility macros

> On Fri, Jan 1, 2021 at 8:09 AM Derrick Stolee via GitGitGadget
> <gitgitgadget@gmail.com> wrote:
>> My strategy for update-index was to create static globals "repo" and
>> "istate" that point to the_repository and the_index, respectively. Then, I
>> was able to remove macros one-by-one without changing method prototypes
>> within the file.
>>
>> I had started trying to keep everything local to the method signatures, but
>> I hit a snag when reaching the command-line parsing callbacks, which I could
>> not modify their call signature. [...]
> 
> You should be able to do this, not by modifying the callback
> signature, but by taking advantage of the `extra` member of `struct
> option` which is available to callback functions or arbitrary use. If
> you need to access the index in a callback, then assign a `struct
> index_state *` to `extra`; likewise assign a `struct repository *` to
> `extra` to access the repository. If you need access to both the index
> and the repository, then just store the repository in `extra` since
> the repository has an `index` field.
> 
> You won't be able to use any of the canned OPT_FOO() macros to
> initialize an entry in the update-index.c `options[]` array which
> needs `extra`-initialization since the macros don't let you specify
> `extra`, but you can easily bypass the macro and initialize the
> `struct option` manually. (After all, the macros exist for
> convenience; they are not a hard requirement.)
> 
> Within the callback, extract the `repository` or `index_state` as you
> would any other field. For instance:
> 
>     const struct repository *repo = opt->extra;