Re: [PATCH v3 8/8] builtin/checkout-index: stop using `the_repository`

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 7, 2025, 14:15 UTC
Message-ID: <xmqqfrjpexks.fsf@gitster.g>
In-Reply-To: <CAPSxiM_fLNUvg86TtCHZTbsE_VnhKdMiDa3pfLPsLa3ThZfxmw@mail.gmail.com>

Usman Akinyemi <usmanakinyemi202@gmail.com> writes:

Show 8 quoted lines

>> Hmph, if we are passing anything down to these code paths, I would
>> have expected that it would be an instance of "struct index_state".
>>
>> Do these two helper functions need anything other than that from the
>> repository instance?
> No, they do not. They could possibly do in the future and is there any
> reason why we might want to pass the "struct index_state" instead of
> the whole "struct repository" ?

You are asking a wrong question.

When there are two things, A and B, where B is a piece of data with a smaller scope that is contained within A, unless your function needs to access parts of A that is impossible to access if you fed it only B, limiting the interface to the smallest piece necessary (in this case, B) is always the better design. A potential caller that only has B and not A can still call your function that takes B if you designed the API that way. If you insist that the caller pass A (and infer which B to use as part of A), a potential caller that only has B cannot call your function.

So, with that understanding of a general principle in mind, you need a stronger justification to pass the larger object A that goes against the best practice, saying "Even though we do not have to pass A because only B is necessary, we pass A because ...". And you just said there is no such reason why you need a repository and for these functions an index_state is sufficient.

There are many functions that only need "struct index_state" instance and obtains one from their callers. They do not have a repository object, other than the one that is referred to from the index_state (to keep track of which repository it originated from).

But this pointer is not designed to round-trip. There can be, and there indeed are, code paths that use multiple index_state instances associated with one repository. But a repository instance has only one ".index" member. It means if you pass a repository, you are making it impossible for your potential callers that has secondary index_state instances.