Re: [PATCH v3 3/3] rev-list: add commit object support in `--missing` option

From: Karthik Nayak <karthik.188@gmail.com>
Date: Oct 20, 2023, 11:14 UTC
Message-ID: <CAOLa=ZR4Qd9x-zVv9OtL7KTU00W2uT-kxdhKOLFZbi5cnHdiSQ@mail.gmail.com>
In-Reply-To: <xmqqttqmtcc2.fsf@gitster.g>

On Fri, Oct 20, 2023 at 1:35 AM Junio C Hamano <gitster@pobox.com> wrote:

Show 19 quoted lines

>
> Junio C Hamano <gitster@pobox.com> writes:
>
> > Do we really need to allocate a new bit in the object flags, which
> > is already a scarce resource?
>
> Clarification.  I was *not* wondering if we can steal and (re|ab)use
> a bit that is used for other purposes, in order to avoid allocating
> a new bit.
>
> Rather, I was wondering if we need to use object flags to mark these
> objects, or can do what we want to do without using any object flags
> at all.  For the purpose of reporting "missing" objects, wouldn't it
> be sufficient to walk the object graph and report our findings as we
> go?  To avoid reporting the same object twice, as we reasonably can
> expect that the missing objects are minority (compared to the total
> number of objects), perhaps the codepath that makes such a report
> can use a hashmap of object_ids or something, for example.
>

This is kind of hard to do since the revision walking happens in revision.c and list-objects.c but the missing commits are needed in builtin/rev-list.c. So even if we build a list of missing commits in list-objects.c, there is no nice way to send this back to rev-list.c.

The only way we communicate between these layers is through callbacks, i.e. the show_commit() function in rev-list.c is called for every commit that the revision walk traverses over.

I'm not entirely sure what the best path to take here to be honest. I will look and see if there are any bits where overlapping doesn't cause any issues in the meantime.