Re: [PATCH v4 04/10] sparse-index: use WRITE_TREE_MISSING_OK

[Elijah Newren <newren@gmail.com>]

On Mon, Aug 30, 2021 at 6:19 AM Derrick Stolee <stolee@gmail.com> wrote:
>
> On 8/27/2021 5:33 PM, Elijah Newren wrote:
> > On Tue, Aug 24, 2021 at 2:51 PM Derrick Stolee via GitGitGadget
> > <gitgitgadget@gmail.com> wrote:
> >>
> >> From: Derrick Stolee <dstolee@microsoft.com>
> >>
> >> When updating the cache tree in convert_to_sparse(), the
> >> WRITE_TREE_MISSING_OK flag indicates that trees might be computed that
> >> do not already exist within the object database.
> >
> > Okay.
> >
> >> This happens in cases
> >> such as 'git add' creating new trees that it wants to store in
> >> anticipation of a following 'git commit'.
> >
> > This doesn't make any sense to me.  Does 'git add' call
> > convert_to_sparse()?  I don't see why it would; wouldn't the calls to
> > convert_to_sparse() come via sparse-checkout init/set commands?  If
> > I'm correct on that, and 'git add' wants to create new trees, then by
> > the time convert_to_sparse() is called in some subsequent git
> > operation, then convert_to_sparse would already have the trees it
> > needs.
>
> If someone adds a change outside the sparse-checkout cone, then the
> index is expanded in-memory, then is converted to sparse when the
> index is written again.

Ah, I missed that convert_to_sparse() is called from write_index(),
and that we are dealing with a single operation that does a
sparse->full (in memory)->sparse roundtrip.

Thanks for clearing that up.

> > I thought the reason you would need this is someone modified and
> > staged a change to a file underneath a directory that will be
> > sparsified away; at the time of convert_to_sparse(), a tree object may
> > not have yet been written for the new tree with the newly modified
> > file (because those tend to be written at commit time), but you'd need
> > it at the time you sparsified.
>
> Yes. I think we are trying to say the same thing.

While writing this, I was thinking in terms of the `git add` being
done when the index was full (both in memory and on disk), and then a
later `git sparse-checkout ...` command would invoke the
convert_to_sparse().  I didn't fully catch the nuances here.

Thanks for bringing me up to speed.