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From:   Junio C Hamano <gitster@pobox.com>
To:     Elijah Newren <newren@gmail.com>
Cc:     Elijah Newren via GitGitGadget <gitgitgadget@gmail.com>,
        Git Mailing List <git@vger.kernel.org>,
        Christian Couder <chriscool@tuxfamily.org>,
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        Johannes Sixt <j6t@kdbg.org>
Subject: Re: [PATCH v3 04/15] merge-tree: implement real merges
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Date:   Thu, 24 Feb 2022 15:36:55 -0800
In-Reply-To: <xmqqee3skp3x.fsf@gitster.g> (Junio C. Hamano's message of "Thu,
        24 Feb 2022 12:04:50 -0800")
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Junio C Hamano <gitster@pobox.com> writes:

> It is more like
> ...

Actually, I misspoke.  It is a bit different.

In my mind, the building block hierarchy would have been

 (1) Take three tree objects A, B, and O, and do the three-way
     merge.  No history relation is assumed among them.

 (2) Take two tree objects A and B, with one or more commit objects
     Os; use (2) recursively to reduce Os into a single O and then
     apply (1) on A, B and O.

 (3) Take two commit objects A and B.  Compute Os out of A and B and
     use (2) once to merge A and B.

I think the basic primitive that should be exposed to an external
world (read: plumbing) this year, after all years of experience with
merge-recursive, should be (2), not (1).  

If you have (2), then (3) is trivially possible (it is just a single
call to get_merge_base()).  "git merge-tree A B" without having to
spell out bases is so convenent and you do not have to write
"git merge-tree A B $(git merge-base --all A B)", so I am OK for it
to exist, but it is not essential.

If you have (2) and exposed (2) as the primitive plumbing,
cherry-pick and revert would be a narrow special case of passing
only one O to the machinery.

And coming from the above point of view, exposing (3) as the
primitive plumbing to scripters and system builders, and later
having to _add_ support to allow (2), felt backwards.  It should be
trivial for us to make (2) available before we can even offer (3),
but what is happening to this new plumbing command goes in the
opposite order.

It may be, as you said, the problem the underlying ort API has that
somehow makes it harder to expose (2), in which case, yes, I think
that is what bugs me.