Re: [GIT PULL] GPIO bulk changes for kernel v4.6

[Linus Torvalds]

On Fri, Mar 18, 2016 at 9:37 AM, Junio C Hamano <gitster@xxxxxxxxx> wrote:
>
>> I don't think the original "resolve" did it, for example. You can't do
>> a three-way merge without a base.
>
> Yes, and that continues to this day:

Yeah, "octopus" also refuses it cleanly:

        common=$(git merge-base --all $SHA1 $MRC) ||
                die "Unable to find common commit with $pretty_name"

The code in the recursive merge that allows this to happen is this:

        if (merged_common_ancestors == NULL) {
                /* if there is no common ancestor, use an empty tree */
                struct tree *tree;

                tree = lookup_tree(EMPTY_TREE_SHA1_BIN);
                merged_common_ancestors = make_virtual_commit(tree, "ancestor");
        }

so the "no common ancestors" is just considered to be an empty merge base.

And I do think that's right, and I think it's clever, and it goes back to 2006:

  934d9a24078e merge-recur: if there is no common ancestor, fake empty one

but I think there should be an option there.

> This is a tangent but I wonder if we should say why we refuse to
> the standard error before calling these two "exit"s.

As mentioned, Octopus does.

That said, there's probably no reason to ever use the old three-way
merge, so I'm not even sure it's worth fixing the old
git-merge-resolve.

            Linus