Re: [PATCH v3 04/15] merge-tree: implement real merges

From: Junio C Hamano <gitster@pobox.com>
Date: Feb 2, 2022, 21:22 UTC
Message-ID: <xmqqy22tx8t1.fsf@gitster.g>
In-Reply-To: <02c29f920d0d5fde6d85f7b86a69be92e3f0f34d.1643787281.git.gitgitgadget@gmail.com>

"Elijah Newren via GitGitGadget" <gitgitgadget@gmail.com> writes:

Show 36 quoted lines

> @@ -392,7 +395,46 @@ struct merge_tree_options {
>  static int real_merge(struct merge_tree_options *o,
>  		      const char *branch1, const char *branch2)
>  {
> -	die(_("real merges are not yet implemented"));
> +	struct commit *parent1, *parent2;
> +	struct commit_list *common;
> +	struct commit_list *merge_bases = NULL;
> +	struct commit_list *j;
> +	struct merge_options opt;
> +	struct merge_result result = { 0 };
> +
> +	parent1 = get_merge_parent(branch1);
> +	if (!parent1)
> +		help_unknown_ref(branch1, "merge-tree",
> +				 _("not something we can merge"));
> +
> +	parent2 = get_merge_parent(branch2);
> +	if (!parent2)
> +		help_unknown_ref(branch2, "merge-tree",
> +				 _("not something we can merge"));
> +
> +	init_merge_options(&opt, the_repository);
> +
> +	opt.show_rename_progress = 0;
> +
> +	opt.branch1 = branch1;
> +	opt.branch2 = branch2;
> +
> +	/*
> +	 * Get the merge bases, in reverse order; see comment above
> +	 * merge_incore_recursive in merge-ort.h
> +	 */
> +	common = get_merge_bases(parent1, parent2);
> +	if (!common)
> +		die(_("refusing to merge unrelated histories"));

It appears to me that "merge-tree" in this mode, with the above code, cannot be used as a workhorse to implement server-side cherry-pick (or revert), which needs to allow the user to specify an arbitrary "common ancestor", instead of computing on its own.

To replay the change made by commit A on top of commit X (i.e. "cherry-pick A on X"), we have to be able to say "compute the three-way merge between A and X, pretending as if A^ were their common ancestor". The story is the same for revert---we compute three-way merge between A^ and X, pretending as if A were their common ancestor.

The above interface into this function, sadly, does not seem to allow such a request, unless I am missing something.

And if I am correct, it is a shame---after all, the point of the merge-trees command is to take three trees and run a three-way merge, and not being able to merge three "trees" and require "commits" makes this mode much less useful than its potential.