Re: [PATCH 4/8] merge-tree: implement real merges

[Fabian Stelzer <fs@gigacodes.de>]

On 31.12.2021 05:04, Elijah Newren via GitGitGadget wrote:
>From: Elijah Newren <newren@gmail.com>
>
>This adds the ability to perform real merges rather than just trivial
>merges (meaning handling three way content merges, recursive ancestor
>consolidation, renames, proper directory/file conflict handling, and so
>forth).  However, unlike `git merge`, the working tree and index are
>left alone and no branch is updated.
>
>The only output is:
>  - the toplevel resulting tree printed on stdout
>  - exit status of 0 (clean) or 1 (conflicts present)
>
>This output is mean to be used by some higher level script, perhaps in a
>sequence of steps like this:
>
>   NEWTREE=$(git merge-tree --real $BRANCH1 $BRANCH2)
>   test $? -eq 0 || die "There were conflicts..."
>   NEWCOMMIT=$(git commit-tree $NEWTREE -p $BRANCH1 $BRANCH2)
>   git update-ref $BRANCH1 $NEWCOMMIT
>
>Note that higher level scripts may also want to access the
>conflict/warning messages normally output during a merge, or have quick
>access to a list of files with conflicts.  That is not available in this
>preliminary implementation, but subsequent commits will add that
>ability.
>
>Signed-off-by: Elijah Newren <newren@gmail.com>
>---
> Documentation/git-merge-tree.txt | 28 +++++++----
> builtin/merge-tree.c             | 55 +++++++++++++++++++++-
> t/t4301-merge-tree-real.sh       | 81 ++++++++++++++++++++++++++++++++
> 3 files changed, 153 insertions(+), 11 deletions(-)
> create mode 100755 t/t4301-merge-tree-real.sh
>
>diff --git a/Documentation/git-merge-tree.txt b/Documentation/git-merge-tree.txt
>index 58731c19422..5823938937f 100644
>--- a/Documentation/git-merge-tree.txt
>+++ b/Documentation/git-merge-tree.txt
>@@ -3,26 +3,34 @@ git-merge-tree(1)
>
> NAME
> ----
>-git-merge-tree - Show three-way merge without touching index
>+git-merge-tree - Perform merge without touching index or working tree
>
>
> SYNOPSIS
> --------
> [verse]
>+'git merge-tree' --real <branch1> <branch2>
> 'git merge-tree' <base-tree> <branch1> <branch2>
>
> DESCRIPTION
> -----------
>-Reads three tree-ish, and output trivial merge results and
>-conflicting stages to the standard output.  This is similar to
>-what three-way 'git read-tree -m' does, but instead of storing the
>-results in the index, the command outputs the entries to the
>-standard output.
>+Performs a merge, but does not make any new commits and does not read
>+from or write to either the working tree or index.
>
>-This is meant to be used by higher level scripts to compute
>-merge results outside of the index, and stuff the results back into the
>-index.  For this reason, the output from the command omits
>-entries that match the <branch1> tree.
>+The first form will merge the two branches, doing a full recursive
>+merge with rename detection.  If the merge is clean, the exit status
>+will be `0`, and if the merge has conflicts, the exit status will be
>+`1`.  The output will consist solely of the resulting toplevel tree
>+(which may have files including conflict markers).
>+
>+The second form is meant for backward compatibility and will only do a
>+trival merge.  It reads three tree-ish, and outputs trivial merge
>+results and conflicting stages to the standard output in a semi-diff
>+format.  Since this was designed for higher level scripts to consume
>+and merge the results back into the index, it omits entries that match
>+<branch1>.  The result of this second form is is similar to what
>+three-way 'git read-tree -m' does, but instead of storing the results
>+in the index, the command outputs the entries to the standard output.
>
> GIT
> ---
>diff --git a/builtin/merge-tree.c b/builtin/merge-tree.c
>index f04b1eaad0a..c5757bed5bb 100644
>--- a/builtin/merge-tree.c
>+++ b/builtin/merge-tree.c
>@@ -2,6 +2,9 @@
> #include "builtin.h"
> #include "tree-walk.h"
> #include "xdiff-interface.h"
>+#include "help.h"
>+#include "commit-reach.h"
>+#include "merge-ort.h"
> #include "object-store.h"
> #include "parse-options.h"
> #include "repository.h"
>@@ -391,7 +394,57 @@ 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",
>+				 _("not something we can merge"));
>+
>+	parent2 = get_merge_parent(branch2);
>+	if (!parent2)
>+		help_unknown_ref(branch2, "merge",
>+				 _("not something we can merge"));
>+
>+	init_merge_options(&opt, the_repository);
>+	/*
>+	 * TODO: Support subtree and other -X options?
>+	if (use_strategies_nr == 1 &&
>+	    !strcmp(use_strategies[0]->name, "subtree"))
>+		opt.subtree_shift = "";
>+	for (x = 0; x < xopts_nr; x++)
>+		if (parse_merge_opt(&opt, xopts[x]))
>+			die(_("Unknown strategy option: -X%s"), xopts[x]);
>+	*/
>+
>+	opt.show_rename_progress = 0;
>+
>+	opt.branch1 = merge_remote_util(parent1)->name; /* or just branch1? */
>+	opt.branch2 = merge_remote_util(parent2)->name; /* or just branch2? */
>+
>+	/*
>+	 * Get the merge bases, in reverse order; see comment above
>+	 * merge_incore_recursive in merge-ort.h
>+	 */
>+	common = get_merge_bases(parent1, parent2);
>+	for (j = common; j; j = j->next)
>+		commit_list_insert(j->item, &merge_bases);
>+
>+	/*
>+	 * TODO: notify if merging unrelated histories?
>+	if (!common)
>+		fprintf(stderr, _("merging unrelated histories"));
>+	 */
>+
>+	merge_incore_recursive(&opt, merge_bases, parent1, parent2, &result);
>+	printf("%s\n", oid_to_hex(&result.tree->object.oid));
>+	merge_switch_to_result(&opt, NULL, &result, 0, 0);
>+	return result.clean ? 0 : 1;
> }
>
> int cmd_merge_tree(int argc, const char **argv, const char *prefix)
>diff --git a/t/t4301-merge-tree-real.sh b/t/t4301-merge-tree-real.sh
>new file mode 100755
>index 00000000000..9fb617ccc7f
>--- /dev/null
>+++ b/t/t4301-merge-tree-real.sh
>@@ -0,0 +1,81 @@
>+#!/bin/sh
>+
>+test_description='git merge-tree --real'
>+
>+. ./test-lib.sh
>+
>+# This test is ort-specific
>+GIT_TEST_MERGE_ALGORITHM=ort
>+export GIT_TEST_MERGE_ALGORITHM
>+
>+test_expect_success setup '
>+	test_write_lines 1 2 3 4 5 >numbers &&
>+	echo hello >greeting &&
>+	echo foo >whatever &&
>+	git add numbers greeting whatever &&
>+	git commit -m initial &&
>+
>+	git branch side1 &&
>+	git branch side2 &&
>+
>+	git checkout side1 &&
>+	test_write_lines 1 2 3 4 5 6 >numbers &&
>+	echo hi >greeting &&
>+	echo bar >whatever &&
>+	git add numbers greeting whatever &&
>+	git commit -m rename-and-modify &&

The commit implies a rename as well which I think is missing.

>+
>+	git checkout side2 &&
>+	test_write_lines 0 1 2 3 4 5 >numbers &&
>+	echo yo >greeting &&
>+	git rm whatever &&
>+	mkdir whatever &&
>+	>whatever/empty &&
>+	git add numbers greeting whatever/empty &&
>+	git commit -m remove-and-rename

And this looks more like a remove-and-modify. Is it still a rename when we 
empty the files content?

>+'
>+
>+test_expect_success 'Content merge and a few conflicts' '
>+	git checkout side1^0 &&
>+	test_must_fail git merge side2 &&
>+	cp .git/AUTO_MERGE EXPECT &&
>+	E_TREE=$(cat EXPECT) &&
>+
>+	git reset --hard &&
>+	test_must_fail git merge-tree --real side1 side2 >RESULT &&
>+	R_TREE=$(cat RESULT) &&
>+
>+	# Due to differences of e.g. "HEAD" vs "side1", the results will not
>+	# exactly match.  Dig into individual files.
>+
>+	# Numbers should have three-way merged cleanly
>+	test_write_lines 0 1 2 3 4 5 6 >expect &&
>+	git show ${R_TREE}:numbers >actual &&
>+	test_cmp expect actual &&
>+
>+	# whatever and whatever~<branch> should have same HASHES
>+	git rev-parse ${E_TREE}:whatever ${E_TREE}:whatever~HEAD >expect &&
>+	git rev-parse ${R_TREE}:whatever ${R_TREE}:whatever~side1 >actual &&
>+	test_cmp expect actual &&
>+
>+	# greeting should have a merge conflict
>+	git show ${E_TREE}:greeting >tmp &&
>+	cat tmp | sed -e s/HEAD/side1/ >expect &&
>+	git show ${R_TREE}:greeting >actual &&
>+	test_cmp expect actual
>+'
>+
>+test_expect_success 'Barf on misspelled option' '
>+	# Mis-spell with single "s" instead of double "s"
>+	test_expect_code 129 git merge-tree --real --mesages FOOBAR side1 side2 2>expect &&
>+
>+	grep "error: unknown option.*mesages" expect
>+'
>+
>+test_expect_success 'Barf on too many arguments' '
>+	test_expect_code 129 git merge-tree --real side1 side2 side3 2>expect &&
>+
>+	grep "^usage: git merge-tree" expect
>+'
>+
>+test_done
>-- 
>gitgitgadget
>