[RFC PATCH 13/35] merge: add --reverse-parents option

[Felipe Contreras <felipe.contreras@gmail.com>]

It would be useful to make `git update` reverse parents as desired.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
---
 Documentation/merge-options.txt |  4 ++++
 builtin/commit.c                |  5 +++++
 builtin/merge.c                 | 11 ++++++++++-
 t/t7600-merge.sh                | 26 ++++++++++++++++++++++++++
 4 files changed, 45 insertions(+), 1 deletion(-)

diff --git a/Documentation/merge-options.txt b/Documentation/merge-options.txt
index eb0aabd396..8917ed97b2 100644
--- a/Documentation/merge-options.txt
+++ b/Documentation/merge-options.txt
@@ -149,6 +149,10 @@ ifndef::git-pull[]
 	Note that not all merge strategies may support progress
 	reporting.
 
+--reverse-parents::
+--no-reverse-parents::
+	Reverse the order of parents in the merge commit.
+
 endif::git-pull[]
 
 --autostash::
diff --git a/builtin/commit.c b/builtin/commit.c
index f9dd155566..931738eeaf 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1724,6 +1724,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		FILE *fp;
 		int allow_fast_forward = 1;
 		struct commit_list **pptr = &parents;
+		int reverse_parents = 0;
 
 		if (!reflog_msg)
 			reflog_msg = "commit (merge)";
@@ -1744,9 +1745,13 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 				die_errno(_("could not read MERGE_MODE"));
 			if (strstr(sb.buf, "no-ff"))
 				allow_fast_forward = 0;
+			if (strstr(sb.buf, "reverse"))
+				reverse_parents = 1;
 		}
 		if (allow_fast_forward)
 			reduce_heads_replace(&parents);
+		if (reverse_parents)
+			parents = reverse_commit_list(parents);
 	} else {
 		if (!reflog_msg)
 			reflog_msg = is_from_cherry_pick(whence)
diff --git a/builtin/merge.c b/builtin/merge.c
index 1836f98f82..b9c6c43d8f 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -91,6 +91,7 @@ static int signoff;
 static const char *sign_commit;
 static int autostash;
 static int no_verify;
+static int reverse_parents;
 
 static struct strategy all_strategy[] = {
 	{ "recursive",  DEFAULT_TWOHEAD | NO_TRIVIAL },
@@ -306,6 +307,8 @@ static struct option builtin_merge_options[] = {
 	OPT_BOOL(0, "overwrite-ignore", &overwrite_ignore, N_("update ignored files (default)")),
 	OPT_BOOL(0, "signoff", &signoff, N_("add a Signed-off-by trailer")),
 	OPT_BOOL(0, "no-verify", &no_verify, N_("bypass pre-merge-commit and commit-msg hooks")),
+	OPT_BOOL(0, "reverse-parents", &reverse_parents,
+		N_("reverse the order of parents")),
 	OPT_END()
 };
 
@@ -913,6 +916,8 @@ static int merge_trivial(struct commit *head, struct commit_list *remoteheads)
 	pptr = commit_list_append(head, pptr);
 	pptr = commit_list_append(remoteheads->item, pptr);
 	prepare_to_commit(remoteheads);
+	if (reverse_parents)
+		parents = reverse_commit_list(parents);
 	if (commit_tree(merge_msg.buf, merge_msg.len, &result_tree, parents,
 			&result_commit, NULL, sign_commit))
 		die(_("failed to write commit object"));
@@ -937,6 +942,8 @@ static int finish_automerge(struct commit *head,
 	parents = remoteheads;
 	if (!head_subsumed || fast_forward == FF_NO)
 		commit_list_insert(head, &parents);
+	if (reverse_parents)
+		parents = reverse_commit_list(parents);
 	prepare_to_commit(remoteheads);
 	if (commit_tree(merge_msg.buf, merge_msg.len, result_tree, parents,
 			&result_commit, NULL, sign_commit))
@@ -1049,7 +1056,9 @@ static void write_merge_heads(struct commit_list *remoteheads)
 
 	strbuf_reset(&buf);
 	if (fast_forward == FF_NO)
-		strbuf_addstr(&buf, "no-ff");
+		strbuf_addstr(&buf, "no-ff ");
+	if (reverse_parents)
+		strbuf_addstr(&buf, "reverse ");
 	write_file_buf(git_path_merge_mode(the_repository), buf.buf, buf.len);
 	strbuf_release(&buf);
 }
diff --git a/t/t7600-merge.sh b/t/t7600-merge.sh
index 10f8956665..660d13dd51 100755
--- a/t/t7600-merge.sh
+++ b/t/t7600-merge.sh
@@ -563,6 +563,16 @@ test_expect_success 'merge c0 with c1 (no-ff)' '
 
 test_debug 'git log --graph --decorate --oneline --all'
 
+test_expect_success 'merge c0 with c1 (--reverse-parents)' '
+	git reset --hard c0 &&
+	test_tick &&
+	git merge --no-ff --reverse-parents c1 &&
+	verify_merge file result.1 &&
+	verify_parents $c1 $c0
+'
+
+test_debug 'git log --graph --decorate --oneline --all'
+
 test_expect_success 'merge c0 with c1 (merge.ff=false)' '
 	git reset --hard c0 &&
 	test_config merge.ff "false" &&
@@ -940,6 +950,22 @@ test_expect_success 'merge annotated/signed tag w/ tracking' '
 	)
 '
 
+test_expect_success 'merge --reverse-parents --no-commit && commit' '
+	git reset --hard c0 &&
+	git merge --no-ff --reverse-parents --no-commit c1 &&
+	EDITOR=: git commit &&
+	verify_parents $c1 $c0
+'
+
+test_debug 'git log --graph --decorate --oneline --all'
+
+test_expect_success 'amending reverse merge commit' '
+	EDITOR=: git commit --amend &&
+	verify_parents $c1 $c0
+'
+
+test_debug 'git log --graph --decorate --oneline --all'
+
 test_expect_success GPG 'merge --ff-only tag' '
 	git reset --hard c0 &&
 	git commit --allow-empty -m "A newer commit" &&
-- 
2.32.0.36.g70aac2b1aa