[PATCH 1/3] rebase -r: do create merge commit after empty resolution

["Philippe Blain via GitGitGadget" <gitgitgadget@gmail.com>]

From: Philippe Blain <levraiphilippeblain@gmail.com>

When a user runs 'git rebase --continue' to conclude a conflicted merge
during a 'git rebase -r' invocation, we do not create a merge commit if
the resolution was empty (i.e. if the index and HEAD are identical). We
simply continue the rebase as if no 'merge' instruction had been given.
This is confusing since all commits from the side branch are absent from
the rebased history. What's more, if that 'merge' is the last
instruction in the todo list, we fail to remove the merge state, such
that running 'git status' shows we are still merging after the rebase
has concluded.

This happens because in 'sequencer.c::commit_staged_changes', we exit
early before calling 'run_git_commit' if 'is_clean' is true, i.e. if
nothing is staged. Fix this by also checking for the presence of
MERGE_HEAD before exiting early, such that we do call 'run_git_commit'
when MERGE_HEAD is present. This also ensures that we unlink
git_path_merge_head later in 'commit_staged_changes' to clear the merge
state.

Make sure to also remove MERGE_HEAD when a merge command fails to start.
We already remove MERGE_MSG since e032abd5a0 (rebase: fix rewritten list
for failed pick, 2023-09-06). Removing MERGE_HEAD ensures that in this
situation, upon 'git rebase --continue' we still exit early in
'commit_staged_changes', without calling 'run_git_commit'. This is
already covered by t5407.11, which fails without this change because we
enter 'run_git_commit' and then fail to find 'rebase_path_message'.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
---
 sequencer.c                |  3 ++-
 t/t3418-rebase-continue.sh | 24 ++++++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/sequencer.c b/sequencer.c
index ad0ab75c8d4..2baaf716a3c 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -4349,6 +4349,7 @@ static int do_merge(struct repository *r,
 		error(_("could not even attempt to merge '%.*s'"),
 		      merge_arg_len, arg);
 		unlink(git_path_merge_msg(r));
+		unlink(git_path_merge_head(r));
 		goto leave_merge;
 	}
 	/*
@@ -5364,7 +5365,7 @@ static int commit_staged_changes(struct repository *r,
 		flags |= AMEND_MSG;
 	}
 
-	if (is_clean) {
+	if (is_clean && !file_exists(git_path_merge_head(r))) {
 		if (refs_ref_exists(get_main_ref_store(r),
 				    "CHERRY_PICK_HEAD") &&
 		    refs_delete_ref(get_main_ref_store(r), "",
diff --git a/t/t3418-rebase-continue.sh b/t/t3418-rebase-continue.sh
index 127216f7225..f4b459fea16 100755
--- a/t/t3418-rebase-continue.sh
+++ b/t/t3418-rebase-continue.sh
@@ -111,6 +111,30 @@ test_expect_success 'rebase -r passes merge strategy options correctly' '
 	git rebase --continue
 '
 
+test_expect_success '--continue creates merge commit after empty resolution' '
+	git reset --hard main &&
+	git checkout -b rebase_i_merge &&
+	test_commit unrelated &&
+	git checkout -b rebase_i_merge_side &&
+	test_commit side2 main.txt &&
+	git checkout rebase_i_merge &&
+	test_commit side1 main.txt &&
+	PICK=$(git rev-parse --short rebase_i_merge) &&
+	test_must_fail git merge rebase_i_merge_side &&
+	echo side1 >main.txt &&
+	git add main.txt &&
+	test_tick &&
+	git commit --no-edit &&
+	FAKE_LINES="1 2 3 5 6 7 8 9 10 11" &&
+	export FAKE_LINES &&
+	test_must_fail git rebase -ir main &&
+	echo side1 >main.txt &&
+	git add main.txt &&
+	git rebase --continue &&
+	git log --merges >out &&
+	test_grep "Merge branch '\''rebase_i_merge_side'\''" out
+'
+
 test_expect_success '--skip after failed fixup cleans commit message' '
 	test_when_finished "test_might_fail git rebase --abort" &&
 	git checkout -b with-conflicting-fixup &&
-- 
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