From dc23839ef31bdf7e7a26242ea5c5d208982d0ba3 Mon Sep 17 00:00:00 2001
From: Phillip Wood <phillip.wood@dunelm.org.uk>
Date: Tue, 17 Dec 2024 14:50:27 +0000
Subject: [PATCH] rebase -i: reword empty commit after fast-forward

When rebasing git tries to reuse existing commits by fast-forwarding if
the parents are unchanged. Rewording an empty commit that has been
fast-forwarded fails because "git commit --amend" is called without
"--allow-empty". This happens because when a commit is fast-forwarded
the logic that checks whether we should pass "--allow-empty" is
skipped. Fix this by always passing "--allow-empty" when rewording a
commit. This is safe when we fast-forward because we know that the
commit cannot have become empty due to the rebase as we are reusing an
existing commit object. It is also safe when we do not fast-forward
because the call to run_git_commit() that created the commit we're
rewording would have failed if the commit had become empty.

As "git commit" will happily create empty merge commits without
"--allow-empty" we do not need to pass that flag when rewording merge
commits.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
---
 sequencer.c                   |  5 ++---
 t/t3404-rebase-interactive.sh | 14 ++++++++++++++
 t/t3430-rebase-merges.sh      | 20 ++++++++++++++++++++
 3 files changed, 36 insertions(+), 3 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index 407ee4e90fea68..763bef1c8983e4 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -2510,9 +2510,8 @@ static int do_pick_commit(struct repository *r,
 		*check_todo = !!(flags & EDIT_MSG);
 		if (!res && reword) {
 fast_forward_edit:
-			res = run_git_commit(NULL, opts, EDIT_MSG |
-					     VERIFY_MSG | AMEND_MSG |
-					     (flags & ALLOW_EMPTY));
+			flags = EDIT_MSG | VERIFY_MSG | AMEND_MSG | ALLOW_EMPTY;
+			res = run_git_commit(NULL, opts, flags);
 			*check_todo = 1;
 		}
 	}
diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index ecfc02062cd046..2aee9789a2fae2 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -791,6 +791,20 @@ test_expect_success 'reword' '
 	grep "C changed" actual
 '
 
+test_expect_success 'reword fast-forwarded empty commit' '
+	git commit --allow-empty -m "empty commit" --only &&
+	(
+		set_fake_editor &&
+		FAKE_COMMIT_AMEND=edited FAKE_LINES="reword 1" \
+			git rebase -i HEAD^
+	) &&
+	test_commit_message HEAD <<-\EOF
+	empty commit
+
+	edited
+	EOF
+'
+
 test_expect_success 'no uncommitted changes when rewording and the todo list is reloaded' '
 	git checkout E &&
 	test_when_finished "git checkout @{-1}" &&
diff --git a/t/t3430-rebase-merges.sh b/t/t3430-rebase-merges.sh
index 2593711fecdc9f..b84d68c4b96bc9 100755
--- a/t/t3430-rebase-merges.sh
+++ b/t/t3430-rebase-merges.sh
@@ -610,4 +610,24 @@ test_expect_success 'truncate label names' '
 	grep "label 0123456789-$" out
 '
 
+test_expect_success 'reword fast-forwarded empty merge commit' '
+	oid="$(git commit-tree -m "D1" -p A D^{tree})" &&
+	oid="$(git commit-tree -m "empty merge" -p D -p $oid D^{tree})" &&
+
+	write_script sequence-editor.sh <<-\EOF &&
+	sed /^merge/s/-C/-c/ "$1" >"$1.tmp"
+	mv "$1.tmp" "$1"
+	EOF
+
+	(
+		test_set_sequence_editor "$(pwd)/sequence-editor.sh" &&
+		GIT_EDITOR="echo edited >>" git rebase -i -r D $oid
+	) &&
+	test_commit_message HEAD <<-\EOF
+	empty merge
+
+	edited
+	EOF
+'
+
 test_done