Subject: [PATCH v2] rebase -i: reword empty commit after fast-forward

From: "Phillip Wood via GitGitGadget" <[email-masked]>
Date: Tue, 11 Feb 2025 15:59:08 +0000

From: Phillip Wood <[email-masked]>

When rebase rewords a commit it picks the commit and then runs "git
commit --amend" to reword it. When the commit is picked the sequencer
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 because we
are amending a commit that has already been picked so if it had become
empty when it was picked we'd have already returned an error.

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 <[email-masked]>
---
    rebase -i: reword empty commit after fast-forward
    
    Thanks to Junio for his comments on V1. I've added a comment explaining
    why we use a fixed set of commits flags as he suggested.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1860%2Fphillipwood%2Frebase-reword-empty-commit-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1860/phillipwood/rebase-reword-empty-commit-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1860

Range-diff vs v1:

 1:  06de47a91d8 ! 1:  826edd3ba48 rebase -i: reword empty commit after fast-forward
     @@ sequencer.c: static int do_pick_commit(struct repository *r,
      -			res = run_git_commit(NULL, opts, EDIT_MSG |
      -					     VERIFY_MSG | AMEND_MSG |
      -					     (flags & ALLOW_EMPTY));
     ++			/*
     ++			 * To reword we amend the commit we just
     ++			 * picked or fast-forwarded. As the commit has
     ++			 * already been picked we want to use the same
     ++			 * set of commit flags regardless of how we
     ++			 * got here.
     ++			 */
      +			flags = EDIT_MSG | VERIFY_MSG | AMEND_MSG | ALLOW_EMPTY;
      +			res = run_git_commit(NULL, opts, flags);
       			*check_todo = 1;

 sequencer.c                   | 12 +++++++++---
 t/t3404-rebase-interactive.sh | 14 ++++++++++++++
 t/t3430-rebase-merges.sh      | 20 ++++++++++++++++++++
 3 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index 407ee4e90fe..ad0ab75c8d4 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -2510,9 +2510,15 @@ 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));
+			/*
+			 * To reword we amend the commit we just
+			 * picked or fast-forwarded. As the commit has
+			 * already been picked we want to use the same
+			 * set of commit flags regardless of how we
+			 * got here.
+			 */
+			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 ecfc02062cd..2aee9789a2f 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 2593711fecd..b84d68c4b96 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

base-commit: 58b5801aa94ad5031978f8e42c1be1230b3d352f
-- 
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