Subject: [PATCH 2/2] [WIP] sequencer.c: carry forward notes on HEAD across "rebase -x"

From: Junio C Hamano <[email-masked]>
Date: Thu, 11 Mar 2021 19:01:07 -0800

When the external command invoked by "rebase -x" replaces the HEAD,
we seem to lose the notes attached to the original HEAD, which is
why we have been losing the amlog mappings in the git core project.

Here is a half-successful attempt to fix it.  For whatever reason,
when the external command is "git commit --amend --no-edit", the
updated code carries notes forward correctly, but when the command
is changed to "git commit --amend -m tweak", it fails to do so, and
I do not have more time to work on this, so I'd stop here with an
expected failure in the test.

Help is appreciated.

Signed-off-by: Junio C Hamano <[email-masked]>
---
 sequencer.c                   | 10 +++++++++-
 t/t3404-rebase-interactive.sh | 18 ++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/sequencer.c b/sequencer.c
index 92a4871997..e0bdc39e4d 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -3276,7 +3276,10 @@ static int do_exec(struct repository *r, const char *command_line)
 {
 	struct strvec child_env = STRVEC_INIT;
 	const char *child_argv[] = { NULL, NULL };
-	int dirty, status;
+	int dirty, status, bad_head;
+	struct object_id old_head_oid, new_head_oid;
+
+	bad_head = get_oid("HEAD", &old_head_oid);
 
 	fprintf(stderr, _("Executing: %s\n"), command_line);
 	child_argv[0] = command_line;
@@ -3286,6 +3289,11 @@ static int do_exec(struct repository *r, const char *command_line)
 	status = run_command_v_opt_cd_env(child_argv, RUN_USING_SHELL, NULL,
 					  child_env.v);
 
+	bad_head |= get_oid("HEAD", &new_head_oid);
+
+	if (!bad_head && !oideq(&old_head_oid, &new_head_oid))
+		commit_post_rewrite(r, &old_head_oid, &new_head_oid);
+
 	/* force re-reading of the cache */
 	if (discard_index(r->index) < 0 || repo_read_index(r) < 0)
 		return error(_("could not read index"));
diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index 66bcbbf952..3222c594ab 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -155,6 +155,8 @@ test_expect_success 'rebase -i with the exec command checks tree cleanness' '
 	git rebase --continue
 '
 
+# NEEDSWORK: Fix c762aada1ab3a2c428c with s/@/HEAD/;
+
 test_expect_success 'rebase -x with empty command fails' '
 	test_when_finished "git rebase --abort ||:" &&
 	test_must_fail env git rebase -x "" @ 2>actual &&
@@ -867,6 +869,22 @@ test_expect_success 'rebase -i can copy notes over a fixup' '
 	test_cmp expect output
 '
 
+test_expect_success 'notes are copied even rebase -x changes HEAD' '
+	git reset --hard n3 &&
+	git rebase -x "git commit --amend --no-edit" n1^1 &&
+	git log --format="%s <%N>" n1^1..n3 >expect &&
+	git log --format="%s <%N>" n1^1..HEAD >actual &&
+	test_cmp expect actual
+'
+
+test_expect_failure 'notes are copied even rebase -x changes HEAD' '
+	git reset --hard n3 &&
+	git rebase -x "git commit --amend -m tweak" n1^1 &&
+	git log --format="tweak <%N>" n1^1..n3 >expect &&
+	git log --format="%s <%N>" n1^1..HEAD >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'rebase while detaching HEAD' '
 	git symbolic-ref HEAD &&
 	grandparent=$(git rev-parse HEAD~2) &&
-- 
2.31.0-rc2-175-g3820f1c72e