Subject: [PATCH] [RFC] rebase -m: partial support for copying extra commit headers

From: "Phillip Wood via GitGitGadget" <[email-masked]>
Date: Mon, 07 Apr 2025 15:52:43 +0000

From: Phillip Wood <[email-masked]>

There are external projects such as GitButler that add extra headers to
the commit objects that they create. Unfortunately these headers are
lost when the user runs "git rebase". In the absence of merge conflicts
copying these headers across to the rebased commit is relatively
straight forward as the sequencer creates the rebased commits using
commit_tree_extended() rather than forking "git commit". Preserving them
in the presence of merge conflict would mean that we either need to add
a way to creating extra headers when running "git commit" or modifying
the sequencer to stop using git commit when the commit message needs to
be edited. Both of these options involve a significant amount of more
work.

While losing the extra headers if there are merge conflicts is a
significant shortcoming for users rebasing their branches it is not such
a problem on the server where forges typically reject a rebase if it has
conflicts. Therefore even though this commit is far from a complete
solution it is a significant improvement for forges that have not yet
moved to using "git replay" which already preserves extra commit
headers.

In the long run we would also want to preserve extra headers when
cherry-picking a commit. As we cannot currently preserve extra headers
when the user wishes to edit the commit message of the cherry-picked
commit this patch only changes the behavior of "git rebase"

Signed-off-by: Phillip Wood <[email-masked]>
---
    [RFC] rebase -m: partial support for copying extra commit headers
    
    This patch is largely a response to
    https://lore.kernel.org/git/[email-masked]/ . I'm in two minds
    about whether we should consider merging such partial support but if it
    helps forges preserve extra commit headers then it may well be worth it.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1902%2Fphillipwood%2Frebase-preserve-extra-commit-headers-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1902/phillipwood/rebase-preserve-extra-commit-headers-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1902

 sequencer.c                   | 25 +++++++++++++-----
 t/t3404-rebase-interactive.sh | 48 +++++++++++++++++++++++++++++++++++
 2 files changed, 66 insertions(+), 7 deletions(-)

diff --git a/sequencer.c b/sequencer.c
index ad0ab75c8d4..5b92f77b660 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -1527,6 +1527,9 @@ static int parse_head(struct repository *r, struct commit **head)
 	return 0;
 }
 
+/* Headers to exclude when copying extra commit headers */
+static const char *exclude_gpgsig[] = { "gpgsig", "gpgsig-sha256", NULL };
+
 /*
  * Try to commit without forking 'git commit'. In some cases we need
  * to run 'git commit' to display an error message
@@ -1538,6 +1541,7 @@ static int parse_head(struct repository *r, struct commit **head)
  */
 static int try_to_commit(struct repository *r,
 			 struct strbuf *msg, const char *author,
+			 struct commit_extra_header *extra_header,
 			 struct replay_opts *opts, unsigned int flags,
 			 struct object_id *oid)
 {
@@ -1545,7 +1549,7 @@ static int try_to_commit(struct repository *r,
 	struct object_id tree;
 	struct commit *current_head = NULL;
 	struct commit_list *parents = NULL;
-	struct commit_extra_header *extra = NULL;
+	struct commit_extra_header *extra = extra_header;
 	struct strbuf err = STRBUF_INIT;
 	struct strbuf commit_msg = STRBUF_INIT;
 	char *amend_author = NULL;
@@ -1561,7 +1565,6 @@ static int try_to_commit(struct repository *r,
 		return -1;
 
 	if (flags & AMEND_MSG) {
-		const char *exclude_gpgsig[] = { "gpgsig", "gpgsig-sha256", NULL };
 		const char *out_enc = get_commit_output_encoding();
 		const char *message = repo_logmsg_reencode(r, current_head,
 							   NULL, out_enc);
@@ -1714,7 +1717,8 @@ static int try_to_commit(struct repository *r,
 		commit_post_rewrite(r, current_head, oid);
 
 out:
-	free_commit_extra_headers(extra);
+	if (extra != extra_header)
+		free_commit_extra_headers(extra);
 	free_commit_list(parents);
 	strbuf_release(&err);
 	strbuf_release(&commit_msg);
@@ -1734,6 +1738,7 @@ static int write_rebase_head(struct object_id *oid)
 
 static int do_commit(struct repository *r,
 		     const char *msg_file, const char *author,
+		     struct commit_extra_header *extra_header,
 		     struct replay_opts *opts, unsigned int flags,
 		     struct object_id *oid)
 {
@@ -1749,7 +1754,7 @@ static int do_commit(struct repository *r,
 					   msg_file);
 
 		res = try_to_commit(r, msg_file ? &sb : NULL,
-				    author, opts, flags, &oid);
+				    author, extra_header, opts, flags, &oid);
 		strbuf_release(&sb);
 		if (!res) {
 			refs_delete_ref(get_main_ref_store(r), "",
@@ -2251,6 +2256,7 @@ static int do_pick_commit(struct repository *r,
 	int res, unborn = 0, reword = 0, allow, drop_commit;
 	enum todo_command command = item->command;
 	struct commit *commit = item->commit;
+	struct commit_extra_header *extra_header = NULL;
 
 	if (opts->no_commit) {
 		/*
@@ -2391,8 +2397,12 @@ static int do_pick_commit(struct repository *r,
 			strbuf_addstr(&ctx->message, oid_to_hex(&commit->object.oid));
 			strbuf_addstr(&ctx->message, ")\n");
 		}
-		if (!is_fixup(command))
+		if (!is_fixup(command)) {
 			author = get_author(msg.message);
+			if (is_rebase_i(opts))
+				extra_header = read_commit_extra_headers(commit,
+								exclude_gpgsig);
+		}
 	}
 	ctx->have_message = 1;
 
@@ -2503,8 +2513,8 @@ static int do_pick_commit(struct repository *r,
 	} /* else allow == 0 and there's nothing special to do */
 	if (!opts->no_commit && !drop_commit) {
 		if (author || command == TODO_REVERT || (flags & AMEND_MSG))
-			res = do_commit(r, msg_file, author, opts, flags,
-					commit? &commit->object.oid : NULL);
+			res = do_commit(r, msg_file, author, extra_header, opts,
+					flags, commit? &commit->object.oid : NULL);
 		else
 			res = error(_("unable to parse commit author"));
 		*check_todo = !!(flags & EDIT_MSG);
@@ -2535,6 +2545,7 @@ fast_forward_edit:
 leave:
 	free_message(commit, &msg);
 	free(author);
+	free_commit_extra_headers(extra_header);
 	update_abort_safety_file();
 
 	return res;
diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index 2aee9789a2f..200f55824c7 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -2297,6 +2297,54 @@ test_expect_success 'non-merge commands reject merge commits' '
 	test_cmp expect actual
 '
 
+test_expect_success 'unconflicted pick copies extra commit headers' '
+	tree="$(git rev-parse C^{tree})" &&
+	parent="$(git rev-parse C^{commit})" &&
+	for i in 1 2 3 4 5 6 7
+	do
+		cat >commit <<-EOF &&
+		tree $tree
+		parent $parent
+		author $GIT_AUTHOR_NAME <$GIT_AUTHOR_EMAIL> $GIT_AUTHOR_DATE
+		committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+		x-extra-header value $i
+
+		An empty commit with an extra header $i
+		EOF
+
+		parent="$(git hash-object -t commit -w commit)" &&
+		eval "oid$i=\$parent" &&
+		test_tick || return 1
+	done &&
+
+	cat >todo <<-EOF &&
+	pick $oid1
+	pick $oid2
+	fixup $oid3
+	reword $oid4
+	edit $oid5
+	pick $oid6
+	squash $oid7
+	EOF
+
+	(
+		set_replace_editor todo &&
+		FAKE_COMMIT_AMEND=EDITED git rebase -i --onto A $oid1^ $oid5
+	) &&
+	echo changed >file2 &&
+	git add file2 &&
+	FAKE_COMMIT_AMEND=EDITED git rebase --continue &&
+	j=4 &&
+	for i in 1 2 4 5 6
+	do
+		git cat-file commit HEAD~$j >actual &&
+		sed -n -e /^\$/q -e /^x-extra/p actual >actual.extra-header &&
+		echo "x-extra-header value $i" >expect &&
+		test_cmp expect actual.extra-header &&
+		j=$((j-1)) || return 1
+	done
+'
+
 # This must be the last test in this file
 test_expect_success '$EDITOR and friends are unchanged' '
 	test_editor_unchanged

base-commit: a36e024e989f4d35f35987a60e3af8022cac3420
-- 
gitgitgadget