Subject: [PATCH v5 09/29] rebase: support --trailer

From: Li Chen <[email-masked]>
Date: Wed, 22 Oct 2025 13:39:29 +0800

From: Li Chen <[email-masked]>

Implement a new `--trailer <text>` option for `git rebase`
(support merge backend only now), which appends arbitrary
trailer lines to each rebased commit message.

Reject it if the user passes an option that requires the
apply backend (git am) since it lacks message‑filter/trailer
hook. otherwise we can just use the merge backend.

Automatically set REBASE_FORCE when any trailer is supplied.

And reject invalid input before user edits the interactive file.

Signed-off-by: Li Chen <[email-masked]>
---
 Documentation/git-rebase.adoc |  9 +++-
 builtin/rebase.c              | 89 +++++++++++++++++++++++++++++++++
 sequencer.c                   | 13 +++++
 sequencer.h                   |  4 +-
 t/meson.build                 |  1 +
 t/t3440-rebase-trailer.sh     | 94 +++++++++++++++++++++++++++++++++++
 6 files changed, 208 insertions(+), 2 deletions(-)
 create mode 100755 t/t3440-rebase-trailer.sh

diff --git a/Documentation/git-rebase.adoc b/Documentation/git-rebase.adoc
index 005caf6164..4d2fe4be6e 100644
--- a/Documentation/git-rebase.adoc
+++ b/Documentation/git-rebase.adoc
@@ -487,9 +487,16 @@ See also INCOMPATIBLE OPTIONS below.
 	Add a `Signed-off-by` trailer to all the rebased commits. Note
 	that if `--interactive` is given then only commits marked to be
 	picked, edited or reworded will have the trailer added.
-+
+
 See also INCOMPATIBLE OPTIONS below.
 
+--trailer=<trailer>::
+       Append the given trailer line(s) to every rebased commit
+       message, processed via linkgit:git-interpret-trailers[1].
+       When this option is present *rebase automatically implies*
+       `--force-rebase` so that fast‑forwarded commits are also
+       rewritten.
+
 -i::
 --interactive::
 	Make a list of the commits which are about to be rebased.  Let the
diff --git a/builtin/rebase.c b/builtin/rebase.c
index c468828189..3db1439b52 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -36,6 +36,7 @@
 #include "reset.h"
 #include "trace2.h"
 #include "hook.h"
+#include "trailer.h"
 
 static char const * const builtin_rebase_usage[] = {
 	N_("git rebase [-i] [options] [--exec <cmd>] "
@@ -113,6 +114,7 @@ struct rebase_options {
 	enum action action;
 	char *reflog_action;
 	int signoff;
+	struct strvec trailer_args;
 	int allow_rerere_autoupdate;
 	int keep_empty;
 	int autosquash;
@@ -143,6 +145,7 @@ struct rebase_options {
 		.flags = REBASE_NO_QUIET, 		\
 		.git_am_opts = STRVEC_INIT,		\
 		.exec = STRING_LIST_INIT_NODUP,		\
+		.trailer_args = STRVEC_INIT,  \
 		.git_format_patch_opt = STRBUF_INIT,	\
 		.fork_point = -1,			\
 		.reapply_cherry_picks = -1,             \
@@ -166,6 +169,7 @@ static void rebase_options_release(struct rebase_options *opts)
 	free(opts->strategy);
 	string_list_clear(&opts->strategy_opts, 0);
 	strbuf_release(&opts->git_format_patch_opt);
+	strvec_clear(&opts->trailer_args);
 }
 
 static struct replay_opts get_replay_opts(const struct rebase_options *opts)
@@ -177,6 +181,10 @@ static struct replay_opts get_replay_opts(const struct rebase_options *opts)
 	sequencer_init_config(&replay);
 
 	replay.signoff = opts->signoff;
+
+	for (size_t i = 0; i < opts->trailer_args.nr; i++)
+		strvec_push(&replay.trailer_args, opts->trailer_args.v[i]);
+
 	replay.allow_ff = !(opts->flags & REBASE_FORCE);
 	if (opts->allow_rerere_autoupdate)
 		replay.allow_rerere_auto = opts->allow_rerere_autoupdate;
@@ -434,6 +442,8 @@ static int read_basic_state(struct rebase_options *opts)
 	struct strbuf head_name = STRBUF_INIT;
 	struct strbuf buf = STRBUF_INIT;
 	struct object_id oid;
+	const char trailer_state_name[] = "trailer";
+	const char *path = state_dir_path(trailer_state_name, opts);
 
 	if (!read_oneliner(&head_name, state_dir_path("head-name", opts),
 			   READ_ONELINER_WARN_MISSING) ||
@@ -502,11 +512,31 @@ static int read_basic_state(struct rebase_options *opts)
 
 	strbuf_release(&buf);
 
+	if (strbuf_read_file(&buf, path, 0) >= 0) {
+		const char *p = buf.buf, *end = buf.buf + buf.len;
+
+		while (p < end) {
+			char *nl = memchr(p, '\n', end - p);
+			if (!nl)
+				die("nl shouldn't be NULL");
+			*nl = '\0';
+
+			if (*p)
+				strvec_push(&opts->trailer_args, p);
+
+			p = nl + 1;
+		}
+		strbuf_release(&buf);
+	}
+	strbuf_release(&buf);
+
 	return 0;
 }
 
 static int rebase_write_basic_state(struct rebase_options *opts)
 {
+	const char trailer_state_name[] = "trailer";
+
 	write_file(state_dir_path("head-name", opts), "%s",
 		   opts->head_name ? opts->head_name : "detached HEAD");
 	write_file(state_dir_path("onto", opts), "%s",
@@ -528,6 +558,21 @@ static int rebase_write_basic_state(struct rebase_options *opts)
 	if (opts->signoff)
 		write_file(state_dir_path("signoff", opts), "--signoff");
 
+	/*
+	 * save opts->trailer_args into state_dir/trailer
+	 */
+	if (opts->trailer_args.nr) {
+		struct strbuf buf = STRBUF_INIT;
+
+		for (size_t i = 0; i < opts->trailer_args.nr; i++) {
+				strbuf_addstr(&buf, opts->trailer_args.v[i]);
+				strbuf_addch(&buf, '\n');
+		}
+		write_file(state_dir_path(trailer_state_name, opts),
+				   "%s", buf.buf);
+		strbuf_release(&buf);
+	}
+
 	return 0;
 }
 
@@ -1084,6 +1129,35 @@ static int check_exec_cmd(const char *cmd)
 	return 0;
 }
 
+static int validate_trailer_args_after_config(const struct strvec *cli_args,
+					      struct strbuf *err)
+{
+	for (size_t i = 0; i < cli_args->nr; i++) {
+		const char *raw = cli_args->v[i];
+		const char *txt; // Key[:=]Val
+		const char *sep;
+
+		if (!skip_prefix(raw, "--trailer=", &txt))
+			txt = raw;
+
+		if (!*txt) {
+			strbuf_addstr(err, _("empty --trailer argument"));
+			return -1;
+		}
+
+		sep = strpbrk(txt, ":=");
+
+		/* there must be key bfore seperator */
+		if (sep && sep == txt) {
+			strbuf_addf(err,
+				    _("invalid trailer '%s': missing key before separator"),
+				    txt);
+			return -1;
+		}
+	}
+	return 0;
+}
+
 int cmd_rebase(int argc,
 	       const char **argv,
 	       const char *prefix,
@@ -1132,6 +1206,8 @@ int cmd_rebase(int argc,
 			.flags = PARSE_OPT_NOARG,
 			.defval = REBASE_DIFFSTAT,
 		},
+		OPT_STRVEC(0, "trailer", &options.trailer_args, N_("trailer"),
+				N_("add custom trailer(s)")),
 		OPT_BOOL(0, "signoff", &options.signoff,
 			 N_("add a Signed-off-by trailer to each commit")),
 		OPT_BOOL(0, "committer-date-is-author-date",
@@ -1285,6 +1361,16 @@ int cmd_rebase(int argc,
 			     builtin_rebase_options,
 			     builtin_rebase_usage, 0);
 
+	if (options.trailer_args.nr) {
+		struct strbuf err = STRBUF_INIT;
+
+		if (validate_trailer_args_after_config(&options.trailer_args, &err))
+			die("%s", err.buf);
+
+		options.flags |= REBASE_FORCE;
+		strbuf_release(&err);
+	}
+
 	if (preserve_merges_selected)
 		die(_("--preserve-merges was replaced by --rebase-merges\n"
 			"Note: Your `pull.rebase` configuration may also be set to 'preserve',\n"
@@ -1542,6 +1628,9 @@ int cmd_rebase(int argc,
 	if (options.root && !options.onto_name)
 		imply_merge(&options, "--root without --onto");
 
+	if (options.trailer_args.nr)
+		imply_merge(&options, "--trailer");
+
 	if (isatty(2) && options.flags & REBASE_NO_QUIET)
 		strbuf_addstr(&options.git_format_patch_opt, " --progress");
 
diff --git a/sequencer.c b/sequencer.c
index 5476d39ba9..5103ae786c 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -420,6 +420,7 @@ void replay_opts_release(struct replay_opts *opts)
 	if (opts->revs)
 		release_revisions(opts->revs);
 	free(opts->revs);
+	strvec_clear(&opts->trailer_args);
 	replay_ctx_release(ctx);
 	free(opts->ctx);
 }
@@ -2517,6 +2518,18 @@ static int do_pick_commit(struct repository *r,
 			_("dropping %s %s -- patch contents already upstream\n"),
 			oid_to_hex(&commit->object.oid), msg.subject);
 	} /* else allow == 0 and there's nothing special to do */
+
+	if (!res && opts->trailer_args.nr && !drop_commit) {
+		const char *trailer_file =
+			msg_file ? msg_file : git_path_merge_msg(r);
+
+		if (amend_file_with_trailers(trailer_file,
+						&opts->trailer_args)) {
+			res = error(_("unable to add trailers to commit message"));
+			goto leave;
+		}
+	}
+
 	if (!opts->no_commit && !drop_commit) {
 		if (author || command == TODO_REVERT || (flags & AMEND_MSG))
 			res = do_commit(r, msg_file, author, reflog_action,
diff --git a/sequencer.h b/sequencer.h
index 719684c8a9..e21835c5a0 100644
--- a/sequencer.h
+++ b/sequencer.h
@@ -44,6 +44,7 @@ struct replay_opts {
 	int record_origin;
 	int no_commit;
 	int signoff;
+	struct strvec trailer_args;
 	int allow_ff;
 	int allow_rerere_auto;
 	int allow_empty;
@@ -82,8 +83,9 @@ struct replay_opts {
 	struct replay_ctx *ctx;
 };
 #define REPLAY_OPTS_INIT {			\
-	.edit = -1,				\
 	.action = -1,				\
+	.edit = -1,				\
+	.trailer_args = STRVEC_INIT, \
 	.xopts = STRVEC_INIT,			\
 	.ctx = replay_ctx_new(),		\
 }
diff --git a/t/meson.build b/t/meson.build
index 401b24e50e..990e8ad4be 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -384,6 +384,7 @@ integration_tests = [
   't3436-rebase-more-options.sh',
   't3437-rebase-fixup-options.sh',
   't3438-rebase-broken-files.sh',
+  't3440-rebase-trailer.sh',
   't3500-cherry.sh',
   't3501-revert-cherry-pick.sh',
   't3502-cherry-pick-merge.sh',
diff --git a/t/t3440-rebase-trailer.sh b/t/t3440-rebase-trailer.sh
new file mode 100755
index 0000000000..b1c7b03330
--- /dev/null
+++ b/t/t3440-rebase-trailer.sh
@@ -0,0 +1,94 @@
+#!/bin/sh
+#
+
+test_description='git rebase --trailer integration tests
+We verify that --trailer works with the merge backend,
+and that it is rejected early when the apply backend is requested.'
+
+GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME=main
+export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
+
+. ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-rebase.sh # test_commit_message, helpers
+
+create_expect() {
+	cat >"$1" <<-EOF
+	$2
+
+	Reviewed-by: Dev <[email-masked]>
+	EOF
+}
+
+test_expect_success 'setup repo with a small history' '
+	git commit --allow-empty -m "Initial empty commit" &&
+	test_commit first file a &&
+	test_commit second file &&
+	git checkout -b conflict-branch first &&
+	test_commit file-2 file-2 &&
+	test_commit conflict file &&
+	test_commit third file &&
+	ident="$GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL>" &&
+	create_expect initial-signed  "Initial empty commit" &&
+	create_expect first-signed    "first"                 &&
+	create_expect second-signed   "second"                &&
+	create_expect file2-signed    "file-2"                &&
+	create_expect third-signed    "third"                 &&
+	create_expect conflict-signed "conflict"
+'
+
+test_expect_success 'apply backend is rejected with --trailer' '
+	head_before=$(git rev-parse HEAD) &&
+	test_expect_code 128 \
+	git rebase --apply --trailer "Reviewed-by: Dev <[email-masked]>" \
+				HEAD^ 2>err &&
+	test_grep "requires the merge backend" err &&
+	test_cmp_rev HEAD $head_before
+'
+
+test_expect_success 'reject empty --trailer argument' '
+	git reset --hard third &&
+	test_expect_code 128 git rebase -m --trailer "" HEAD^ 2>err &&
+	test_grep "empty --trailer" err
+'
+
+test_expect_success 'reject trailer with missing key before separator' '
+	git reset --hard third &&
+	test_expect_code 128 git rebase -m --trailer ": no-key" HEAD^ 2>err &&
+	test_grep "missing key before separator" err
+'
+
+test_expect_success 'CLI trailer duplicates allowed; replace policy keeps last' '
+	git reset --hard third &&
+	git -c trailer.Bug.ifexists=replace -c trailer.Bug.ifmissing=add rebase -m --trailer "Bug: 123" --trailer "Bug: 456" HEAD~1 &&
+	git cat-file commit HEAD | grep "^Bug: 456" &&
+	git cat-file commit HEAD | grep -v "^Bug: 123"
+'
+
+test_expect_success 'multiple Signed-off-by trailers all preserved' '
+	git reset --hard third &&
+	git rebase -m \
+		--trailer "Signed-off-by: Dev A <[email-masked]>" \
+		--trailer "Signed-off-by: Dev B <[email-masked]>" HEAD~1 &&
+	git cat-file commit HEAD | grep -c "^Signed-off-by:" >count &&
+	test "$(cat count)" = 2   # two new commits
+'
+
+test_expect_success 'rebase -m --trailer adds trailer after conflicts' '
+	git reset --hard third &&
+	test_must_fail git rebase -m \
+		--trailer "Reviewed-by: Dev <[email-masked]>" \
+		second third &&
+	git checkout --theirs file &&
+	git add file &&
+	git rebase --continue &&
+	test_commit_message HEAD~2 file2-signed
+'
+
+test_expect_success 'rebase --root --trailer updates every commit' '
+	git checkout first &&
+	git rebase --root --keep-empty \
+		--trailer "Reviewed-by: Dev <[email-masked]>" &&
+	test_commit_message HEAD   first-signed &&
+	test_commit_message HEAD^  initial-signed
+'
+test_done
-- 
2.51.0