Merge branch 'lc/rebase-trailer'

"git rebase" learns "--trailer" command to drive the
interpret-trailers machinery.

* lc/rebase-trailer:
  rebase: support --trailer
  commit, tag: parse --trailer with OPT_STRVEC
  trailer: append trailers without fork/exec
  trailer: libify a couple of functions
  interpret-trailers: refactor create_in_place_tempfile()
  interpret-trailers: factor trailer rewriting

11 files changed, 459 insertions, 87 deletions

diff --git a/Documentation/git-rebase.adoc b/Documentation/git-rebase.adoc
index e177808004..f6c22d1598 100644
--- a/Documentation/git-rebase.adoc
+++ b/Documentation/git-rebase.adoc
@@ -497,6 +497,13 @@ See also INCOMPATIBLE OPTIONS below.
 +
 See also INCOMPATIBLE OPTIONS below.
 
+--trailer=<trailer>::
+	Append the given trailer to every rebased commit message, processed
+	via linkgit:git-interpret-trailers[1]. This option implies
+	`--force-rebase`.
++
+See also INCOMPATIBLE OPTIONS below.
+
 -i::
 --interactive::
 	Make a list of the commits which are about to be rebased.  Let the
@@ -653,6 +660,7 @@ are incompatible with the following options:
  * --[no-]reapply-cherry-picks when used without --keep-base
  * --update-refs
  * --root when used without --onto
+ * --trailer
 
 In addition, the following pairs of options are incompatible:
 
diff --git a/builtin/commit.c b/builtin/commit.c
index 7b23c1f883..02a04f68be 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1720,7 +1720,8 @@ int cmd_commit(int argc,
 		OPT_STRING(0, "fixup", &fixup_message, N_("[(amend|reword):]commit"), N_("use autosquash formatted message to fixup or amend/reword specified commit")),
 		OPT_STRING(0, "squash", &squash_message, N_("commit"), N_("use autosquash formatted message to squash specified commit")),
 		OPT_BOOL(0, "reset-author", &renew_authorship, N_("the commit is authored by me now (used with -C/-c/--amend)")),
-		OPT_PASSTHRU_ARGV(0, "trailer", &trailer_args, N_("trailer"), N_("add custom trailer(s)"), PARSE_OPT_NONEG),
+		OPT_STRVEC(0, "trailer", &trailer_args, N_("trailer"),
+			   N_("add custom trailer(s)")),
 		OPT_BOOL('s', "signoff", &signoff, N_("add a Signed-off-by trailer")),
 		OPT_FILENAME('t', "template", &template_file, N_("use specified template file")),
 		OPT_BOOL('e', "edit", &edit_flag, N_("force edit of commit")),
@@ -1820,6 +1821,9 @@ int cmd_commit(int argc,
 	argc = parse_and_validate_options(argc, argv, builtin_commit_options,
 					  builtin_commit_usage,
 					  prefix, current_head, &s);
+	if (trailer_args.nr)
+		trailer_config_init();
+
 	if (verbose == -1)
 		verbose = (config_commit_verbose < 0) ? 0 : config_commit_verbose;
 
diff --git a/builtin/interpret-trailers.c b/builtin/interpret-trailers.c
index 41b0750e5a..acaf42b2d9 100644
--- a/builtin/interpret-trailers.c
+++ b/builtin/interpret-trailers.c
@@ -93,37 +93,6 @@ static int parse_opt_parse(const struct option *opt, const char *arg,
 	return 0;
 }
 
-static struct tempfile *trailers_tempfile;
-
-static FILE *create_in_place_tempfile(const char *file)
-{
-	struct stat st;
-	struct strbuf filename_template = STRBUF_INIT;
-	const char *tail;
-	FILE *outfile;
-
-	if (stat(file, &st))
-		die_errno(_("could not stat %s"), file);
-	if (!S_ISREG(st.st_mode))
-		die(_("file %s is not a regular file"), file);
-	if (!(st.st_mode & S_IWUSR))
-		die(_("file %s is not writable by user"), file);
-
-	/* Create temporary file in the same directory as the original */
-	tail = strrchr(file, '/');
-	if (tail)
-		strbuf_add(&filename_template, file, tail - file + 1);
-	strbuf_addstr(&filename_template, "git-interpret-trailers-XXXXXX");
-
-	trailers_tempfile = xmks_tempfile_m(filename_template.buf, st.st_mode);
-	strbuf_release(&filename_template);
-	outfile = fdopen_tempfile(trailers_tempfile, "w");
-	if (!outfile)
-		die_errno(_("could not open temporary file"));
-
-	return outfile;
-}
-
 static void read_input_file(struct strbuf *sb, const char *file)
 {
 	if (file) {
@@ -140,55 +109,31 @@ static void interpret_trailers(const struct process_trailer_options *opts,
 			       struct list_head *new_trailer_head,
 			       const char *file)
 {
-	LIST_HEAD(head);
-	struct strbuf sb = STRBUF_INIT;
-	struct strbuf trailer_block_sb = STRBUF_INIT;
-	struct trailer_block *trailer_block;
-	FILE *outfile = stdout;
+	struct strbuf input = STRBUF_INIT;
+	struct strbuf out = STRBUF_INIT;
+	struct tempfile *tempfile = NULL;
+	int fd = 1;
 
 	trailer_config_init();
 
-	read_input_file(&sb, file);
+	read_input_file(&input, file);
 
-	if (opts->in_place)
-		outfile = create_in_place_tempfile(file);
-
-	trailer_block = parse_trailers(opts, sb.buf, &head);
-
-	/* Print the lines before the trailer block */
-	if (!opts->only_trailers)
-		fwrite(sb.buf, 1, trailer_block_start(trailer_block), outfile);
-
-	if (!opts->only_trailers && !blank_line_before_trailer_block(trailer_block))
-		fprintf(outfile, "\n");
-
-
-	if (!opts->only_input) {
-		LIST_HEAD(config_head);
-		LIST_HEAD(arg_head);
-		parse_trailers_from_config(&config_head);
-		parse_trailers_from_command_line_args(&arg_head, new_trailer_head);
-		list_splice(&config_head, &arg_head);
-		process_trailers_lists(&head, &arg_head);
+	if (opts->in_place) {
+		tempfile = trailer_create_in_place_tempfile(file);
+		if (!tempfile)
+			die(NULL);
+		fd = tempfile->fd;
 	}
+	process_trailers(opts, new_trailer_head, &input, &out);
 
-	/* Print trailer block. */
-	format_trailers(opts, &head, &trailer_block_sb);
-	free_trailers(&head);
-	fwrite(trailer_block_sb.buf, 1, trailer_block_sb.len, outfile);
-	strbuf_release(&trailer_block_sb);
-
-	/* Print the lines after the trailer block as is. */
-	if (!opts->only_trailers)
-		fwrite(sb.buf + trailer_block_end(trailer_block), 1,
-		       sb.len - trailer_block_end(trailer_block), outfile);
-	trailer_block_release(trailer_block);
-
+	if (write_in_full(fd, out.buf, out.len) < 0)
+		die_errno(_("could not write to temporary file '%s'"), file);
 	if (opts->in_place)
-		if (rename_tempfile(&trailers_tempfile, file))
+		if (rename_tempfile(&tempfile, file))
 			die_errno(_("could not rename temporary file to %s"), file);
 
-	strbuf_release(&sb);
+	strbuf_release(&input);
+	strbuf_release(&out);
 }
 
 int cmd_interpret_trailers(int argc,
diff --git a/builtin/rebase.c b/builtin/rebase.c
index 8c1316db38..a1c7d78196 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;
@@ -1134,6 +1142,8 @@ 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",
@@ -1287,6 +1297,12 @@ int cmd_rebase(int argc,
 			     builtin_rebase_options,
 			     builtin_rebase_usage, 0);
 
+	if (options.trailer_args.nr) {
+		if (validate_trailer_args(&options.trailer_args))
+			die(NULL);
+		options.flags |= REBASE_FORCE;
+	}
+
 	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"
@@ -1544,6 +1560,9 @@ 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/builtin/tag.c b/builtin/tag.c
index aeb04c487f..e0f05f94fd 100644
--- a/builtin/tag.c
+++ b/builtin/tag.c
@@ -499,8 +499,8 @@ int cmd_tag(int argc,
 		OPT_CALLBACK_F('m', "message", &msg, N_("message"),
 			       N_("tag message"), PARSE_OPT_NONEG, parse_msg_arg),
 		OPT_FILENAME('F', "file", &msgfile, N_("read message from file")),
-		OPT_PASSTHRU_ARGV(0, "trailer", &trailer_args, N_("trailer"),
-				  N_("add custom trailer(s)"), PARSE_OPT_NONEG),
+		OPT_STRVEC(0, "trailer", &trailer_args, N_("trailer"),
+			   N_("add custom trailer(s)")),
 		OPT_BOOL('e', "edit", &edit_flag, N_("force edit of tag message")),
 		OPT_BOOL('s', "sign", &opt.sign, N_("annotated and GPG-signed tag")),
 		OPT_CLEANUP(&cleanup_arg),
@@ -568,6 +568,9 @@ int cmd_tag(int argc,
 	if (cmdmode == 'l')
 		setup_auto_pager("tag", 1);
 
+	if (trailer_args.nr)
+		trailer_config_init();
+
 	if (opt.sign == -1)
 		opt.sign = cmdmode ? 0 : config_sign_tag > 0;
 
diff --git a/sequencer.c b/sequencer.c
index aafd0bc959..e5af49cecd 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -209,6 +209,7 @@ static GIT_PATH_FUNC(rebase_path_reschedule_failed_exec, "rebase-merge/reschedul
 static GIT_PATH_FUNC(rebase_path_no_reschedule_failed_exec, "rebase-merge/no-reschedule-failed-exec")
 static GIT_PATH_FUNC(rebase_path_drop_redundant_commits, "rebase-merge/drop_redundant_commits")
 static GIT_PATH_FUNC(rebase_path_keep_redundant_commits, "rebase-merge/keep_redundant_commits")
+static GIT_PATH_FUNC(rebase_path_trailer, "rebase-merge/trailer")
 
 /*
  * A 'struct replay_ctx' represents the private state of the sequencer.
@@ -420,6 +421,7 @@ 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);
 }
@@ -2027,12 +2029,15 @@ static int append_squash_message(struct strbuf *buf, const char *body,
 	if (is_fixup_flag(command, flag) && !seen_squash(ctx)) {
 		/*
 		 * We're replacing the commit message so we need to
-		 * append the Signed-off-by: trailer if the user
-		 * requested '--signoff'.
+		 * append any trailers if the user requested
+		 * '--signoff' or '--trailer'.
 		 */
 		if (opts->signoff)
 			append_signoff(buf, 0, 0);
 
+		if (opts->trailer_args.nr)
+			amend_strbuf_with_trailers(buf, &opts->trailer_args);
+
 		if ((command == TODO_FIXUP) &&
 		    (flag & TODO_REPLACE_FIXUP_MSG) &&
 		    (file_exists(rebase_path_fixup_msg()) ||
@@ -2451,6 +2456,9 @@ static int do_pick_commit(struct repository *r,
 	if (opts->signoff && !is_fixup(command))
 		append_signoff(&ctx->message, 0, 0);
 
+	if (opts->trailer_args.nr && !is_fixup(command))
+		amend_strbuf_with_trailers(&ctx->message, &opts->trailer_args);
+
 	if (is_rebase_i(opts) && write_author_script(msg.message) < 0)
 		res = -1;
 	else if (!opts->strategy ||
@@ -3180,6 +3188,33 @@ static void read_strategy_opts(struct replay_opts *opts, struct strbuf *buf)
 	parse_strategy_opts(opts, buf->buf);
 }
 
+static int read_trailers(struct replay_opts *opts, struct strbuf *buf)
+{
+	ssize_t len;
+
+	strbuf_reset(buf);
+	len = strbuf_read_file(buf, rebase_path_trailer(), 0);
+	if (len > 0) {
+		char *p = buf->buf, *nl;
+
+		trailer_config_init();
+
+		while ((nl = strchr(p, '\n'))) {
+			*nl = '\0';
+			if (!*p)
+				return error(_("trailers file contains empty line"));
+			strvec_push(&opts->trailer_args, p);
+			p = nl + 1;
+		}
+	} else if (!len) {
+		return error(_("trailers file is empty"));
+	} else if (errno != ENOENT) {
+		return error(_("cannot read trailers files"));
+	}
+
+	return 0;
+}
+
 static int read_populate_opts(struct replay_opts *opts)
 {
 	struct replay_ctx *ctx = opts->ctx;
@@ -3241,6 +3276,11 @@ static int read_populate_opts(struct replay_opts *opts)
 			opts->keep_redundant_commits = 1;
 
 		read_strategy_opts(opts, &buf);
+
+		if (read_trailers(opts, &buf)) {
+			ret = -1;
+			goto done_rebase_i;
+		}
 		strbuf_reset(&buf);
 
 		if (read_oneliner(&ctx->current_fixups,
@@ -3336,6 +3376,14 @@ int write_basic_state(struct replay_opts *opts, const char *head_name,
 		write_file(rebase_path_reschedule_failed_exec(), "%s", "");
 	else
 		write_file(rebase_path_no_reschedule_failed_exec(), "%s", "");
+	if (opts->trailer_args.nr) {
+		struct strbuf buf = STRBUF_INIT;
+
+		for (size_t i = 0; i < opts->trailer_args.nr; i++)
+			strbuf_addf(&buf, "%s\n", opts->trailer_args.v[i]);
+		write_file(rebase_path_trailer(), "%s", buf.buf);
+		strbuf_release(&buf);
+	}
 
 	return 0;
 }
diff --git a/sequencer.h b/sequencer.h
index 719684c8a9..bea20da085 100644
--- a/sequencer.h
+++ b/sequencer.h
@@ -57,6 +57,8 @@ struct replay_opts {
 	int ignore_date;
 	int commit_use_reference;
 
+	struct strvec trailer_args;
+
 	int mainline;
 
 	char *gpg_sign;
@@ -84,6 +86,7 @@ struct replay_opts {
 #define REPLAY_OPTS_INIT {			\
 	.edit = -1,				\
 	.action = -1,				\
+	.trailer_args = STRVEC_INIT,		\
 	.xopts = STRVEC_INIT,			\
 	.ctx = replay_ctx_new(),		\
 }
diff --git a/t/meson.build b/t/meson.build
index 9b2fa4dee8..e539117ca4 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -393,6 +393,7 @@ integration_tests = [
   't3436-rebase-more-options.sh',
   't3437-rebase-fixup-options.sh',
   't3438-rebase-broken-files.sh',
+  't3440-rebase-trailer.sh',
   't3450-history.sh',
   't3451-history-reword.sh',
   't3500-cherry.sh',
diff --git a/t/t3440-rebase-trailer.sh b/t/t3440-rebase-trailer.sh
new file mode 100755
index 0000000000..8b47579566
--- /dev/null
+++ b/t/t3440-rebase-trailer.sh
@@ -0,0 +1,147 @@
+#!/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
+. "$TEST_DIRECTORY"/lib-rebase.sh # test_commit_message, helpers
+
+REVIEWED_BY_TRAILER="Reviewed-by: Dev <dev@example.com>"
+SP=" "
+
+test_expect_success 'setup repo with a small history' '
+	git commit --allow-empty -m "Initial empty commit" &&
+	test_commit first file a &&
+	test_commit second file &&
+	git checkout -b conflict-branch first &&
+	test_commit file-2 file-2 &&
+	test_commit conflict file &&
+	test_commit third file &&
+	git checkout main
+'
+
+test_expect_success 'apply backend is rejected with --trailer' '
+	git checkout -B apply-backend third &&
+	test_expect_code 128 \
+		git rebase --apply --trailer "$REVIEWED_BY_TRAILER" HEAD^ 2>err &&
+	test_grep "fatal: --trailer requires the merge backend" err
+'
+
+test_expect_success 'reject empty --trailer argument' '
+	git checkout -B empty-trailer third &&
+	test_expect_code 128 git rebase --trailer "" HEAD^ 2>err &&
+	test_grep "empty --trailer" err
+'
+
+test_expect_success 'reject trailer with missing key before separator' '
+	git checkout -B missing-key third &&
+	test_expect_code 128 git rebase --trailer ": no-key" HEAD^ 2>err &&
+	test_grep "missing key before separator" err
+'
+
+test_expect_success 'allow trailer with missing value after separator' '
+	git checkout -B missing-value third &&
+	git rebase --trailer "Acked-by:" HEAD^ &&
+	test_commit_message HEAD <<-EOF
+	third
+
+	Acked-by:${SP}
+	EOF
+'
+
+test_expect_success 'CLI trailer duplicates allowed; replace policy keeps last' '
+	git checkout -B replace-policy third &&
+	git -c trailer.Bug.ifexists=replace -c trailer.Bug.ifmissing=add \
+		rebase --trailer "Bug: 123" --trailer "Bug: 456" HEAD^ &&
+	test_commit_message HEAD <<-EOF
+	third
+
+	Bug: 456
+	EOF
+'
+
+test_expect_success 'multiple Signed-off-by trailers all preserved' '
+	git checkout -B multiple-signoff third &&
+	git rebase --trailer "Signed-off-by: Dev A <a@example.com>" \
+		--trailer "Signed-off-by: Dev B <b@example.com>" HEAD^ &&
+	test_commit_message HEAD <<-EOF
+	third
+
+	Signed-off-by: Dev A <a@example.com>
+	Signed-off-by: Dev B <b@example.com>
+	EOF
+'
+
+test_expect_success 'rebase --trailer adds trailer after conflicts' '
+	git checkout -B trailer-conflict third &&
+	test_commit fourth file &&
+	test_must_fail git rebase --trailer "$REVIEWED_BY_TRAILER" second &&
+	git checkout --theirs file &&
+	git add file &&
+	git rebase --continue &&
+	test_commit_message HEAD <<-EOF &&
+	fourth
+
+	$REVIEWED_BY_TRAILER
+	EOF
+	test_commit_message HEAD^ <<-EOF
+	third
+
+	$REVIEWED_BY_TRAILER
+	EOF
+'
+
+test_expect_success '--trailer handles fixup commands in todo list' '
+	git checkout -B fixup-trailer third &&
+	test_commit fixup-base base &&
+	test_commit fixup-second second &&
+	cat >todo <<-\EOF &&
+	pick fixup-base fixup-base
+	fixup fixup-second fixup-second
+	EOF
+	(
+		set_replace_editor todo &&
+		git rebase -i --trailer "$REVIEWED_BY_TRAILER" HEAD~2
+	) &&
+	test_commit_message HEAD <<-EOF &&
+	fixup-base
+
+	$REVIEWED_BY_TRAILER
+	EOF
+	git reset --hard fixup-second &&
+	cat >todo <<-\EOF &&
+	pick fixup-base fixup-base
+	fixup -C fixup-second fixup-second
+	EOF
+	(
+		set_replace_editor todo &&
+		git rebase -i --trailer "$REVIEWED_BY_TRAILER" HEAD~2
+	) &&
+	test_commit_message HEAD <<-EOF
+	fixup-second
+
+	$REVIEWED_BY_TRAILER
+	EOF
+'
+
+test_expect_success 'rebase --root honors trailer.<name>.key' '
+	git checkout -B root-trailer first &&
+	git -c trailer.review.key=Reviewed-by rebase --root \
+		--trailer=review="Dev <dev@example.com>" &&
+	test_commit_message HEAD <<-EOF &&
+	first
+
+	Reviewed-by: Dev <dev@example.com>
+	EOF
+	test_commit_message HEAD^ <<-EOF
+	Initial empty commit
+
+	Reviewed-by: Dev <dev@example.com>
+	EOF
+'
+test_done
diff --git a/trailer.c b/trailer.c
index 911a81ed99..ca8abd1882 100644
--- a/trailer.c
+++ b/trailer.c
@@ -7,8 +7,11 @@
 #include "string-list.h"
 #include "run-command.h"
 #include "commit.h"
+#include "strvec.h"
 #include "trailer.h"
 #include "list.h"
+#include "tempfile.h"
+
 /*
  * Copyright (c) 2013, 2014 Christian Couder <chriscool@tuxfamily.org>
  */
@@ -772,6 +775,35 @@ void parse_trailers_from_command_line_args(struct list_head *arg_head,
 	free(cl_separators);
 }
 
+int validate_trailer_args(const struct strvec *cli_args)
+{
+	char *cl_separators;
+	int ret = 0;
+
+	trailer_config_init();
+
+	cl_separators = xstrfmt("=%s", separators);
+
+	for (size_t i = 0; i < cli_args->nr; i++) {
+		const char *txt = cli_args->v[i];
+		ssize_t separator_pos;
+
+		if (!*txt) {
+			ret = error(_("empty --trailer argument"));
+			goto out;
+		}
+		separator_pos = find_separator(txt, cl_separators);
+		if (separator_pos == 0) {
+			ret = error(_("invalid trailer '%s': missing key before separator"),
+				    txt);
+			goto out;
+		}
+	}
+out:
+	free(cl_separators);
+	return ret;
+}
+
 static const char *next_line(const char *str)
 {
 	const char *nl = strchrnul(str, '\n');
@@ -1224,14 +1256,146 @@ void trailer_iterator_release(struct trailer_iterator *iter)
 	strbuf_release(&iter->key);
 }
 
-int amend_file_with_trailers(const char *path, const struct strvec *trailer_args)
+struct tempfile *trailer_create_in_place_tempfile(const char *file)
+{
+	struct tempfile *tempfile = NULL;
+	struct stat st;
+	struct strbuf filename_template = STRBUF_INIT;
+	const char *tail;
+
+	if (stat(file, &st)) {
+		error_errno(_("could not stat %s"), file);
+		return NULL;
+	}
+	if (!S_ISREG(st.st_mode)) {
+		error(_("file %s is not a regular file"), file);
+		return NULL;
+	}
+	if (!(st.st_mode & S_IWUSR)) {
+		error(_("file %s is not writable by user"), file);
+		return NULL;
+	}
+	/* Create temporary file in the same directory as the original */
+	tail = find_last_dir_sep(file);
+	if (tail)
+		strbuf_add(&filename_template, file, tail - file + 1);
+	strbuf_addstr(&filename_template, "git-interpret-trailers-XXXXXX");
+
+	tempfile = mks_tempfile_m(filename_template.buf, st.st_mode);
+
+	strbuf_release(&filename_template);
+
+	return tempfile;
+}
+
+int amend_strbuf_with_trailers(struct strbuf *buf,
+				const struct strvec *trailer_args)
 {
-	struct child_process run_trailer = CHILD_PROCESS_INIT;
+	struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
+	LIST_HEAD(new_trailer_head);
+	struct strbuf out = STRBUF_INIT;
+	size_t i;
+	int ret = 0;
+
+	opts.no_divider = 1;
+
+	for (i = 0; i < trailer_args->nr; i++) {
+		const char *text = trailer_args->v[i];
+		struct new_trailer_item *item;
+
+		if (!*text) {
+			ret = error(_("empty --trailer argument"));
+			goto out;
+		}
+		item = xcalloc(1, sizeof(*item));
+		item->text = xstrdup(text);
+		list_add_tail(&item->list, &new_trailer_head);
+	}
 
-	run_trailer.git_cmd = 1;
-	strvec_pushl(&run_trailer.args, "interpret-trailers",
-		     "--in-place", "--no-divider",
-		     path, NULL);
-	strvec_pushv(&run_trailer.args, trailer_args->v);
-	return run_command(&run_trailer);
+	process_trailers(&opts, &new_trailer_head, buf, &out);
+
+	strbuf_swap(buf, &out);
+out:
+	strbuf_release(&out);
+	free_trailers(&new_trailer_head);
+
+	return ret;
+}
+
+static int write_file_in_place(const char *path, const struct strbuf *buf)
+{
+	struct tempfile *tempfile = trailer_create_in_place_tempfile(path);
+	if (!tempfile)
+		return -1;
+
+	if (write_in_full(tempfile->fd, buf->buf, buf->len) < 0)
+		return error_errno(_("could not write to temporary file"));
+
+	if (rename_tempfile(&tempfile, path))
+		return error_errno(_("could not rename temporary file to %s"), path);
+
+	return 0;
+}
+
+int amend_file_with_trailers(const char *path,
+			     const struct strvec *trailer_args)
+{
+	struct strbuf buf = STRBUF_INIT;
+	int ret = 0;
+
+	if (!trailer_args)
+		BUG("amend_file_with_trailers called with NULL trailer_args");
+	if (!trailer_args->nr)
+		return 0;
+
+	if (validate_trailer_args(trailer_args)) {
+		ret = -1;
+		goto out;
+	}
+	if (strbuf_read_file(&buf, path, 0) < 0)
+		ret = error_errno(_("could not read '%s'"), path);
+	else
+		amend_strbuf_with_trailers(&buf, trailer_args);
+
+	if (!ret)
+		ret = write_file_in_place(path, &buf);
+out:
+	strbuf_release(&buf);
+	return ret;
+}
+
+void process_trailers(const struct process_trailer_options *opts,
+		      struct list_head *new_trailer_head,
+		      struct strbuf *input, struct strbuf *out)
+{
+	LIST_HEAD(head);
+	struct trailer_block *trailer_block;
+
+	trailer_block = parse_trailers(opts, input->buf, &head);
+
+	/* Print the lines before the trailer block */
+	if (!opts->only_trailers)
+		strbuf_add(out, input->buf, trailer_block_start(trailer_block));
+
+	if (!opts->only_trailers && !blank_line_before_trailer_block(trailer_block))
+		strbuf_addch(out, '\n');
+
+	if (!opts->only_input) {
+		LIST_HEAD(config_head);
+		LIST_HEAD(arg_head);
+		parse_trailers_from_config(&config_head);
+		parse_trailers_from_command_line_args(&arg_head, new_trailer_head);
+		list_splice(&config_head, &arg_head);
+		process_trailers_lists(&head, &arg_head);
+	}
+
+	/* Print trailer block. */
+	format_trailers(opts, &head, out);
+	free_trailers(&head);
+
+	/* Print the lines after the trailer block as is. */
+	if (!opts->only_trailers)
+		strbuf_add(out, input->buf + trailer_block_end(trailer_block),
+			   input->len - trailer_block_end(trailer_block));
+	trailer_block_release(trailer_block);
 }
diff --git a/trailer.h b/trailer.h
index 4740549586..b49338858c 100644
--- a/trailer.h
+++ b/trailer.h
@@ -68,6 +68,8 @@ void parse_trailers_from_config(struct list_head *config_head);
 void parse_trailers_from_command_line_args(struct list_head *arg_head,
 					   struct list_head *new_trailer_head);
 
+int validate_trailer_args(const struct strvec *cli_args);
+
 void process_trailers_lists(struct list_head *head,
 			    struct list_head *arg_head);
 
@@ -196,10 +198,38 @@ int trailer_iterator_advance(struct trailer_iterator *iter);
 void trailer_iterator_release(struct trailer_iterator *iter);
 
 /*
- * Augment a file to add trailers to it by running git-interpret-trailers.
- * This calls run_command() and its return value is the same (i.e. 0 for
- * success, various non-zero for other errors). See run-command.h.
+ * Append trailers specified in trailer_args to buf in-place.
+ *
+ * Each element of trailer_args should be in the same format as the value
+ * accepted by --trailer=<trailer> (i.e., without the --trailer= prefix).
+ */
+int amend_strbuf_with_trailers(struct strbuf *buf,
+				const struct strvec *trailer_args);
+
+/*
+ * Augment a file by appending trailers specified in trailer_args.
+ *
+ * Each element of trailer_args should be in the same format as the value
+ * accepted by --trailer=<trailer> (i.e., without the --trailer= prefix).
+ *
+ * Returns 0 on success or a non-zero error code on failure.
  */
 int amend_file_with_trailers(const char *path, const struct strvec *trailer_args);
 
+/*
+ * Create a tempfile ""git-interpret-trailers-XXXXXX" in the same
+ * directory as file.
+ */
+struct tempfile *trailer_create_in_place_tempfile(const char *file);
+
+/*
+ * Rewrite the contents of input by processing its trailer block according to
+ * opts and (optionally) appending trailers from new_trailer_head.
+ *
+ * The rewritten message is appended to out (callers should strbuf_reset()
+ * first if needed).
+ */
+void process_trailers(const struct process_trailer_options *opts,
+		      struct list_head *new_trailer_head,
+		      struct strbuf *input, struct strbuf *out);
 #endif /* TRAILER_H */