v5, 29 partsrebase: support --trailer

builtin/interpret-trailers.c, trailer.c, trailer.h

diff --git a/builtin/interpret-trailers.c b/builtin/interpret-trailers.c
index 41b0750e5a..be0fa83f79 100644
--- a/builtin/interpret-trailers.c
+++ b/builtin/interpret-trailers.c
@@ -10,7 +10,6 @@
 #include "gettext.h"
 #include "parse-options.h"
 #include "string-list.h"
-#include "tempfile.h"
 #include "trailer.h"
 #include "config.h"
 
@@ -93,37 +92,6 @@ 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) {
@@ -136,61 +104,6 @@ static void read_input_file(struct strbuf *sb, const char *file)
 	strbuf_complete_line(sb);
 }
 
-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;
-
-	trailer_config_init();
-
-	read_input_file(&sb, 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);
-	}
-
-	/* 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 (opts->in_place)
-		if (rename_tempfile(&trailers_tempfile, file))
-			die_errno(_("could not rename temporary file to %s"), file);
-
-	strbuf_release(&sb);
-}
-
 int cmd_interpret_trailers(int argc,
 			   const char **argv,
 			   const char *prefix,
@@ -232,14 +145,37 @@ int cmd_interpret_trailers(int argc,
 			git_interpret_trailers_usage,
 			options);
 
+	trailer_config_init();
+
 	if (argc) {
 		int i;
-		for (i = 0; i < argc; i++)
-			interpret_trailers(&opts, &trailers, argv[i]);
+		for (i = 0; i < argc; i++) {
+			struct strbuf in_buf = STRBUF_INIT;
+			struct strbuf out_buf = STRBUF_INIT;
+
+			read_input_file(&in_buf, argv[i]);
+			if (trailer_process(&opts, in_buf.buf, &trailers, &out_buf) < 0)
+				die(_("failed to process trailers for %s"), argv[i]);
+			if (opts.in_place)
+				write_file_buf(argv[i], out_buf.buf, out_buf.len);
+			else
+				fwrite(out_buf.buf, 1, out_buf.len, stdout);
+			strbuf_release(&in_buf);
+			strbuf_release(&out_buf);
+		}
 	} else {
+		struct strbuf in_buf = STRBUF_INIT;
+		struct strbuf out_buf = STRBUF_INIT;
+
 		if (opts.in_place)
 			die(_("no input file given for in-place editing"));
-		interpret_trailers(&opts, &trailers, NULL);
+
+		read_input_file(&in_buf, NULL);
+		if (trailer_process(&opts, in_buf.buf, &trailers, &out_buf) < 0)
+			die(_("failed to process trailers"));
+		fwrite(out_buf.buf, 1, out_buf.len, stdout);
+		strbuf_release(&in_buf);
+		strbuf_release(&out_buf);
 	}
 
 	new_trailers_clear(&trailers);
diff --git a/trailer.c b/trailer.c
index 911a81ed99..2fe49df23a 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1224,14 +1224,121 @@ 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)
+static int amend_strbuf_with_trailers(struct strbuf *buf,
+									  const struct strvec *trailer_args)
 {
-	struct child_process run_trailer = CHILD_PROCESS_INIT;
-
-	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);
+	struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
+	LIST_HEAD(new_trailer_head);
+	struct strbuf out = STRBUF_INIT;
+	size_t i;
+
+	opts.no_divider = 1;
+
+	for (i = 0; i < trailer_args->nr; i++) {
+		const char *arg = trailer_args->v[i];
+		const char *text;
+		struct new_trailer_item *item;
+
+		if (!skip_prefix(arg, "--trailer=", &text))
+			text = arg;
+		if (!*text)
+			continue;
+		item = xcalloc(1, sizeof(*item));
+		INIT_LIST_HEAD(&item->list);
+		item->text = text;
+		list_add_tail(&item->list, &new_trailer_head);
+	}
+	if (trailer_process(&opts, buf->buf, &new_trailer_head, &out) < 0)
+		return -1;
+	strbuf_swap(buf, &out);
+	strbuf_release(&out);
+	while (!list_empty(&new_trailer_head)) {
+		struct new_trailer_item *item =
+			list_first_entry(&new_trailer_head, struct new_trailer_item, list);
+		list_del(&item->list);
+		free(item);
+	}
+	return 0;
+}
+
+int trailer_process(const struct process_trailer_options *opts,
+		    const char *msg,
+		    struct list_head *new_trailer_head,
+		    struct strbuf *out)
+{
+	struct trailer_block *blk;
+	LIST_HEAD(orig_head);
+	LIST_HEAD(config_head);
+	LIST_HEAD(arg_head);
+	struct strbuf trailers_sb = STRBUF_INIT;
+	int had_trailer_before;
+
+	blk = parse_trailers(opts, msg, &orig_head);
+	had_trailer_before = !list_empty(&orig_head);
+	if (!opts->only_input) {
+		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(&orig_head, &arg_head);
+	}
+	format_trailers(opts, &orig_head, &trailers_sb);
+	if (!opts->only_trailers && !opts->only_input && !opts->unfold &&
+	    !opts->trim_empty && list_empty(&orig_head) &&
+	    (list_empty(new_trailer_head) || opts->only_input)) {
+		size_t split = trailer_block_start(blk); /* end-of-log-msg */
+		if (!blank_line_before_trailer_block(blk)) {
+			strbuf_add(out, msg, split);
+			strbuf_addch(out, '\n');
+			strbuf_addstr(out, msg + split);
+		} else
+			strbuf_addstr(out, msg);
+
+		strbuf_release(&trailers_sb);
+		trailer_block_release(blk);
+		return 0;
+	}
+	if (opts->only_trailers) {
+		strbuf_addbuf(out, &trailers_sb);
+	} else if (had_trailer_before) {
+		strbuf_add(out, msg, trailer_block_start(blk));
+		if (!blank_line_before_trailer_block(blk))
+			strbuf_addch(out, '\n');
+		strbuf_addbuf(out, &trailers_sb);
+		strbuf_add(out, msg + trailer_block_end(blk),
+			   strlen(msg) - trailer_block_end(blk));
+	} else {
+		size_t cpos = trailer_block_start(blk);
+		strbuf_add(out, msg, cpos);
+		if (cpos == 0) /* empty body → just one \n */
+			strbuf_addch(out, '\n');
+		else if (!blank_line_before_trailer_block(blk))
+			strbuf_addch(out, '\n'); /* body without trailing blank */
+
+		strbuf_addbuf(out, &trailers_sb);
+		strbuf_add(out, msg + cpos, strlen(msg) - cpos);
+	}
+	strbuf_release(&trailers_sb);
+	free_trailers(&orig_head);
+	trailer_block_release(blk);
+	return 0;
+}
+
+int amend_file_with_trailers(const char *path,
+			     const struct strvec *trailer_args)
+{
+	struct strbuf buf = STRBUF_INIT;
+
+	if (!trailer_args || !trailer_args->nr)
+		return 0;
+
+	if (strbuf_read_file(&buf, path, 0) < 0)
+		return error_errno("could not read '%s'", path);
+
+	if (amend_strbuf_with_trailers(&buf, trailer_args))
+		die("failed to append trailers");
+
+	/* `write_file_buf()` aborts on error internally */
+	write_file_buf(path, buf.buf, buf.len);
+	strbuf_release(&buf);
+	return 0;
 }
diff --git a/trailer.h b/trailer.h
index 4740549586..b4f28bfd65 100644
--- a/trailer.h
+++ b/trailer.h
@@ -196,10 +196,22 @@ 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.
+ * Augment a file to add trailers to it (similar to 'git interpret-trailers').
+ * 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);
 
+/*
+ * Process trailer lines for a commit message in-memory.
+ * @opts: trailer processing options (e.g. from parse-options)
+ * @msg: the input message string
+ * @new_trailer_head: list of new trailers to add (struct new_trailer_item)
+ * @out: strbuf to store the resulting message (must be initialized)
+ *
+ * Returns 0 on success, <0 on error.
+ */
+int trailer_process(const struct process_trailer_options *opts,
+		    const char *msg,
+		    struct list_head *new_trailer_head,
+		    struct strbuf *out);
 #endif /* TRAILER_H */

diff --git a/builtin/interpret-trailers.c b/builtin/interpret-trailers.c
index 41b0750e5af..4c90580ffff 100644
--- a/builtin/interpret-trailers.c
+++ b/builtin/interpret-trailers.c
@@ -136,32 +136,21 @@ static void read_input_file(struct strbuf *sb, const char *file)
  	strbuf_complete_line(sb);
  }
  
-static void interpret_trailers(const struct process_trailer_options *opts,
-			       struct list_head *new_trailer_head,
-			       const char *file)
+static void process_trailers(const struct process_trailer_options *opts,
+			     struct list_head *new_trailer_head,
+			     struct strbuf *sb, struct strbuf *out)
  {
  	LIST_HEAD(head);
-	struct strbuf sb = STRBUF_INIT;
-	struct strbuf trailer_block_sb = STRBUF_INIT;
  	struct trailer_block *trailer_block;
-	FILE *outfile = stdout;
-
-	trailer_config_init();
-
-	read_input_file(&sb, file);
-
-	if (opts->in_place)
-		outfile = create_in_place_tempfile(file);
-
-	trailer_block = parse_trailers(opts, sb.buf, &head);
+
+	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);
+		strbuf_add(out, sb->buf, trailer_block_start(trailer_block));
  
  	if (!opts->only_trailers && !blank_line_before_trailer_block(trailer_block))
-		fprintf(outfile, "\n");
-
+		strbuf_addch(out, '\n');
  
  	if (!opts->only_input) {
  		LIST_HEAD(config_head);
@@ -173,22 +162,40 @@ static void interpret_trailers(const struct process_trailer_options *opts,
  	}
  
  	/* Print trailer block. */
-	format_trailers(opts, &head, &trailer_block_sb);
+	format_trailers(opts, &head, out);
  	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);
+		strbuf_add(out, sb->buf + trailer_block_end(trailer_block),
+			   sb->len - trailer_block_end(trailer_block));
  	trailer_block_release(trailer_block);
-
+}
+
+static void interpret_trailers(const struct process_trailer_options *opts,
+			       struct list_head *new_trailer_head,
+			       const char *file)
+{
+	struct strbuf sb = STRBUF_INIT;
+	struct strbuf out = STRBUF_INIT;
+	FILE *outfile = stdout;
+
+	trailer_config_init();
+
+	read_input_file(&sb, file);
+
+	if (opts->in_place)
+		outfile = create_in_place_tempfile(file);
+
+	process_trailers(opts, new_trailer_head, &sb, &out);
+
+	fwrite(out.buf, out.len, 1, outfile);
  	if (opts->in_place)
  		if (rename_tempfile(&trailers_tempfile, file))
  			die_errno(_("could not rename temporary file to %s"), file);
  
  	strbuf_release(&sb);
+	strbuf_release(&out);
  }
  
  int cmd_interpret_trailers(int argc,

diff --git a/builtin/interpret-trailers.c b/builtin/interpret-trailers.c
index be0fa83f79..2c8b6fc3b9 100644
--- a/builtin/interpret-trailers.c
+++ b/builtin/interpret-trailers.c
@@ -104,6 +104,30 @@ static void read_input_file(struct strbuf *sb, const char *file)
 	strbuf_complete_line(sb);
 }
 
+static void interpret_trailers(const struct process_trailer_options *opts,
+			       struct list_head *new_trailer_head,
+			       const char *file)
+{
+	struct strbuf in_buf = STRBUF_INIT;
+	struct strbuf out_buf = STRBUF_INIT;
+
+	read_input_file(&in_buf, file);
+	if (trailer_process(opts, in_buf.buf, new_trailer_head, &out_buf) < 0) {
+		if (file)
+			die(_("failed to process trailers for %s"), file);
+		else
+			die(_("failed to process trailers"));
+	}
+
+	if (opts->in_place)
+		write_file_buf(file, out_buf.buf, out_buf.len);
+	else
+		fwrite(out_buf.buf, 1, out_buf.len, stdout);
+
+	strbuf_release(&in_buf);
+	strbuf_release(&out_buf);
+}
+
 int cmd_interpret_trailers(int argc,
 			   const char **argv,
 			   const char *prefix,
@@ -149,33 +173,13 @@ int cmd_interpret_trailers(int argc,
 
 	if (argc) {
 		int i;
-		for (i = 0; i < argc; i++) {
-			struct strbuf in_buf = STRBUF_INIT;
-			struct strbuf out_buf = STRBUF_INIT;
-
-			read_input_file(&in_buf, argv[i]);
-			if (trailer_process(&opts, in_buf.buf, &trailers, &out_buf) < 0)
-				die(_("failed to process trailers for %s"), argv[i]);
-			if (opts.in_place)
-				write_file_buf(argv[i], out_buf.buf, out_buf.len);
-			else
-				fwrite(out_buf.buf, 1, out_buf.len, stdout);
-			strbuf_release(&in_buf);
-			strbuf_release(&out_buf);
-		}
+		for (i = 0; i < argc; i++)
+			interpret_trailers(&opts, &trailers, argv[i]);
 	} else {
-		struct strbuf in_buf = STRBUF_INIT;
-		struct strbuf out_buf = STRBUF_INIT;
-
 		if (opts.in_place)
 			die(_("no input file given for in-place editing"));
 
-		read_input_file(&in_buf, NULL);
-		if (trailer_process(&opts, in_buf.buf, &trailers, &out_buf) < 0)
-			die(_("failed to process trailers"));
-		fwrite(out_buf.buf, 1, out_buf.len, stdout);
-		strbuf_release(&in_buf);
-		strbuf_release(&out_buf);
+		interpret_trailers(&opts, &trailers, NULL);
 	}
 
 	new_trailers_clear(&trailers);

builtin/commit.c, builtin/tag.c, trailer.c

diff --git a/builtin/commit.c b/builtin/commit.c
index 0243f17d53..67070d6a54 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1719,7 +1719,7 @@ 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_CALLBACK_F(0, "trailer", &trailer_args, N_("trailer"), N_("add custom trailer(s)"), PARSE_OPT_NONEG, parse_opt_strvec),
 		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")),
diff --git a/builtin/tag.c b/builtin/tag.c
index f0665af3ac..65c4a0b36b 100644
--- a/builtin/tag.c
+++ b/builtin/tag.c
@@ -499,8 +499,7 @@ 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_CALLBACK_F(0, "trailer", &trailer_args, N_("trailer"), N_("add custom trailer(s)"), PARSE_OPT_NONEG, parse_opt_strvec),
 		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),
diff --git a/trailer.c b/trailer.c
index 2fe49df23a..b7b0029e05 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1235,12 +1235,9 @@ static int amend_strbuf_with_trailers(struct strbuf *buf,
 	opts.no_divider = 1;
 
 	for (i = 0; i < trailer_args->nr; i++) {
-		const char *arg = trailer_args->v[i];
-		const char *text;
+		const char *text = trailer_args->v[i];
 		struct new_trailer_item *item;
 
-		if (!skip_prefix(arg, "--trailer=", &text))
-			text = arg;
 		if (!*text)
 			continue;
 		item = xcalloc(1, sizeof(*item));

diff --git a/trailer.c b/trailer.c
index b7b0029e05..9abb5a522a 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1265,14 +1265,15 @@ int trailer_process(const struct process_trailer_options *opts,
 {
 	struct trailer_block *blk;
 	LIST_HEAD(orig_head);
-	LIST_HEAD(config_head);
-	LIST_HEAD(arg_head);
 	struct strbuf trailers_sb = STRBUF_INIT;
 	int had_trailer_before;
 
 	blk = parse_trailers(opts, msg, &orig_head);
 	had_trailer_before = !list_empty(&orig_head);
 	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);

diff --git a/trailer.c b/trailer.c
index 9abb5a522a..a448380327 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1266,7 +1266,7 @@ int trailer_process(const struct process_trailer_options *opts,
 	struct trailer_block *blk;
 	LIST_HEAD(orig_head);
 	struct strbuf trailers_sb = STRBUF_INIT;
-	int had_trailer_before;
+	bool had_trailer_before;
 
 	blk = parse_trailers(opts, msg, &orig_head);
 	had_trailer_before = !list_empty(&orig_head);

diff --git a/builtin/interpret-trailers.c b/builtin/interpret-trailers.c
index 2c8b6fc3b9..cdf39dbca8 100644
--- a/builtin/interpret-trailers.c
+++ b/builtin/interpret-trailers.c
@@ -122,7 +122,7 @@ static void interpret_trailers(const struct process_trailer_options *opts,
 	if (opts->in_place)
 		write_file_buf(file, out_buf.buf, out_buf.len);
 	else
-		fwrite(out_buf.buf, 1, out_buf.len, stdout);
+		strbuf_write(&out_buf, stdout);
 
 	strbuf_release(&in_buf);
 	strbuf_release(&out_buf);

diff --git a/trailer.c b/trailer.c
index a448380327..ac6ac2ac20 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1266,10 +1266,8 @@ int trailer_process(const struct process_trailer_options *opts,
 	struct trailer_block *blk;
 	LIST_HEAD(orig_head);
 	struct strbuf trailers_sb = STRBUF_INIT;
-	bool had_trailer_before;
 
 	blk = parse_trailers(opts, msg, &orig_head);
-	had_trailer_before = !list_empty(&orig_head);
 	if (!opts->only_input) {
 		LIST_HEAD(config_head);
 		LIST_HEAD(arg_head);
@@ -1280,40 +1278,18 @@ int trailer_process(const struct process_trailer_options *opts,
 		process_trailers_lists(&orig_head, &arg_head);
 	}
 	format_trailers(opts, &orig_head, &trailers_sb);
-	if (!opts->only_trailers && !opts->only_input && !opts->unfold &&
-	    !opts->trim_empty && list_empty(&orig_head) &&
-	    (list_empty(new_trailer_head) || opts->only_input)) {
-		size_t split = trailer_block_start(blk); /* end-of-log-msg */
-		if (!blank_line_before_trailer_block(blk)) {
-			strbuf_add(out, msg, split);
-			strbuf_addch(out, '\n');
-			strbuf_addstr(out, msg + split);
-		} else
-			strbuf_addstr(out, msg);
-
-		strbuf_release(&trailers_sb);
-		trailer_block_release(blk);
-		return 0;
-	}
 	if (opts->only_trailers) {
 		strbuf_addbuf(out, &trailers_sb);
-	} else if (had_trailer_before) {
-		strbuf_add(out, msg, trailer_block_start(blk));
-		if (!blank_line_before_trailer_block(blk))
-			strbuf_addch(out, '\n');
-		strbuf_addbuf(out, &trailers_sb);
-		strbuf_add(out, msg + trailer_block_end(blk),
-			   strlen(msg) - trailer_block_end(blk));
 	} else {
-		size_t cpos = trailer_block_start(blk);
-		strbuf_add(out, msg, cpos);
-		if (cpos == 0) /* empty body → just one \n */
-			strbuf_addch(out, '\n');
-		else if (!blank_line_before_trailer_block(blk))
-			strbuf_addch(out, '\n'); /* body without trailing blank */
+		size_t block_start = trailer_block_start(blk);
+		size_t block_end = trailer_block_end(blk);
+		bool need_blank_line = !blank_line_before_trailer_block(blk);
 
+		strbuf_add(out, msg, block_start);
+		if (need_blank_line)
+			strbuf_addch(out, '\n');
 		strbuf_addbuf(out, &trailers_sb);
-		strbuf_add(out, msg + cpos, strlen(msg) - cpos);
+		strbuf_add(out, msg + block_end, strlen(msg) - block_end);
 	}
 	strbuf_release(&trailers_sb);
 	free_trailers(&orig_head);

trailer.c, wrapper.c, wrapper.h

diff --git a/trailer.c b/trailer.c
index ac6ac2ac20..1f317f4d37 100644
--- a/trailer.c
+++ b/trailer.c
@@ -9,6 +9,7 @@
 #include "commit.h"
 #include "trailer.h"
 #include "list.h"
+#include "wrapper.h"
 /*
  * Copyright (c) 2013, 2014 Christian Couder <chriscool@tuxfamily.org>
  */
@@ -1308,11 +1309,16 @@ int amend_file_with_trailers(const char *path,
 	if (strbuf_read_file(&buf, path, 0) < 0)
 		return error_errno("could not read '%s'", path);
 
-	if (amend_strbuf_with_trailers(&buf, trailer_args))
-		die("failed to append trailers");
+	if (amend_strbuf_with_trailers(&buf, trailer_args)) {
+		strbuf_release(&buf);
+		return error("failed to append trailers");
+	}
+
+	if (write_file_buf_gently(path, buf.buf, buf.len)) {
+		strbuf_release(&buf);
+		return -1;
+	}
 
-	/* `write_file_buf()` aborts on error internally */
-	write_file_buf(path, buf.buf, buf.len);
 	strbuf_release(&buf);
 	return 0;
 }
diff --git a/wrapper.c b/wrapper.c
index 3d507d4204..1f12dbb2fa 100644
--- a/wrapper.c
+++ b/wrapper.c
@@ -688,6 +688,22 @@ void write_file_buf(const char *path, const char *buf, size_t len)
 		die_errno(_("could not close '%s'"), path);
 }
 
+int write_file_buf_gently(const char *path, const char *buf, size_t len)
+{
+	int fd = open(path, O_WRONLY | O_CREAT | O_TRUNC, 0666);
+
+	if (fd < 0)
+		return error_errno(_("could not open '%s'"), path);
+	if (write_in_full(fd, buf, len) < 0) {
+		int ret = error_errno(_("could not write to '%s'"), path);
+		close(fd);
+		return ret;
+	}
+	if (close(fd))
+		return error_errno(_("could not close '%s'"), path);
+	return 0;
+}
+
 void write_file(const char *path, const char *fmt, ...)
 {
 	va_list params;
diff --git a/wrapper.h b/wrapper.h
index 44a8597ac3..e5f867b200 100644
--- a/wrapper.h
+++ b/wrapper.h
@@ -56,6 +56,12 @@ static inline ssize_t write_str_in_full(int fd, const char *str)
  */
 void write_file_buf(const char *path, const char *buf, size_t len);
 
+/**
+ * Like write_file_buf(), but report errors instead of exiting. Returns 0 on
+ * success or a negative value on error after emitting a message.
+ */
+int write_file_buf_gently(const char *path, const char *buf, size_t len);
+
 /**
  * Like write_file_buf(), but format the contents into a buffer first.
  * Additionally, write_file() will append a newline if one is not already

Documentation/git-rebase.adoc, builtin/rebase.c, sequencer.c, sequencer.h, t/meson.build, 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
+	$2
+
+	Reviewed-by: Dev <dev@example.com>
+	EOF
+}
+
+test_expect_success 'setup repo with a small history' '
+	git commit --allow-empty -m "Initial empty commit" &&
+	test_commit first file a &&
+	test_commit second file &&
+	git checkout -b conflict-branch first &&
+	test_commit file-2 file-2 &&
+	test_commit conflict file &&
+	test_commit third file &&
+	ident="$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 <dev@example.com>" \
+				HEAD^ 2>err &&
+	test_grep "requires the merge backend" err &&
+	test_cmp_rev HEAD $head_before
+'
+
+test_expect_success 'reject empty --trailer argument' '
+	git reset --hard third &&
+	test_expect_code 128 git rebase -m --trailer "" HEAD^ 2>err &&
+	test_grep "empty --trailer" err
+'
+
+test_expect_success 'reject trailer with missing key before separator' '
+	git reset --hard third &&
+	test_expect_code 128 git rebase -m --trailer ": no-key" HEAD^ 2>err &&
+	test_grep "missing key before separator" err
+'
+
+test_expect_success 'CLI trailer duplicates allowed; replace policy keeps last' '
+	git reset --hard third &&
+	git -c trailer.Bug.ifexists=replace -c trailer.Bug.ifmissing=add rebase -m --trailer "Bug: 123" --trailer "Bug: 456" HEAD~1 &&
+	git cat-file commit HEAD | grep "^Bug: 456" &&
+	git cat-file commit HEAD | grep -v "^Bug: 123"
+'
+
+test_expect_success 'multiple Signed-off-by trailers all preserved' '
+	git reset --hard third &&
+	git rebase -m \
+		--trailer "Signed-off-by: Dev A <a@ex.com>" \
+		--trailer "Signed-off-by: Dev B <b@ex.com>" HEAD~1 &&
+	git cat-file commit HEAD | grep -c "^Signed-off-by:" >count &&
+	test "$(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 <dev@example.com>" \
+		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 <dev@example.com>" &&
+	test_commit_message HEAD   first-signed &&
+	test_commit_message HEAD^  initial-signed
+'
+test_done

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 3db1439b52..b0f547ef2b 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -442,8 +442,6 @@ 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) ||
@@ -512,7 +510,7 @@ static int read_basic_state(struct rebase_options *opts)
 
 	strbuf_release(&buf);
 
-	if (strbuf_read_file(&buf, path, 0) >= 0) {
+	if (strbuf_read_file(&buf, state_dir_path("trailer", opts), 0) >= 0) {
 		const char *p = buf.buf, *end = buf.buf + buf.len;
 
 		while (p < end) {
@@ -535,8 +533,6 @@ static int read_basic_state(struct rebase_options *opts)
 
 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",
@@ -568,7 +564,7 @@ static int rebase_write_basic_state(struct rebase_options *opts)
 				strbuf_addstr(&buf, opts->trailer_args.v[i]);
 				strbuf_addch(&buf, '\n');
 		}
-		write_file(state_dir_path(trailer_state_name, opts),
+		write_file(state_dir_path("trailer", opts),
 				   "%s", buf.buf);
 		strbuf_release(&buf);
 	}

diff --git a/builtin/rebase.c b/builtin/rebase.c
index b0f547ef2b..89ecb84a31 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -508,7 +508,7 @@ static int read_basic_state(struct rebase_options *opts)
 		opts->gpg_sign_opt = xstrdup(buf.buf);
 	}
 
-	strbuf_release(&buf);
+	strbuf_reset(&buf);
 
 	if (strbuf_read_file(&buf, state_dir_path("trailer", opts), 0) >= 0) {
 		const char *p = buf.buf, *end = buf.buf + buf.len;

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 89ecb84a31..a950005dfc 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -524,7 +524,6 @@ static int read_basic_state(struct rebase_options *opts)
 
 			p = nl + 1;
 		}
-		strbuf_release(&buf);
 	}
 	strbuf_release(&buf);
 

diff --git a/builtin/rebase.c b/builtin/rebase.c
index a950005dfc..3ac1eda61b 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -1128,13 +1128,9 @@ 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 *txt = cli_args->v[i]; // Key[:=]Val
 		const char *sep;
 
-		if (!skip_prefix(raw, "--trailer=", &txt))
-			txt = raw;
-
 		if (!*txt) {
 			strbuf_addstr(err, _("empty --trailer argument"));
 			return -1;

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 3ac1eda61b..872945a897 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -1124,29 +1124,22 @@ 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)
+static void validate_trailer_args_after_config(const struct strvec *cli_args)
 {
 	for (size_t i = 0; i < cli_args->nr; i++) {
 		const char *txt = cli_args->v[i]; // Key[:=]Val
 		const char *sep;
 
-		if (!*txt) {
-			strbuf_addstr(err, _("empty --trailer argument"));
-			return -1;
-		}
+		if (!*txt)
+			die(_("empty --trailer argument"));
 
 		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;
-		}
+		if (sep && sep == txt)
+			die(_("invalid trailer '%s': missing key before separator"),
+			    txt);
 	}
-	return 0;
 }
 
 int cmd_rebase(int argc,
@@ -1353,13 +1346,8 @@ int cmd_rebase(int argc,
 			     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);
-
+		validate_trailer_args_after_config(&options.trailer_args);
 		options.flags |= REBASE_FORCE;
-		strbuf_release(&err);
 	}
 
 	if (preserve_merges_selected)

builtin/rebase.c, trailer.c, trailer.h

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 872945a897..a88abe08b4 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -1124,24 +1124,6 @@ static int check_exec_cmd(const char *cmd)
 	return 0;
 }
 
-static void validate_trailer_args_after_config(const struct strvec *cli_args)
-{
-	for (size_t i = 0; i < cli_args->nr; i++) {
-		const char *txt = cli_args->v[i]; // Key[:=]Val
-		const char *sep;
-
-		if (!*txt)
-			die(_("empty --trailer argument"));
-
-		sep = strpbrk(txt, ":=");
-
-		/* there must be key bfore seperator */
-		if (sep && sep == txt)
-			die(_("invalid trailer '%s': missing key before separator"),
-			    txt);
-	}
-}
-
 int cmd_rebase(int argc,
 	       const char **argv,
 	       const char *prefix,
diff --git a/trailer.c b/trailer.c
index 1f317f4d37..85e42859ca 100644
--- a/trailer.c
+++ b/trailer.c
@@ -7,6 +7,7 @@
 #include "string-list.h"
 #include "run-command.h"
 #include "commit.h"
+#include "strvec.h"
 #include "trailer.h"
 #include "list.h"
 #include "wrapper.h"
@@ -773,6 +774,30 @@ void parse_trailers_from_command_line_args(struct list_head *arg_head,
 	free(cl_separators);
 }
 
+void validate_trailer_args_after_config(const struct strvec *cli_args)
+{
+	char *cl_separators;
+
+	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)
+			die(_("empty --trailer argument"));
+
+		separator_pos = find_separator(txt, cl_separators);
+		if (separator_pos == 0)
+			die(_("invalid trailer '%s': missing key before separator"),
+		    txt);
+	}
+
+	free(cl_separators);
+}
+
 static const char *next_line(const char *str)
 {
 	const char *nl = strchrnul(str, '\n');
diff --git a/trailer.h b/trailer.h
index b4f28bfd65..4654ff9c96 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);
 
+void validate_trailer_args_after_config(const struct strvec *cli_args);
+
 void process_trailers_lists(struct list_head *head,
 			    struct list_head *arg_head);
 

sequencer.c, trailer.c, trailer.h

diff --git a/sequencer.c b/sequencer.c
index 5103ae786c..552e629e4f 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -2444,6 +2444,14 @@ 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)) {
+		if (amend_strbuf_with_trailers(&ctx->message,
+					       &opts->trailer_args)) {
+			res = error(_("unable to add trailers to commit message"));
+			goto leave;
+		}
+	}
+
 	if (is_rebase_i(opts) && write_author_script(msg.message) < 0)
 		res = -1;
 	else if (!opts->strategy ||
@@ -2519,17 +2527,6 @@ static int do_pick_commit(struct repository *r,
 			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/trailer.c b/trailer.c
index 85e42859ca..3e96d1624a 100644
--- a/trailer.c
+++ b/trailer.c
@@ -1250,8 +1250,8 @@ void trailer_iterator_release(struct trailer_iterator *iter)
 	strbuf_release(&iter->key);
 }
 
-static int amend_strbuf_with_trailers(struct strbuf *buf,
-									  const struct strvec *trailer_args)
+int amend_strbuf_with_trailers(struct strbuf *buf,
+			       const struct strvec *trailer_args)
 {
 	struct process_trailer_options opts = PROCESS_TRAILER_OPTIONS_INIT;
 	LIST_HEAD(new_trailer_head);
diff --git a/trailer.h b/trailer.h
index 4654ff9c96..479bc137cd 100644
--- a/trailer.h
+++ b/trailer.h
@@ -197,6 +197,9 @@ int trailer_iterator_advance(struct trailer_iterator *iter);
  */
 void trailer_iterator_release(struct trailer_iterator *iter);
 
+int amend_strbuf_with_trailers(struct strbuf *buf,
+			       const struct strvec *trailer_args);
+
 /*
  * Augment a file to add trailers to it (similar to 'git interpret-trailers').
  * Returns 0 on success or a non-zero error code on failure.

diff --git a/t/t3440-rebase-trailer.sh b/t/t3440-rebase-trailer.sh
index b1c7b03330..a8108f2296 100755
--- a/t/t3440-rebase-trailer.sh
+++ b/t/t3440-rebase-trailer.sh
@@ -26,14 +26,7 @@ test_expect_success 'setup repo with a small history' '
 	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_commit third file
 '
 
 test_expect_success 'apply backend is rejected with --trailer' '
@@ -74,6 +67,7 @@ test_expect_success 'multiple Signed-off-by trailers all preserved' '
 '
 
 test_expect_success 'rebase -m --trailer adds trailer after conflicts' '
+	create_expect file2-signed "file-2" &&
 	git reset --hard third &&
 	test_must_fail git rebase -m \
 		--trailer "Reviewed-by: Dev <dev@example.com>" \
@@ -85,6 +79,8 @@ test_expect_success 'rebase -m --trailer adds trailer after conflicts' '
 '
 
 test_expect_success 'rebase --root --trailer updates every commit' '
+	create_expect initial-signed "Initial empty commit" &&
+	create_expect first-signed "first" &&
 	git checkout first &&
 	git rebase --root --keep-empty \
 		--trailer "Reviewed-by: Dev <dev@example.com>" &&

diff --git a/t/t3440-rebase-trailer.sh b/t/t3440-rebase-trailer.sh
index a8108f2296..6b1c93b4cb 100755
--- a/t/t3440-rebase-trailer.sh
+++ b/t/t3440-rebase-trailer.sh
@@ -34,7 +34,7 @@ test_expect_success 'apply backend is rejected with --trailer' '
 	test_expect_code 128 \
 	git rebase --apply --trailer "Reviewed-by: Dev <dev@example.com>" \
 				HEAD^ 2>err &&
-	test_grep "requires the merge backend" err &&
+	test_grep "fatal: --trailer requires the merge backend" err &&
 	test_cmp_rev HEAD $head_before
 '
 

diff --git a/t/t3440-rebase-trailer.sh b/t/t3440-rebase-trailer.sh
index 6b1c93b4cb..1571dd2c97 100755
--- a/t/t3440-rebase-trailer.sh
+++ b/t/t3440-rebase-trailer.sh
@@ -52,18 +52,28 @@ test_expect_success 'reject trailer with missing key before separator' '
 
 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"
+	git -c trailer.Bug.ifexists=replace -c trailer.Bug.ifmissing=add \
+		rebase -m --trailer "Bug: 123" --trailer "Bug: 456" HEAD~1 &&
+	cat >expect <<-\EOF &&
+	third
+
+	Bug: 456
+	EOF
+	test_commit_message HEAD expect
 '
 
 test_expect_success 'multiple Signed-off-by trailers all preserved' '
 	git reset --hard third &&
 	git rebase -m \
-		--trailer "Signed-off-by: Dev A <a@ex.com>" \
-		--trailer "Signed-off-by: Dev B <b@ex.com>" HEAD~1 &&
-	git cat-file commit HEAD | grep -c "^Signed-off-by:" >count &&
-	test "$(cat count)" = 2   # two new commits
+			--trailer "Signed-off-by: Dev A <a@ex.com>" \
+			--trailer "Signed-off-by: Dev B <b@ex.com>" HEAD~1 &&
+	cat >expect <<-\EOF &&
+	third
+
+	Signed-off-by: Dev A <a@ex.com>
+	Signed-off-by: Dev B <b@ex.com>
+	EOF
+	test_commit_message HEAD expect
 '
 
 test_expect_success 'rebase -m --trailer adds trailer after conflicts' '

diff --git a/t/t3440-rebase-trailer.sh b/t/t3440-rebase-trailer.sh
index 1571dd2c97..504bdd86fc 100755
--- a/t/t3440-rebase-trailer.sh
+++ b/t/t3440-rebase-trailer.sh
@@ -39,21 +39,18 @@ test_expect_success 'apply backend is rejected with --trailer' '
 '
 
 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 &&
+		rebase -m --trailer "Bug: 123" --trailer "Bug: 456" HEAD~1 third &&
 	cat >expect <<-\EOF &&
 	third
 
@@ -63,10 +60,9 @@ test_expect_success 'CLI trailer duplicates allowed; replace policy keeps last'
 '
 
 test_expect_success 'multiple Signed-off-by trailers all preserved' '
-	git reset --hard third &&
 	git rebase -m \
 			--trailer "Signed-off-by: Dev A <a@ex.com>" \
-			--trailer "Signed-off-by: Dev B <b@ex.com>" HEAD~1 &&
+			--trailer "Signed-off-by: Dev B <b@ex.com>" HEAD~1 third &&
 	cat >expect <<-\EOF &&
 	third
 
@@ -78,7 +74,6 @@ test_expect_success 'multiple Signed-off-by trailers all preserved' '
 
 test_expect_success 'rebase -m --trailer adds trailer after conflicts' '
 	create_expect file2-signed "file-2" &&
-	git reset --hard third &&
 	test_must_fail git rebase -m \
 		--trailer "Reviewed-by: Dev <dev@example.com>" \
 		second third &&

diff --git a/t/t3440-rebase-trailer.sh b/t/t3440-rebase-trailer.sh
index 504bdd86fc..fbc6f209f1 100755
--- a/t/t3440-rebase-trailer.sh
+++ b/t/t3440-rebase-trailer.sh
@@ -73,14 +73,14 @@ test_expect_success 'multiple Signed-off-by trailers all preserved' '
 '
 
 test_expect_success 'rebase -m --trailer adds trailer after conflicts' '
-	create_expect file2-signed "file-2" &&
+	create_expect third-signed "third" &&
 	test_must_fail git rebase -m \
 		--trailer "Reviewed-by: Dev <dev@example.com>" \
 		second third &&
 	git checkout --theirs file &&
 	git add file &&
 	git rebase --continue &&
-	test_commit_message HEAD~2 file2-signed
+	test_commit_message HEAD third-signed
 '
 
 test_expect_success 'rebase --root --trailer updates every commit' '

diff --git a/sequencer.c b/sequencer.c
index 552e629e4f..c02364cfce 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.
@@ -3244,6 +3245,17 @@ static int read_populate_opts(struct replay_opts *opts)
 
 		read_strategy_opts(opts, &buf);
 		strbuf_reset(&buf);
+		if (strbuf_read_file(&buf, rebase_path_trailer(), 0) >= 0) {
+			char *p = buf.buf, *nl;
+
+			while ((nl = strchr(p, '\n'))) {
+				*nl = '\0';
+				if (*p)
+					strvec_push(&opts->trailer_args, p);
+				p = nl + 1;
+			}
+			strbuf_reset(&buf);
+		}
 
 		if (read_oneliner(&ctx->current_fixups,
 				  rebase_path_current_fixups(),
@@ -3338,6 +3350,14 @@ 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/t/t3440-rebase-trailer.sh b/t/t3440-rebase-trailer.sh
index fbc6f209f1..4687be3a21 100755
--- a/t/t3440-rebase-trailer.sh
+++ b/t/t3440-rebase-trailer.sh
@@ -87,7 +87,7 @@ test_expect_success 'rebase --root --trailer updates every commit' '
 	create_expect initial-signed "Initial empty commit" &&
 	create_expect first-signed "first" &&
 	git checkout first &&
-	git rebase --root --keep-empty \
+	git rebase --root \
 		--trailer "Reviewed-by: Dev <dev@example.com>" &&
 	test_commit_message HEAD   first-signed &&
 	test_commit_message HEAD^  initial-signed

diff --git a/t/t3440-rebase-trailer.sh b/t/t3440-rebase-trailer.sh
index 4687be3a21..4b0b0ee2d3 100755
--- a/t/t3440-rebase-trailer.sh
+++ b/t/t3440-rebase-trailer.sh
@@ -11,11 +11,13 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 . ./test-lib.sh
 . "$TEST_DIRECTORY"/lib-rebase.sh # test_commit_message, helpers
 
-create_expect() {
-	cat >"$1" <<-EOF
+REVIEWED_BY_TRAILER="Reviewed-by: Dev <dev@example.com>"
+
+expect_trailer_msg() {
+	test_commit_message "$1" <<-EOF
 	$2
 
-	Reviewed-by: Dev <dev@example.com>
+	${3:-$REVIEWED_BY_TRAILER}
 	EOF
 }
 
@@ -32,7 +34,7 @@ test_expect_success 'setup repo with a small history' '
 test_expect_success 'apply backend is rejected with --trailer' '
 	head_before=$(git rev-parse HEAD) &&
 	test_expect_code 128 \
-	git rebase --apply --trailer "Reviewed-by: Dev <dev@example.com>" \
+	git rebase --apply --trailer "$REVIEWED_BY_TRAILER" \
 				HEAD^ 2>err &&
 	test_grep "fatal: --trailer requires the merge backend" err &&
 	test_cmp_rev HEAD $head_before
@@ -73,23 +75,20 @@ test_expect_success 'multiple Signed-off-by trailers all preserved' '
 '
 
 test_expect_success 'rebase -m --trailer adds trailer after conflicts' '
-	create_expect third-signed "third" &&
 	test_must_fail git rebase -m \
-		--trailer "Reviewed-by: Dev <dev@example.com>" \
+		--trailer "$REVIEWED_BY_TRAILER" \
 		second third &&
 	git checkout --theirs file &&
 	git add file &&
 	git rebase --continue &&
-	test_commit_message HEAD third-signed
+	expect_trailer_msg HEAD "third"
 '
 
 test_expect_success 'rebase --root --trailer updates every commit' '
-	create_expect initial-signed "Initial empty commit" &&
-	create_expect first-signed "first" &&
 	git checkout first &&
 	git rebase --root \
-		--trailer "Reviewed-by: Dev <dev@example.com>" &&
-	test_commit_message HEAD   first-signed &&
-	test_commit_message HEAD^  initial-signed
+		--trailer "$REVIEWED_BY_TRAILER" &&
+	expect_trailer_msg HEAD  "first" &&
+	expect_trailer_msg HEAD^ "Initial empty commit"
 '
 test_done

diff --git a/t/t3440-rebase-trailer.sh b/t/t3440-rebase-trailer.sh
index 4b0b0ee2d3..bed6955001 100755
--- a/t/t3440-rebase-trailer.sh
+++ b/t/t3440-rebase-trailer.sh
@@ -50,6 +50,16 @@ test_expect_success 'reject trailer with missing key before separator' '
 	test_grep "missing key before separator" err
 '
 
+test_expect_success 'allow trailer with missing value after separator' '
+	git rebase -m --trailer "Acked-by:" HEAD~1 third &&
+	sed -e "s/_/ /g" <<-\EOF >expect &&
+	third
+
+	Acked-by:_
+	EOF
+	test_commit_message HEAD expect
+'
+
 test_expect_success 'CLI trailer duplicates allowed; replace policy keeps last' '
 	git -c trailer.Bug.ifexists=replace -c trailer.Bug.ifmissing=add \
 		rebase -m --trailer "Bug: 123" --trailer "Bug: 456" HEAD~1 third &&

diff --git a/t/t3440-rebase-trailer.sh b/t/t3440-rebase-trailer.sh
index bed6955001..bea98d08c6 100755
--- a/t/t3440-rebase-trailer.sh
+++ b/t/t3440-rebase-trailer.sh
@@ -73,13 +73,13 @@ test_expect_success 'CLI trailer duplicates allowed; replace policy keeps last'
 
 test_expect_success 'multiple Signed-off-by trailers all preserved' '
 	git rebase -m \
-			--trailer "Signed-off-by: Dev A <a@ex.com>" \
-			--trailer "Signed-off-by: Dev B <b@ex.com>" HEAD~1 third &&
+			--trailer "Signed-off-by: Dev A <a@example.com>" \
+			--trailer "Signed-off-by: Dev B <b@example.com>" HEAD~1 third &&
 	cat >expect <<-\EOF &&
 	third
 
-	Signed-off-by: Dev A <a@ex.com>
-	Signed-off-by: Dev B <b@ex.com>
+	Signed-off-by: Dev A <a@example.com>
+	Signed-off-by: Dev B <b@example.com>
 	EOF
 	test_commit_message HEAD expect
 '

diff --git a/t/t3440-rebase-trailer.sh b/t/t3440-rebase-trailer.sh
index bea98d08c6..35d2054716 100755
--- a/t/t3440-rebase-trailer.sh
+++ b/t/t3440-rebase-trailer.sh
@@ -85,13 +85,16 @@ test_expect_success 'multiple Signed-off-by trailers all preserved' '
 '
 
 test_expect_success 'rebase -m --trailer adds trailer after conflicts' '
+	git checkout -B conflict-branch third &&
+	test_commit fourth file &&
 	test_must_fail git rebase -m \
-		--trailer "$REVIEWED_BY_TRAILER" \
-		second third &&
+			--trailer "$REVIEWED_BY_TRAILER" \
+			second &&
 	git checkout --theirs file &&
 	git add file &&
 	git rebase --continue &&
-	expect_trailer_msg HEAD "third"
+	expect_trailer_msg HEAD "fourth" &&
+	expect_trailer_msg HEAD^ "third"
 '
 
 test_expect_success 'rebase --root --trailer updates every commit' '

diff --git a/t/t3440-rebase-trailer.sh b/t/t3440-rebase-trailer.sh
index 35d2054716..d697bf558b 100755
--- a/t/t3440-rebase-trailer.sh
+++ b/t/t3440-rebase-trailer.sh
@@ -99,8 +99,8 @@ test_expect_success 'rebase -m --trailer adds trailer after conflicts' '
 
 test_expect_success 'rebase --root --trailer updates every commit' '
 	git checkout first &&
-	git rebase --root \
-		--trailer "$REVIEWED_BY_TRAILER" &&
+	git -c trailer.review.key=Reviewed-by rebase --root \
+		--trailer=review="Dev <dev@example.com>" &&
 	expect_trailer_msg HEAD  "first" &&
 	expect_trailer_msg HEAD^ "Initial empty commit"
 '

sequencer.c, t/t3440-rebase-trailer.sh

diff --git a/sequencer.c b/sequencer.c
index c02364cfce..fbf35cb474 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -2027,6 +2027,10 @@ static int append_squash_message(struct strbuf *buf, const char *body,
 		if (opts->signoff)
 			append_signoff(buf, 0, 0);
 
+		if (opts->trailer_args.nr &&
+			amend_strbuf_with_trailers(buf, &opts->trailer_args))
+			return error(_("unable to add trailers to commit message"));
+
 		if ((command == TODO_FIXUP) &&
 		    (flag & TODO_REPLACE_FIXUP_MSG) &&
 		    (file_exists(rebase_path_fixup_msg()) ||
diff --git a/t/t3440-rebase-trailer.sh b/t/t3440-rebase-trailer.sh
index d697bf558b..d0e0434664 100755
--- a/t/t3440-rebase-trailer.sh
+++ b/t/t3440-rebase-trailer.sh
@@ -97,6 +97,33 @@ test_expect_success 'rebase -m --trailer adds trailer after conflicts' '
 	expect_trailer_msg HEAD^ "third"
 '
 
+test_expect_success '--trailer handles fixup commands in todo list' '
+	git checkout -B fixup-trailer HEAD &&
+	test_commit fixup-base base &&
+	test_commit fixup-second second &&
+	first_short=$(git rev-parse --short fixup-base) &&
+	second_short=$(git rev-parse --short fixup-second) &&
+	cat >todo <<EOF &&
+pick $first_short fixup-base
+fixup $second_short fixup-second
+EOF
+	(
+		set_replace_editor todo &&
+		git rebase -i --trailer "$REVIEWED_BY_TRAILER" HEAD~2
+	) &&
+	expect_trailer_msg HEAD "fixup-base" &&
+	git reset --hard fixup-second &&
+	cat >todo <<EOF &&
+pick $first_short fixup-base
+fixup -C $second_short fixup-second
+EOF
+	(
+		set_replace_editor todo &&
+		git rebase -i --trailer "$REVIEWED_BY_TRAILER" HEAD~2
+	) &&
+	expect_trailer_msg HEAD "fixup-second"
+'
+
 test_expect_success 'rebase --root --trailer updates every commit' '
 	git checkout first &&
 	git -c trailer.review.key=Reviewed-by rebase --root \