interpret-trailer: easy parse trailer value

index 739110c5a7f6..4b06672bd07d 100644
@@ -113,6 +113,7 @@ static int config_commit_verbose = -1; /* unspecified */
 static int no_post_rewrite, allow_empty_message, pathspec_file_nul;
 static char *untracked_files_arg, *force_date, *ignore_submodule_arg, *ignored_arg;
 static char *sign_commit, *pathspec_from_file;
+static struct strvec trailer_args = STRVEC_INIT;
 
 /*
  * The default commit message cleanup mode will remove the lines

@@ -131,6 +132,14 @@ static struct strbuf message = STRBUF_INIT;
 
 static enum wt_status_format status_format = STATUS_FORMAT_UNSPECIFIED;
 
+static int opt_pass_trailer(const struct option *opt, const char *arg, int unset)
+{
+	BUG_ON_OPT_NEG(unset);
+
+	strvec_pushl(&trailer_args, "--trailer", arg, NULL);
+	return 0;
+}
+
 static int opt_parse_porcelain(const struct option *opt, const char *arg, int unset)
 {
 	enum wt_status_format *value = (enum wt_status_format *)opt->value;

@@ -958,6 +967,18 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 
 	fclose(s->fp);
 
+	if (trailer_args.nr) {
+		struct child_process run_trailer = CHILD_PROCESS_INIT;
+
+		strvec_pushl(&run_trailer.args, "interpret-trailers",
+			     "--in-place", git_path_commit_editmsg(), NULL);
+		strvec_pushv(&run_trailer.args, trailer_args.v);
+		run_trailer.git_cmd = 1;
+		if (run_command(&run_trailer))
+			die(_("unable to pass trailers to --trailers"));
+		strvec_clear(&trailer_args);
+	}
+
 	/*
 	 * Reject an attempt to record a non-merge empty commit without
 	 * explicit --allow-empty. In the cherry-pick case, it may be

@@ -1507,6 +1528,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		OPT_STRING(0, "fixup", &fixup_message, N_("commit"), N_("use autosquash formatted message to fixup 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_CALLBACK_F(0, "trailer", NULL, N_("trailer"), N_("add custom trailer(s)"), PARSE_OPT_NONEG, opt_pass_trailer),
 		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")),

index 4b06672bd07d..58c020e3cfbf 100644
@@ -1043,39 +1043,6 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 	return 1;
 }
 
-static const char *find_author_by_nickname(const char *name)
-{
-	struct rev_info revs;
-	struct commit *commit;
-	struct strbuf buf = STRBUF_INIT;
-	struct string_list mailmap = STRING_LIST_INIT_NODUP;
-	const char *av[20];
-	int ac = 0;
-
-	repo_init_revisions(the_repository, &revs, NULL);
-	strbuf_addf(&buf, "--author=%s", name);
-	av[++ac] = "--all";
-	av[++ac] = "-i";
-	av[++ac] = buf.buf;
-	av[++ac] = NULL;
-	setup_revisions(ac, av, &revs, NULL);
-	revs.mailmap = &mailmap;
-	read_mailmap(revs.mailmap);
-
-	if (prepare_revision_walk(&revs))
-		die(_("revision walk setup failed"));
-	commit = get_revision(&revs);
-	if (commit) {
-		struct pretty_print_context ctx = {0};
-		ctx.date_mode.type = DATE_NORMAL;
-		strbuf_release(&buf);
-		format_commit_message(commit, "%aN <%aE>", &buf, &ctx);
-		clear_mailmap(&mailmap);
-		return strbuf_detach(&buf, NULL);
-	}
-	die(_("--author '%s' is not 'Name <email>' and matches no existing author"), name);
-}
-
 static void handle_ignored_arg(struct wt_status *s)
 {
 	if (!ignored_arg)

index 6ccd774841c6..e1aad52d2c4f 100644
@@ -21,6 +21,7 @@
 #include "commit-reach.h"
 #include "run-command.h"
 #include "shallow.h"
+#include "mailmap.h"
 
 static struct commit_extra_header *read_commit_extra_header_lines(const char *buf, size_t len, const char **);
 

@@ -1703,3 +1704,36 @@ int run_commit_hook(int editor_is_used, const char *index_file,
 
 	return ret;
 }
+
+const char *find_author_by_nickname(const char *name)
+{
+	struct rev_info revs;
+	struct commit *commit;
+	struct strbuf buf = STRBUF_INIT;
+	struct string_list mailmap = STRING_LIST_INIT_NODUP;
+	const char *av[20];
+	int ac = 0;
+
+	repo_init_revisions(the_repository, &revs, NULL);
+	strbuf_addf(&buf, "--author=%s", name);
+	av[++ac] = "--all";
+	av[++ac] = "-i";
+	av[++ac] = buf.buf;
+	av[++ac] = NULL;
+	setup_revisions(ac, av, &revs, NULL);
+	revs.mailmap = &mailmap;
+	read_mailmap(revs.mailmap);
+
+	if (prepare_revision_walk(&revs))
+		die(_("revision walk setup failed"));
+	commit = get_revision(&revs);
+	if (commit) {
+		struct pretty_print_context ctx = {0};
+		ctx.date_mode.type = DATE_NORMAL;
+		strbuf_release(&buf);
+		format_commit_message(commit, "%aN <%aE>", &buf, &ctx);
+		clear_mailmap(&mailmap);
+		return strbuf_detach(&buf, NULL);
+	}
+	die(_("--author '%s' is not 'Name <email>' and matches no existing author"), name);
+}

index 49c0f503964e..970a73ccd5be 100644
@@ -371,4 +371,6 @@ int parse_buffer_signed_by_header(const char *buffer,
 				  struct strbuf *signature,
 				  const struct git_hash_algo *algop);
 
+const char *find_author_by_nickname(const char *name);
+
 #endif /* COMMIT_H */

index 249ed618ed8e..21f367e7b761 100644
@@ -6,6 +6,7 @@
 #include "tempfile.h"
 #include "trailer.h"
 #include "list.h"
+#include "revision.h"
 /*
  * Copyright (c) 2013, 2014 Christian Couder <[email protected]>
  */

@@ -633,11 +634,21 @@ static void parse_trailer(struct strbuf *tok, struct strbuf *val,
 	struct arg_item *item;
 	size_t tok_len;
 	struct list_head *pos;
+	const char *ae = NULL;
 
 	if (separator_pos != -1) {
 		strbuf_add(tok, trailer, separator_pos);
 		strbuf_trim(tok);
-		strbuf_addstr(val, trailer + separator_pos + 1);
+		if (trailer[separator_pos + 1] == '@') {
+			ae = find_author_by_nickname(trailer + separator_pos + 2);
+			reset_revision_walk();
+			if (ae) {
+				strbuf_addstr(val, ae);
+				free((char*)ae);
+			} else
+				strbuf_addstr(val, trailer + separator_pos + 1);
+		} else
+			strbuf_addstr(val, trailer + separator_pos + 1);
 		strbuf_trim(val);
 	} else {
 		strbuf_addstr(tok, trailer);

index 739110c5a7f6..4b06672bd07d 100644
@@ -113,6 +113,7 @@ static int config_commit_verbose = -1; /* unspecified */
 static int no_post_rewrite, allow_empty_message, pathspec_file_nul;
 static char *untracked_files_arg, *force_date, *ignore_submodule_arg, *ignored_arg;
 static char *sign_commit, *pathspec_from_file;
+static struct strvec trailer_args = STRVEC_INIT;
 
 /*
  * The default commit message cleanup mode will remove the lines

@@ -131,6 +132,14 @@ static struct strbuf message = STRBUF_INIT;
 
 static enum wt_status_format status_format = STATUS_FORMAT_UNSPECIFIED;
 
+static int opt_pass_trailer(const struct option *opt, const char *arg, int unset)
+{
+	BUG_ON_OPT_NEG(unset);
+
+	strvec_pushl(&trailer_args, "--trailer", arg, NULL);
+	return 0;
+}
+
 static int opt_parse_porcelain(const struct option *opt, const char *arg, int unset)
 {
 	enum wt_status_format *value = (enum wt_status_format *)opt->value;

@@ -958,6 +967,18 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 
 	fclose(s->fp);
 
+	if (trailer_args.nr) {
+		struct child_process run_trailer = CHILD_PROCESS_INIT;
+
+		strvec_pushl(&run_trailer.args, "interpret-trailers",
+			     "--in-place", git_path_commit_editmsg(), NULL);
+		strvec_pushv(&run_trailer.args, trailer_args.v);
+		run_trailer.git_cmd = 1;
+		if (run_command(&run_trailer))
+			die(_("unable to pass trailers to --trailers"));
+		strvec_clear(&trailer_args);
+	}
+
 	/*
 	 * Reject an attempt to record a non-merge empty commit without
 	 * explicit --allow-empty. In the cherry-pick case, it may be

@@ -1507,6 +1528,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		OPT_STRING(0, "fixup", &fixup_message, N_("commit"), N_("use autosquash formatted message to fixup 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_CALLBACK_F(0, "trailer", NULL, N_("trailer"), N_("add custom trailer(s)"), PARSE_OPT_NONEG, opt_pass_trailer),
 		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")),

index 4b06672bd07d..58c020e3cfbf 100644
@@ -1043,39 +1043,6 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 	return 1;
 }
 
-static const char *find_author_by_nickname(const char *name)
-{
-	struct rev_info revs;
-	struct commit *commit;
-	struct strbuf buf = STRBUF_INIT;
-	struct string_list mailmap = STRING_LIST_INIT_NODUP;
-	const char *av[20];
-	int ac = 0;
-
-	repo_init_revisions(the_repository, &revs, NULL);
-	strbuf_addf(&buf, "--author=%s", name);
-	av[++ac] = "--all";
-	av[++ac] = "-i";
-	av[++ac] = buf.buf;
-	av[++ac] = NULL;
-	setup_revisions(ac, av, &revs, NULL);
-	revs.mailmap = &mailmap;
-	read_mailmap(revs.mailmap);
-
-	if (prepare_revision_walk(&revs))
-		die(_("revision walk setup failed"));
-	commit = get_revision(&revs);
-	if (commit) {
-		struct pretty_print_context ctx = {0};
-		ctx.date_mode.type = DATE_NORMAL;
-		strbuf_release(&buf);
-		format_commit_message(commit, "%aN <%aE>", &buf, &ctx);
-		clear_mailmap(&mailmap);
-		return strbuf_detach(&buf, NULL);
-	}
-	die(_("--author '%s' is not 'Name <email>' and matches no existing author"), name);
-}
-
 static void handle_ignored_arg(struct wt_status *s)
 {
 	if (!ignored_arg)

index 6ccd774841c6..e1aad52d2c4f 100644
@@ -21,6 +21,7 @@
 #include "commit-reach.h"
 #include "run-command.h"
 #include "shallow.h"
+#include "mailmap.h"
 
 static struct commit_extra_header *read_commit_extra_header_lines(const char *buf, size_t len, const char **);
 

@@ -1703,3 +1704,36 @@ int run_commit_hook(int editor_is_used, const char *index_file,
 
 	return ret;
 }
+
+const char *find_author_by_nickname(const char *name)
+{
+	struct rev_info revs;
+	struct commit *commit;
+	struct strbuf buf = STRBUF_INIT;
+	struct string_list mailmap = STRING_LIST_INIT_NODUP;
+	const char *av[20];
+	int ac = 0;
+
+	repo_init_revisions(the_repository, &revs, NULL);
+	strbuf_addf(&buf, "--author=%s", name);
+	av[++ac] = "--all";
+	av[++ac] = "-i";
+	av[++ac] = buf.buf;
+	av[++ac] = NULL;
+	setup_revisions(ac, av, &revs, NULL);
+	revs.mailmap = &mailmap;
+	read_mailmap(revs.mailmap);
+
+	if (prepare_revision_walk(&revs))
+		die(_("revision walk setup failed"));
+	commit = get_revision(&revs);
+	if (commit) {
+		struct pretty_print_context ctx = {0};
+		ctx.date_mode.type = DATE_NORMAL;
+		strbuf_release(&buf);
+		format_commit_message(commit, "%aN <%aE>", &buf, &ctx);
+		clear_mailmap(&mailmap);
+		return strbuf_detach(&buf, NULL);
+	}
+	die(_("--author '%s' is not 'Name <email>' and matches no existing author"), name);
+}

index 49c0f503964e..cb9f7cd13f09 100644
@@ -370,5 +370,15 @@ int parse_buffer_signed_by_header(const char *buffer,
 				  struct strbuf *payload,
 				  struct strbuf *signature,
 				  const struct git_hash_algo *algop);
+/*
+ * Calling `find_author_by_nickname` to find the "author <email>" pair
+ * in the most recent commit which matches "--author=name".
+ *
+ * Note that `find_author_by_nickname` is not reusable, because it haven't
+ * reset flags for parsed objects. The only safe way to use `find_author_by_nickname`
+ * (without rewriting the revision traversal machinery) is to spawn a
+ * subprocess and do find_author_by_nickname() in it.
+ */
+const char *find_author_by_nickname(const char *name);
 
 #endif /* COMMIT_H */

index 249ed618ed8e..21f367e7b761 100644
@@ -6,6 +6,7 @@
 #include "tempfile.h"
 #include "trailer.h"
 #include "list.h"
+#include "revision.h"
 /*
  * Copyright (c) 2013, 2014 Christian Couder <[email protected]>
  */

@@ -633,11 +634,21 @@ static void parse_trailer(struct strbuf *tok, struct strbuf *val,
 	struct arg_item *item;
 	size_t tok_len;
 	struct list_head *pos;
+	const char *ae = NULL;
 
 	if (separator_pos != -1) {
 		strbuf_add(tok, trailer, separator_pos);
 		strbuf_trim(tok);
-		strbuf_addstr(val, trailer + separator_pos + 1);
+		if (trailer[separator_pos + 1] == '@') {
+			ae = find_author_by_nickname(trailer + separator_pos + 2);
+			reset_revision_walk();
+			if (ae) {
+				strbuf_addstr(val, ae);
+				free((char*)ae);
+			} else
+				strbuf_addstr(val, trailer + separator_pos + 1);
+		} else
+			strbuf_addstr(val, trailer + separator_pos + 1);
 		strbuf_trim(val);
 	} else {
 		strbuf_addstr(tok, trailer);