builtin/tag.c: add --trailer option

git-tag currently supports interpreting trailers from an annotated tag
message, using --list --format="%(trailers)". However, to add a trailer
to a tag message, you must do so using `-F` or by editing the message.

In a previous patch, we moved git-commit's implementation of its
--trailer option to the trailer.h API. Let's use that new function to
teach git-tag the same --trailer option, emulating as much of
git-commit's behavior as much as possible.

Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: John Passaro <john.a.passaro@gmail.com>

Documentation/git-tag.txt

@@ -10,6 +10,7 @@ SYNOPSIS
 --------
 [verse]
 'git tag' [-a | -s | -u <key-id>] [-f] [-m <msg> | -F <file>] [-e]
+	[(--trailer <token>[(=|:)<value>])...]
 	<tagname> [<commit> | <object>]
 'git tag' -d <tagname>...
 'git tag' [-n[<num>]] -l [--contains <commit>] [--no-contains <commit>]
@@ -31,8 +32,8 @@ creates a 'tag' object, and requires a tag message.  Unless
 `-m <msg>` or `-F <file>` is given, an editor is started for the user to type
 in the tag message.
 
-If `-m <msg>` or `-F <file>` is given and `-a`, `-s`, and `-u <key-id>`
-are absent, `-a` is implied.
+If `-m <msg>` or `-F <file>` or `--trailer <token>[=<value>]` is given
+and `-a`, `-s`, and `-u <key-id>` are absent, `-a` is implied.
 
 Otherwise, a tag reference that points directly at the given object
 (i.e., a lightweight tag) is created.
@@ -178,6 +179,19 @@ This option is only applicable when listing tags without annotation lines.
 	Implies `-a` if none of `-a`, `-s`, or `-u <key-id>`
 	is given.
 
+--trailer <token>[(=|:)<value>]::
+	Specify a (<token>, <value>) pair that should be applied as a
+	trailer. (e.g. `git tag --trailer "Signed-off-by:T A Ger \
+	<tagger@example.com>" --trailer "Helped-by:C O Mitter \
+	<committer@example.com>"` will add the "Signed-off-by" trailer
+	and the "Helped-by" trailer to the tag message.)
+	The `trailer.*` configuration variables
+	(linkgit:git-interpret-trailers[1]) can be used to define if
+	a duplicated trailer is omitted, where in the run of trailers
+	each trailer would appear, and other details.
+	The trailers can be seen in `git tag --list` using
+	`--format="%(trailers)"` placeholder.
+
 -e::
 --edit::
 	The message taken from file with `-F` and command line with

builtin/tag.c

@@ -28,9 +28,11 @@
 #include "date.h"
 #include "write-or-die.h"
 #include "object-file-convert.h"
+#include "trailer.h"
 
 static const char * const git_tag_usage[] = {
 	N_("git tag [-a | -s | -u <key-id>] [-f] [-m <msg> | -F <file>] [-e]\n"
+	   "        [(--trailer <token>[(=|:)<value>])...]\n"
 	   "        <tagname> [<commit> | <object>]"),
 	N_("git tag -d <tagname>..."),
 	N_("git tag [-n[<num>]] -l [--contains <commit>] [--no-contains <commit>]\n"
@@ -290,10 +292,12 @@ static const char message_advice_nested_tag[] =
 static void create_tag(const struct object_id *object, const char *object_ref,
 		       const char *tag,
 		       struct strbuf *buf, struct create_tag_options *opt,
-		       struct object_id *prev, struct object_id *result, char *path)
+		       struct object_id *prev, struct object_id *result,
+		       struct strvec *trailer_args, char *path)
 {
 	enum object_type type;
 	struct strbuf header = STRBUF_INIT;
+	int should_edit;
 
 	type = oid_object_info(the_repository, object, NULL);
 	if (type <= OBJ_NONE)
@@ -313,13 +317,15 @@ static void create_tag(const struct object_id *object, const char *object_ref,
 		    tag,
 		    git_committer_info(IDENT_STRICT));
 
-	if (!opt->message_given || opt->use_editor) {
+	should_edit = opt->use_editor || !opt->message_given;
+	if (should_edit || trailer_args->nr) {
 		int fd;
 
 		/* write the template message before editing: */
 		fd = xopen(path, O_CREAT | O_TRUNC | O_WRONLY, 0600);
 
-		if (opt->message_given) {
+		if (opt->message_given && buf->len) {
+			strbuf_complete(buf, '\n');
 			write_or_die(fd, buf->buf, buf->len);
 			strbuf_reset(buf);
 		} else if (!is_null_oid(prev)) {
@@ -338,10 +344,22 @@ static void create_tag(const struct object_id *object, const char *object_ref,
 		}
 		close(fd);
 
-		if (launch_editor(path, buf, NULL)) {
-			fprintf(stderr,
-			_("Please supply the message using either -m or -F option.\n"));
-			exit(1);
+		if (trailer_args->nr && amend_file_with_trailers(path, trailer_args))
+			die(_("unable to pass trailers to --trailers"));
+
+		if (should_edit) {
+			if (launch_editor(path, buf, NULL)) {
+				fprintf(stderr,
+					_("Please supply the message using either -m or -F option.\n"));
+				exit(1);
+			}
+		} else if (trailer_args->nr) {
+			strbuf_reset(buf);
+			if (strbuf_read_file(buf, path, 0) < 0) {
+				fprintf(stderr,
+					_("Please supply the message using either -m or -F option.\n"));
+				exit(1);
+			}
 		}
 	}
 
@@ -463,6 +481,7 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 	struct ref_sorting *sorting;
 	struct string_list sorting_options = STRING_LIST_INIT_DUP;
 	struct ref_format format = REF_FORMAT_INIT;
+	struct strvec trailer_args = STRVEC_INIT;
 	int icase = 0;
 	int edit_flag = 0;
 	struct option options[] = {
@@ -479,6 +498,8 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 		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_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),
@@ -548,7 +569,8 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 		opt.sign = 1;
 		set_signing_key(keyid);
 	}
-	create_tag_object = (opt.sign || annotate || msg.given || msgfile);
+	create_tag_object = (opt.sign || annotate || msg.given || msgfile ||
+			     edit_flag || trailer_args.nr);
 
 	if ((create_tag_object || force) && (cmdmode != 0))
 		usage_with_options(git_tag_usage, options);
@@ -654,7 +676,7 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 			opt.sign = 1;
 		path = git_pathdup("TAG_EDITMSG");
 		create_tag(&object, object_ref, tag, &buf, &opt, &prev, &object,
-			   path);
+			   &trailer_args, path);
 	}
 
 	transaction = ref_transaction_begin(&err);
@@ -686,6 +708,7 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 	strbuf_release(&reflog_msg);
 	strbuf_release(&msg.buf);
 	strbuf_release(&err);
+	strvec_clear(&trailer_args);
 	free(msgfile);
 	return ret;
 }

t/t7004-tag.sh

@@ -668,6 +668,115 @@ test_expect_success \
 	test_cmp expect actual
 '
 
+# trailers
+
+test_expect_success 'create tag with -m and --trailer' '
+	get_tag_header tag-with-inline-message-and-trailers $commit commit $time >expect &&
+	cat >>expect <<-\EOF &&
+	create tag with trailers
+
+	my-trailer: here
+	alt-trailer: there
+	EOF
+	git tag -m "create tag with trailers" \
+		--trailer my-trailer=here \
+		--trailer alt-trailer=there \
+		tag-with-inline-message-and-trailers &&
+	get_tag_msg tag-with-inline-message-and-trailers >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'list tag extracting trailers' '
+	cat >expect <<-\EOF &&
+	my-trailer: here
+	alt-trailer: there
+
+	EOF
+	git tag --list --format="%(trailers)" tag-with-inline-message-and-trailers >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'create tag with -F and --trailer' '
+	echo "create tag from message file using --trailer" >messagefilewithnotrailers &&
+	get_tag_header tag-with-file-message-and-trailers $commit commit $time >expect &&
+	cat >>expect <<-\EOF &&
+	create tag from message file using --trailer
+
+	my-trailer: here
+	alt-trailer: there
+	EOF
+	git tag -F messagefilewithnotrailers \
+		--trailer my-trailer=here \
+		--trailer alt-trailer=there \
+		tag-with-file-message-and-trailers &&
+	get_tag_msg tag-with-file-message-and-trailers >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'create tag with -m and --trailer and --edit' '
+	write_script fakeeditor <<-\EOF &&
+	sed -e "1s/^/EDITED: /g" <"$1" >"$1-"
+	mv "$1-" "$1"
+	EOF
+	get_tag_header tag-with-edited-inline-message-and-trailers $commit commit $time >expect &&
+	cat >>expect <<-\EOF &&
+	EDITED: create tag with trailers
+
+	my-trailer: here
+	alt-trailer: there
+	EOF
+	GIT_EDITOR=./fakeeditor git tag --edit \
+		-m "create tag with trailers" \
+		--trailer my-trailer=here \
+		--trailer alt-trailer=there \
+		tag-with-edited-inline-message-and-trailers &&
+	get_tag_msg tag-with-edited-inline-message-and-trailers >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'create tag with -F and --trailer and --edit' '
+	echo "create tag from message file using --trailer" >messagefilewithnotrailers &&
+	get_tag_header tag-with-edited-file-message-and-trailers $commit commit $time >expect &&
+	cat >>expect <<-\EOF &&
+	EDITED: create tag from message file using --trailer
+
+	my-trailer: here
+	alt-trailer: there
+	EOF
+	GIT_EDITOR=./fakeeditor git tag --edit \
+		-F messagefilewithnotrailers \
+		--trailer my-trailer=here \
+		--trailer alt-trailer=there \
+		tag-with-edited-file-message-and-trailers &&
+	get_tag_msg tag-with-edited-file-message-and-trailers >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'create annotated tag and force editor when only --trailer is given' '
+	write_script fakeeditor <<-\EOF &&
+	echo "add a line" >"$1-"
+	cat <"$1" >>"$1-"
+	mv "$1-" "$1"
+	EOF
+	get_tag_header tag-with-trailers-and-no-message $commit commit $time >expect &&
+	cat >>expect <<-\EOF &&
+	add a line
+
+	my-trailer: here
+	alt-trailer: there
+	EOF
+	GIT_EDITOR=./fakeeditor git tag \
+		--trailer my-trailer=here \
+		--trailer alt-trailer=there \
+		tag-with-trailers-and-no-message &&
+	get_tag_msg tag-with-trailers-and-no-message >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'bad editor causes panic when only --trailer is given' '
+	test_must_fail env GIT_EDITOR=false git tag --trailer my-trailer=here tag-will-not-exist
+'
+
 # listing messages for annotated non-signed tags:
 
 test_expect_success \
@@ -810,6 +919,11 @@ test_expect_success 'git tag --format with ahead-behind' '
 	refs/tags/tag-lines 0 1 !
 	refs/tags/tag-one-line 0 1 !
 	refs/tags/tag-right 0 0 !
+	refs/tags/tag-with-edited-file-message-and-trailers 0 1 !
+	refs/tags/tag-with-edited-inline-message-and-trailers 0 1 !
+	refs/tags/tag-with-file-message-and-trailers 0 1 !
+	refs/tags/tag-with-inline-message-and-trailers 0 1 !
+	refs/tags/tag-with-trailers-and-no-message 0 1 !
 	refs/tags/tag-zero-lines 0 1 !
 	EOF
 	git tag -l --format="%(refname) %(ahead-behind:HEAD) !" >actual 2>err &&