[RFC/PATCH 8/9] tag.c: implement '--format' option

[Karthik Nayak <karthik.188@gmail.com>]

Implement the '--format' option provided by 'ref-filter'.
This lets the user list tags as per desired format similar
to the implementation in 'git for-each-ref'.

Add tests and documentation for the same.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
---
 Documentation/git-tag.txt | 15 ++++++++++++++-
 builtin/tag.c             | 11 +++++++----
 t/t7004-tag.sh            | 16 ++++++++++++++++
 3 files changed, 37 insertions(+), 5 deletions(-)

diff --git a/Documentation/git-tag.txt b/Documentation/git-tag.txt
index 1950d94..16e396c 100644
--- a/Documentation/git-tag.txt
+++ b/Documentation/git-tag.txt
@@ -13,7 +13,7 @@ SYNOPSIS
 	<tagname> [<commit> | <object>]
 'git tag' -d <tagname>...
 'git tag' [-n[<num>]] -l [--contains <commit>] [--points-at <object>]
-	[--column[=<options>] | --no-column] [--sort=<key>] [<pattern>...]
+	[--column[=<options>] | --no-column] [--sort=<key>] [--format=<format>]
 	[<pattern>...]
 'git tag' -v <tagname>...
 
@@ -156,6 +156,19 @@ This option is only applicable when listing tags without annotation lines.
 	The object that the new tag will refer to, usually a commit.
 	Defaults to HEAD.
 
+<format>::
+	A string that interpolates `%(fieldname)` from the
+	object pointed at by a ref being shown.  If `fieldname`
+	is prefixed with an asterisk (`*`) and the ref points
+	at a tag object, the value for the field in the object
+	tag refers is used.  When unspecified, defaults to
+	`%(objectname) SPC %(objecttype) TAB %(refname)`.
+	It also interpolates `%%` to `%`, and `%xx` where `xx`
+	are hex digits interpolates to character with hex code
+	`xx`; for example `%00` interpolates to `\0` (NUL),
+	`%09` to `\t` (TAB) and `%0a` to `\n` (LF).
+	The fields are same as those in `git for-each-ref`.
+
 
 CONFIGURATION
 -------------
diff --git a/builtin/tag.c b/builtin/tag.c
index d80120e..257526b 100644
--- a/builtin/tag.c
+++ b/builtin/tag.c
@@ -30,10 +30,9 @@ static const char * const git_tag_usage[] = {
 
 static unsigned int colopts;
 
-static int list_tags(struct ref_filter *filter, struct ref_sorting *sorting)
+static int list_tags(struct ref_filter *filter, struct ref_sorting *sorting, const char *format)
 {
 	struct ref_array array;
-	char *format;
 	int i;
 
 	memset(&array, 0, sizeof(array));
@@ -43,7 +42,7 @@ static int list_tags(struct ref_filter *filter, struct ref_sorting *sorting)
 
 	if (filter->lines)
 		format = "%(refname:lalign16)";
-	else
+	else if (!format)
 		format = "%(refname:short)";
 
 	verify_ref_format(format);
@@ -324,6 +323,7 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 	struct strbuf err = STRBUF_INIT;
 	struct ref_filter filter;
 	static struct ref_sorting *sorting = NULL, **sorting_tail = &sorting;
+	const char *format = NULL;
 	struct option options[] = {
 		OPT_CMDMODE('l', "list", &cmdmode, N_("list tag names"), 'l'),
 		{ OPTION_INTEGER, 'n', NULL, &filter.lines, N_("n"),
@@ -355,6 +355,7 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 			OPTION_CALLBACK, 0, "points-at", &filter.points_at, N_("object"),
 			N_("print only tags of the object"), 0, parse_opt_object_name
 		},
+		OPT_STRING(  0 , "format", &format, N_("format"), N_("format to use for the output")),
 		OPT_END()
 	};
 
@@ -394,8 +395,10 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 			copts.padding = 2;
 			run_column_filter(colopts, &copts);
 		}
+		if (format && (filter.lines != -1))
+			die(_("--format and -n are incompatible"));
 		filter.name_patterns = argv;
-		ret = list_tags(&filter, sorting);
+		ret = list_tags(&filter, sorting, format);
 		if (column_active(colopts))
 			stop_column_filter();
 		return ret;
diff --git a/t/t7004-tag.sh b/t/t7004-tag.sh
index 51a233f..e8cebb6 100755
--- a/t/t7004-tag.sh
+++ b/t/t7004-tag.sh
@@ -1507,4 +1507,20 @@ EOF"
 	test_cmp expect actual
 '
 
+test_expect_success '--format cannot be used with -n' '
+	test_must_fail git tag -l -n4 --format="%(refname)"
+'
+
+test_expect_success '--format should list tags as per format given' '
+	cat >expect <<-\EOF &&
+	foo1.10
+	foo1.3
+	foo1.6
+	foo1.6-rc1
+	foo1.6-rc2
+	EOF
+	git tag -l --format="%(refname)" "foo*" >actual &&
+	test_cmp expect actual
+'
+
 test_done
-- 
2.4.4