tag: disallow '-' as tag name

Disallow '-' as tag name, as well as tag names starting with '-', as it
would be cumbersome to "git checkout tags/-" because "git checkout -" is
to switch to the previous branch.

Add strbuf_check_tag_ref() as helper to check a refname for a tag.

Signed-off-by: Michael Schubert <mschub@elegosoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/tag.c

@@ -352,11 +352,22 @@ static int parse_msg_arg(const struct option *opt, const char *arg, int unset)
 	return 0;
 }
 
+static int strbuf_check_tag_ref(struct strbuf *sb, const char *name)
+{
+	if (name[0] == '-')
+		return CHECK_REF_FORMAT_ERROR;
+
+	strbuf_reset(sb);
+	strbuf_addf(sb, "refs/tags/%s", name);
+
+	return check_ref_format(sb->buf);
+}
+
 int cmd_tag(int argc, const char **argv, const char *prefix)
 {
 	struct strbuf buf = STRBUF_INIT;
+	struct strbuf ref = STRBUF_INIT;
 	unsigned char object[20], prev[20];
-	char ref[PATH_MAX];
 	const char *object_ref, *tag;
 	struct ref_lock *lock;
 
@@ -452,12 +463,10 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 	if (get_sha1(object_ref, object))
 		die(_("Failed to resolve '%s' as a valid ref."), object_ref);
 
-	if (snprintf(ref, sizeof(ref), "refs/tags/%s", tag) > sizeof(ref) - 1)
-		die(_("tag name too long: %.*s..."), 50, tag);
-	if (check_ref_format(ref))
+	if (strbuf_check_tag_ref(&ref, tag))
 		die(_("'%s' is not a valid tag name."), tag);
 
-	if (!resolve_ref(ref, prev, 1, NULL))
+	if (!resolve_ref(ref.buf, prev, 1, NULL))
 		hashclr(prev);
 	else if (!force)
 		die(_("tag '%s' already exists"), tag);
@@ -466,14 +475,15 @@ int cmd_tag(int argc, const char **argv, const char *prefix)
 		create_tag(object, tag, &buf, msg.given || msgfile,
 			   sign, prev, object);
 
-	lock = lock_any_ref_for_update(ref, prev, 0);
+	lock = lock_any_ref_for_update(ref.buf, prev, 0);
 	if (!lock)
-		die(_("%s: cannot lock the ref"), ref);
+		die(_("%s: cannot lock the ref"), ref.buf);
 	if (write_ref_sha1(lock, object, NULL) < 0)
-		die(_("%s: cannot update the ref"), ref);
+		die(_("%s: cannot update the ref"), ref.buf);
 	if (force && hashcmp(prev, object))
 		printf(_("Updated tag '%s' (was %s)\n"), tag, find_unique_abbrev(prev, DEFAULT_ABBREV));
 
 	strbuf_release(&buf);
+	strbuf_release(&ref);
 	return 0;
 }