[PATCH 1/2] fast-import: don't allow to tag empty branch

[Dmitry Ivankov <divanorama@gmail.com>]

'reset' command makes fast-import start a branch from scratch. It's name
is kept in lookup table but it's sha1 is null_sha1 (special value).
'tag' command can be used to tag a branch by it's name. lookup_branch()
is used it that case and it doesn't check for null_sha1. So fast-import
writes a tag for null_sha1 object instead of giving a error.

Add a check to deny tagging an empty branch and add a corresponding test.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
---
 fast-import.c          |    2 ++
 t/t9300-fast-import.sh |   12 ++++++++++++
 2 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/fast-import.c b/fast-import.c
index 742e7da..c44cc11 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -2717,6 +2717,8 @@ static void parse_new_tag(void)
 	from = strchr(command_buf.buf, ' ') + 1;
 	s = lookup_branch(from);
 	if (s) {
+		if (is_null_sha1(s->sha1))
+			die("Can't tag an empty branch.");
 		hashcpy(sha1, s->sha1);
 		type = OBJ_COMMIT;
 	} else if (*from == ':') {
diff --git a/t/t9300-fast-import.sh b/t/t9300-fast-import.sh
index 1a6c066..0b97d7a 100755
--- a/t/t9300-fast-import.sh
+++ b/t/t9300-fast-import.sh
@@ -820,6 +820,18 @@ test_expect_success \
 	'test 1 = `git rev-list J | wc -l` &&
 	 test 0 = `git ls-tree J | wc -l`'
 
+cat >input <<INPUT_END
+reset refs/heads/J2
+
+tag wrong_tag
+from refs/heads/J2
+data <<EOF
+Tag branch that was reset.
+EOF
+INPUT_END
+test_expect_success \
+	'J: tag must fail on empty branch' \
+	'test_must_fail git fast-import <input'
 ###
 ### series K
 ###
-- 
1.7.3.4