[PATCH v2 3/3] fast-import: disallow "merge $itself" command

[Dmitry Ivankov <divanorama@gmail.com>]

"merge $itself" may be used to create commits with previous branch tip
being repeated as n-th parent or even moved from being 1-st to be just
n-th. This is not a documented use case and doesn't look like a common
one.

In presence of "from $some" command "merge $itself" acts the same as
"merge $some" would. Which is completely undocumented and looks like
a bug (caused by parse_from() temporarily rewriting b->sha1 with $some).

Just deny "merge $itself" for now. It was a bit broken and btw "from
$itself" was and is a forbidden command too.

Signed-off-by: Dmitry Ivankov <divanorama@gmail.com>
---
 fast-import.c          |   12 +++++++++---
 t/t9300-fast-import.sh |   13 +++++++++++++
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/fast-import.c b/fast-import.c
index f03da1e..781c614 100644
--- a/fast-import.c
+++ b/fast-import.c
@@ -2611,7 +2611,7 @@ static int parse_from(struct branch *b)
 	return 1;
 }
 
-static struct hash_list *parse_merge(unsigned int *count)
+static struct hash_list *parse_merge(unsigned int *count, struct branch *b)
 {
 	struct hash_list *list = NULL, *n, *e = e;
 	const char *from;
@@ -2622,7 +2622,13 @@ static struct hash_list *parse_merge(unsigned int *count)
 		from = strchr(command_buf.buf, ' ') + 1;
 		n = xmalloc(sizeof(*n));
 		s = lookup_branch(from);
-		if (s)
+		if (b == s)
+			/*
+			 * Also if there were a 'from' command, b will point to
+			 * 'from' commit, because parse_from stores it there.
+			 */
+			die("Can't merge a branch with itself: %s", b->name);
+		else if (s)
 			hashcpy(n->sha1, s->sha1);
 		else if (*from == ':') {
 			uintmax_t idnum = parse_mark_ref_eol(from);
@@ -2686,7 +2692,7 @@ static void parse_new_commit(void)
 	parse_data(&msg, 0, NULL);
 	read_next_command();
 	parse_from(b);
-	merge_list = parse_merge(&merge_count);
+	merge_list = parse_merge(&merge_count, b);
 
 	/* ensure the branch is active/loaded */
 	if (!b->branch_tree.tree || !max_active_branches) {
diff --git a/t/t9300-fast-import.sh b/t/t9300-fast-import.sh
index 6f4c988..79cb72a 100755
--- a/t/t9300-fast-import.sh
+++ b/t/t9300-fast-import.sh
@@ -872,6 +872,19 @@ test_expect_success \
 	'git fast-import <input &&
 	git rev-parse --verify J5 &&
 	test_must_fail git rev-parse --verify J5^'
+
+cat >input <<INPUT_END
+commit refs/heads/J5
+committer $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> $GIT_COMMITTER_DATE
+data <<COMMIT
+Merge J5 with itself.
+COMMIT
+merge refs/heads/J5
+
+INPUT_END
+test_expect_success \
+	'J: disallow merge with itself' \
+	'test_must_fail git fast-import <input'
 ###
 ### series K
 ###
-- 
1.7.3.4