Fix fragile git_oid_ncmp

git_oid_ncmp was making some assumptions about the length of
the data - this shifts the check to the top of the loop so it
will work more robustly, limits the max, and adds some tests
to verify the functionality.

1 files changed, 5 insertions, 2 deletions

diff --git a/src/oid.c b/src/oid.c
index 59c1546d7..4b6699009 100644
--- a/src/oid.c
+++ b/src/oid.c
@@ -176,13 +176,16 @@ int git_oid_ncmp(const git_oid *oid_a, const git_oid *oid_b, size_t len)
 	const unsigned char *a = oid_a->id;
 	const unsigned char *b = oid_b->id;
 
-	do {
+	if (len > GIT_OID_HEXSZ)
+		len = GIT_OID_HEXSZ;
+
+	while (len > 1) {
 		if (*a != *b)
 			return 1;
 		a++;
 		b++;
 		len -= 2;
-	} while (len > 1);
+	};
 
 	if (len)
 		if ((*a ^ *b) & 0xf0)