sha1_file: introduce an nth_packed_object_oid function

There are places in the code where we would like to provide a struct
object_id *, yet read the hash directly from the pack.  Provide an
nth_packed_object_oid function that is similar to the
nth_packed_object_sha1 function.

In order to avoid a potentially invalid cast, nth_packed_object_oid
provides a variable into which to store the value, which it returns on
success; on error, it returns NULL, as nth_packed_object_sha1 does.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1608,6 +1608,12 @@ extern void check_pack_index_ptr(const struct packed_git *p, const void *ptr);
  * error.
  */
 extern const unsigned char *nth_packed_object_sha1(struct packed_git *, uint32_t n);
+/*
+ * Like nth_packed_object_sha1, but write the data into the object specified by
+ * the the first argument.  Returns the first argument on success, and NULL on
+ * error.
+ */
+extern const struct object_id *nth_packed_object_oid(struct object_id *, struct packed_git *, uint32_t n);
 
 /*
  * Return the offset of the nth object within the specified packfile.

sha1_file.c

@@ -2628,6 +2628,17 @@ const unsigned char *nth_packed_object_sha1(struct packed_git *p,
 	}
 }
 
+const struct object_id *nth_packed_object_oid(struct object_id *oid,
+					      struct packed_git *p,
+					      uint32_t n)
+{
+	const unsigned char *hash = nth_packed_object_sha1(p, n);
+	if (!hash)
+		return NULL;
+	hashcpy(oid->hash, hash);
+	return oid;
+}
+
 void check_pack_index_ptr(const struct packed_git *p, const void *vptr)
 {
 	const unsigned char *ptr = vptr;
@@ -3788,13 +3799,13 @@ static int for_each_object_in_pack(struct packed_git *p, each_packed_object_fn c
 	int r = 0;
 
 	for (i = 0; i < p->num_objects; i++) {
-		const unsigned char *sha1 = nth_packed_object_sha1(p, i);
+		struct object_id oid;
 
-		if (!sha1)
+		if (!nth_packed_object_oid(&oid, p, i))
 			return error("unable to get sha1 of object %u in %s",
 				     i, p->pack_name);
 
-		r = cb(sha1, p, i, data);
+		r = cb(oid.hash, p, i, data);
 		if (r)
 			break;
 	}