Fix rbd list lockers crashing

Signed-off-by: Zhi Zhang <zhangz.david@outlook.com>

rbd/rbd.go

@@ -508,30 +508,47 @@ func (image *Image) ListLockers() (tag string, lockers []Locker, err error) {
 
 	var c_exclusive C.int
 	var c_tag_len, c_clients_len, c_cookies_len, c_addrs_len C.size_t
+	var c_locker_cnt C.ssize_t
 
 	C.rbd_list_lockers(image.image, &c_exclusive,
 		nil, (*C.size_t)(&c_tag_len),
 		nil, (*C.size_t)(&c_clients_len),
 		nil, (*C.size_t)(&c_cookies_len),
 		nil, (*C.size_t)(&c_addrs_len))
+	
+	// no locker held on rbd image when either c_clients_len, 
+	// c_cookies_len or c_addrs_len is *0*, so just quickly returned 
+	if int(c_clients_len) == 0 || int(c_cookies_len) == 0 || 
+		int(c_addrs_len) ==0 {
+		lockers = make([]Locker, 0)
+		return "", lockers, nil 
+	}
 
 	tag_buf := make([]byte, c_tag_len)
 	clients_buf := make([]byte, c_clients_len)
 	cookies_buf := make([]byte, c_cookies_len)
 	addrs_buf := make([]byte, c_addrs_len)
-
-	C.rbd_list_lockers(image.image, &c_exclusive,
+	
+	c_locker_cnt = C.rbd_list_lockers(image.image, &c_exclusive,
 		(*C.char)(unsafe.Pointer(&tag_buf[0])), (*C.size_t)(&c_tag_len),
 		(*C.char)(unsafe.Pointer(&clients_buf[0])), (*C.size_t)(&c_clients_len),
 		(*C.char)(unsafe.Pointer(&cookies_buf[0])), (*C.size_t)(&c_cookies_len),
 		(*C.char)(unsafe.Pointer(&addrs_buf[0])), (*C.size_t)(&c_addrs_len))
+	
+	// rbd_list_lockers returns negative value for errors 
+	// and *0* means no locker held on rbd image.
+	// but *0* is unexpected here because first rbd_list_lockers already 
+	// dealt with no locker case 
+	if int(c_locker_cnt) <= 0 {
+		return "", nil, RBDError(int(c_locker_cnt))
+	}
 
 	clients := split(clients_buf)
 	cookies := split(cookies_buf)
 	addrs := split(addrs_buf)
 
-	lockers = make([]Locker, c_clients_len)
-	for i := 0; i < int(c_clients_len); i++ {
+	lockers = make([]Locker, c_locker_cnt)
+	for i := 0; i < int(c_locker_cnt); i++ {
 		lockers[i] = Locker{Client: clients[i],
 			Cookie: cookies[i],
 			Addr:   addrs[i]}