Old Style vs. New Style Dissector

How does Wireshark determine the style of dissector (new or old) ? For example the code below, how is "handle->is_new" true or false?

    /* This function will return
 * old style dissector :
 *   length of the payload or 1 of the payload is empty
 * new dissector :
 *   >0  this protocol was successfully dissected and this was this protocol.
 *   0   this packet did not match this protocol.
 *
 * The only time this function will return 0 is if it is a new style dissector
 * and if the dissector rejected the packet.
 */
call_dissector_through_handle(dissector_handle_t handle, tvbuff_t *tvb,
                  packet_info *pinfo, proto_tree *tree, void *data)
{
……
if (handle->is_new) {
    EP_CHECK_CANARY(("before calling handle->dissector.new_d for %s",handle->name));
    ret = (*handle->dissector.new_d)(tvb, pinfo, tree, data);
    EP_CHECK_CANARY(("after calling handle->dissector.new_d for %s",handle->name));
} else {
    EP_CHECK_CANARY(("before calling handle->dissector.old for %s",handle->name));

subdissector */
(handle->dissector.old)(tvb, pinfo, tree);
EP_CHECK_CANARY(("after calling handle->dissector.old for %s",handle->name));
ret = tvb_length(tvb);
if (ret == 0) {
/
* XXX - a tvbuff can have 0 bytes of data in
* it, so we have to make sure we don't return
* 0.
*/
ret = 1;
}