Ceph » rgw

while (!marker_tracker->can_do_op(key)) {
  if (!updated_status) {
    set_status() << "can't do op, conflicting inflight operation";
    updated_status = true;
  }
  ldout(sync_env->cct, 5) << *this << ": [inc sync] can't do op on key=" << key << " need to wait for conflicting operation to complete" << dendl;
  yield wait_for_child();
  bool again = true;
  while (again) {
    again = collect(&ret, lease_stack);
    if (ret < 0) {
      ldout(sync_env->cct, 0) << "ERROR: a child operation returned error (ret=" << ret << ")" << dendl;
      sync_status = ret;
      /* we have reported this error */
    }
  }
}

the previous batch's RGWBucketSyncSingleEntryCR with the same key is done with error,but we do not find the error because it is in spawn_window.
so the key left in marker_tracker->key_to_marker.
the loop will never break.