It was previously possible to confuse gnbd by sending a process a signal while
    gnbd was writing to the socket (bz 210453). This usually hangs the socket, but
    can possibly cause data corruption. To fix this gnbd now only handles signals
    for gnbd_recvd.  GNBD blocks all signals for all other processes while it
    is going socket IO, so that a partial IO will never be send to the server,
    except when the connection is lost.