start the transaction with a re-used connection
        return a response object (r) upon success or None on failure.
        This DOES not close or remove bad connections in cases where
        it returns.  However, if an unexpected exception occurs, it
        will close and remove the connection before re-raising.

def _reuse_connection(self, h, req, host):
        """start the transaction with a re-used connection
        return a response object (r) upon success or None on failure.
        This DOES not close or remove bad connections in cases where
        it returns.  However, if an unexpected exception occurs, it
        will close and remove the connection before re-raising.
        """
        try:
            self._start_transaction(h, req)
            r = h.getresponse()
            # note: just because we got something back doesn't mean it
            # worked.  We'll check the version below, too.
        except (socket.error, httplib.HTTPException):
            r = None
        except:
            # adding this block just in case we've missed
            # something we will still raise the exception, but
            # lets try and close the connection and remove it
            # first.  We previously got into a nasty loop
            # where an exception was uncaught, and so the
            # connection stayed open.  On the next try, the
            # same exception was raised, etc.  The tradeoff is
            # that it's now possible this call will raise
            # a DIFFERENT exception
            if DEBUG: DEBUG.error("unexpected exception - closing " + \
                                  "connection to %s (%d)", host, id(h))
            self._cm.remove(h)
            h.close()
            raise
                    
        if r is None or r.version == 9:
            # httplib falls back to assuming HTTP 0.9 if it gets a
            # bad header back.  This is most likely to happen if
            # the socket has been closed by the server since we
            # last used the connection.
            if DEBUG: DEBUG.info("failed to re-use connection to %s (%d)",
                                 host, id(h))
            r = None
        else:
            if DEBUG: DEBUG.info("re-using connection to %s (%d)", host, id(h))

        return r