def dispatch_shell(self, stream, msg):
        """dispatch shell requests"""
        # flush control requests first
        if self.control_stream:
            self.control_stream.flush()
        
        idents,msg = self.session.feed_identities(msg, copy=False)
        try:
            msg = self.session.unserialize(msg, content=True, copy=False)
        except:
            self.log.error("Invalid Message", exc_info=True)
            return

        header = msg['header']
        msg_id = header['msg_id']
        msg_type = msg['header']['msg_type']
        
        # Print some info about this message and leave a '--->' marker, so it's
        # easier to trace visually the message chain when debugging.  Each
        # handler prints its message at the end.
        self.log.debug('\n*** MESSAGE TYPE:%s***', msg_type)
        self.log.debug('   Content: %s\n   --->\n   ', msg['content'])

        if msg_id in self.aborted:
            self.aborted.remove(msg_id)
            # is it safe to assume a msg_id will not be resubmitted?
            reply_type = msg_type.split('_')[0] + '_reply'
            status = {'status' : 'aborted'}
            sub = {'engine' : self.ident}
            sub.update(status)
            reply_msg = self.session.send(stream, reply_type, subheader=sub,
                        content=status, parent=msg, ident=idents)
            return
        
        handler = self.shell_handlers.get(msg_type, None)
        if handler is None:
            self.log.error("UNKNOWN MESSAGE TYPE: %r", msg_type)
        else:
            # ensure default_int_handler during handler call
            sig = signal(SIGINT, default_int_handler)
            try:
                handler(stream, idents, msg)
            except Exception:
                self.log.error("Exception in message handler:", exc_info=True)
            finally:
                signal(SIGINT, sig)