Atomically get/gen a Base and incref.
        (requires ctor during BaseRef init)

        Args:
            name (str): The name/iden of the Base instance.

async def gen(self, name):
        '''
        Atomically get/gen a Base and incref.
        (requires ctor during BaseRef init)

        Args:
            name (str): The name/iden of the Base instance.
        '''
        if self.ctor is None:
            raise s_exc.NoSuchCtor(name=name, mesg='BaseRef.gen() requires ctor')

        base = self.base_by_name.get(name)

        if base is None:
            base = await self.ctor(name)
            self.put(name, base)
        else:
            base.incref()

        return base