Re: Error-safe user functions

> +{ oid => '8053',
> +  descr => 'get error message if string is not valid input for data type',
> +  proname => 'pg_input_invalid_message', provolatile => 's',
> +  prorettype => 'text', proargtypes => 'text regtype int4',
> +  prosrc => 'pg_input_invalid_message_mod' },
> +

Is there a guarantee that input functions are stable or immutable? We don't
have any volatile input functions in core PG:

SELECT provolatile, count(*) FROM pg_proc WHERE oid IN (SELECT typinput FROM pg_type) GROUP BY provolatile;