Re: Explicit NULL dereference (src/backend/utils/adt/ruleutils.c)

At Sun, 01 Nov 2020 21:05:29 -0500, Tom Lane <tgl@sss.pgh.pa.us> wrote in
> Kyotaro Horiguchi <horikyota.ntt@gmail.com> writes:
> > We cannot reach there with ev_action == NULL since it comes from a
> > non-nullable column. Since most of the other columns has an assertion
> > that !isnull, I think we should do the same thing for ev_action (and
> > ev_qual).  SPI_getvalue() returns C-NULL for SQL-NULL (or for some
> > other unexpected situations.).
>
> Isn't the comment just above there wrong?
>
>       /* these could be nulls */
>
> I wonder just when that became outdated.

Mmm. I investigated that.

At the very beginning of CREATE RULE (d31084e9d1, 1996), InsertRule()
did the following.

>    template = "INSERT INTO pg_rewrite \
>(rulename, ev_type, ev_class, ev_attr, action, ev_qual, is_instead) VALUES \
>('%s', %d::char, %d::oid, %d::int2, '%s'::text, '%s'::text, \
> '%s'::bool);";
>    if (strlen(template) + strlen(rulname) + strlen(actionbuf) +
>       strlen(qualbuf) + 20 /* fudge fac */ >  RULE_PLAN_SIZE) {
>       elog(WARN, "DefineQueryRewrite: rule plan string too big.");
>    }
>    sprintf(rulebuf, template,
>           rulname, evtype, eventrel_oid, evslot_index, actionbuf,
>           qualbuf, is_instead);

Doesn't seem that ev_qual and ev_action can be NULL.  The same
function in the current converts action list to string using
nodeToSTring so NIL is converted into '<>', which is not NULL.

So I think ev_action cannot be null from the beginning of the history
unless the columns is modified manually.  ev_qual and ev_action are
marked as non-nullable (9b39b799db, in 2018). They could be null if we
modified that columns nullable then set NULL, but that could happen on
all other columns in pg_rewite catalog, which are Assert(!null)ed.

Although ev_action cannot be a empty list using SQL interface. So we
can get rid of the case list_length(action) == 0, but I'm not sure
it's worth doing (but the attaches does..).