Re: Server crash when selecting from pg_cursors

PetSerAl <petseral(at)gmail(dot)com> writes:
> postgres=# CREATE TABLE t (a integer, b integer);
> CREATE TABLE
> postgres=# CREATE FUNCTION f() RETURNS integer
> postgres-# STABLE STRICT LANGUAGE plpgsql
> postgres-# AS $$
> postgres$# BEGIN
> postgres$# PERFORM FROM pg_cursors;
> postgres$# RETURN null;
> postgres$# END
> postgres$# $$;
> CREATE FUNCTION
> postgres=# DO $$
> postgres$# DECLARE
> postgres$# a integer;
> postgres$# BEGIN
> postgres$# FOR a IN SELECT t.a FROM t WHERE t.b = f() LOOP
> postgres$# END LOOP;
> postgres$# END
> postgres$# $$;
> server closed the connection unexpectedly

Huh, nice one. It's not new in v17 though --- it seems quite ancient.
The proximate cause is that pg_cursor() is assuming portal->sourceText
can't be NULL:

values[1] = CStringGetTextDatum(portal->sourceText);

which is not unreasonable considering that portal.h quoth

const char *sourceText; /* text of query (as of 8.4, never NULL) */

But there's a problem: we may be examining a portal that has been
added to the portal hashtable, but for which PortalDefineQuery
hasn't been called yet.

There are a few ways we might try to deal with this, but I think
the most reasonable one is to make pg_cursor() ignore portals
that haven't been defined yet, as attached.

regards, tom lane