Re: Server crash when selecting from pg_cursors

PetSerAl <petseral@gmail.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

diff --git a/src/backend/utils/mmgr/portalmem.c b/src/backend/utils/mmgr/portalmem.c
index 4a24613537..93137820ac 100644
--- a/src/backend/utils/mmgr/portalmem.c
+++ b/src/backend/utils/mmgr/portalmem.c
@@ -1150,6 +1150,9 @@ pg_cursor(PG_FUNCTION_ARGS)
         /* report only "visible" entries */
         if (!portal->visible)
             continue;
+        /* also ignore it if PortalDefineQuery hasn't been called yet */
+        if (!portal->sourceText)
+            continue;

         values[0] = CStringGetTextDatum(portal->name);
         values[1] = CStringGetTextDatum(portal->sourceText);