DROP FUNCTION failure: cache lookup failed for relation X

I've found a situation that causes DROP FUNCTION to fail (tested
in 8.1.6, 8.2.1, and 8.3devel):

CREATE TABLE foo (id integer);

CREATE FUNCTION foofunc() RETURNS trigger AS $$
BEGIN
RETURN NEW;
END;
$$ LANGUAGE plpgsql;

Then in concurrent sessions:

A: BEGIN;

A: CREATE TRIGGER footrig BEFORE INSERT ON foo
FOR EACH ROW EXECUTE PROCEDURE foofunc();

B: DROP TABLE foo; -- blocks pending A's commit

A: COMMIT; -- B's DROP TABLE completes

A: SELECT tgrelid FROM pg_trigger WHERE tgname = 'footrig';
tgrelid
---------
66153
(1 row)

A: DROP FUNCTION foofunc();
ERROR: cache lookup failed for relation 66153

Apparently the row in pg_trigger that A committed wasn't deleted
by B's DROP TABLE, presumably because B didn't have visibility to
to the trigger when its DROP TABLE statement began. This case is
admittedly contrived but I did stumble across it in a test environment.

--
Michael Fuhr