BUG #5621: Insufficient locking of dependent objects

The following bug has been logged online:

Bug reference: 5621
Logged by: Caleb Welton
Email address: cwelton(at)greenplum(dot)com
PostgreSQL version: 9.1devel
Operating system: OSX 10.6.4
Description: Insufficient locking of dependent objects
Details:

There is a general class of concurrency bugs related to insufficient
locking
of dependent objects. This can occur with just about any inter-object
dependency that exists in the system:

Creating a table while dropping the schema it is in
Creating a table while dropping a type it depends on
Creating a function while dropping the language
Creating an aggregate while dropping the transition function.
Etc.

Here are two cases that represent the class of problems:

1) Table/Schema dependencies:

Session 1:
CREATE SCHEMA DROPME;
BEGIN;
CREATE TABLE DROPME.TEST(a int);

Session 2:
DROP SCHEMA DROPME;

Session 1:
COMMIT;

SELECT relname
FROM pg_class c
LEFT OUTER JOIN pg_namespace n
ON (n.oid = c.relnamespace)
WHERE n.oid is null;

Expected output of final query is zero rows (every table should have a
schema)
actual output is:

relname
---------
test
(1 row)

2) Table/Type dependencies

Session 1:
CREATE TYPE dropme as (a int, b int);
BEGIN;
CREATE TABLE test(a dropme);

Session 2:
DROP TYPE dropme;

Session 1:
COMMIT;
SELECT * FROM test;

Output of final select is:

ERROR: cache lookup failed for type 16390

Problem exists in all versions of Postgtres from at least 8.2 through 9.1
devel.