BUG #5621: Insufficient locking of dependent objects

The following bug has been logged online:

Bug reference:      5621
Logged by:          Caleb Welton
Email address:      cwelton@greenplum.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.