Lock conflict behavior?

Hi,

I'm wondering if following behavior of PostgreSQL regarding lock
conflict is an expected one. Here's a scenario:

Session A:BEGIN;SELECT * FROM  pg_class limit 1; -- acquires access share lock

Session B:BEGIN;ALTER TABLE pg_class ....;    -- waits for acquiring access                   exclusive lock(wil fail
anywaythough)
 
Session C:SELECT * FROM pg_class...;    -- whatever query which needs                   to acces pg_class will be
           blocked, too bad...
 

I understand that B should wait for aquiring lock, but Should C wait
for?

Also, it seems that an attacker could do a denial service attack if he
could open session A and B, since other users on session C or
following sessions will be blocked.
--
Tatsuo Ishii
SRA OSS, Inc. Japan