Parsed test spec with 4 sessions

starting permutation: s1_advlock s2_update s3_update s4_update s1_chain s1_begin s1_grablock s1_advunlock1 s1_advunlock2 s1_advunlock3 s1_commit
step s1_advlock: 
    SELECT pg_advisory_lock(142857),
        pg_advisory_lock(285714),
        pg_advisory_lock(571428);

pg_advisory_lock|pg_advisory_lock|pg_advisory_lock
----------------+----------------+----------------
                |                |                
(1 row)

step s2_update: UPDATE pktab SET data = DEFAULT WHERE pg_advisory_lock_shared(142857) IS NOT NULL; <waiting ...>
step s3_update: UPDATE pktab SET data = DEFAULT WHERE pg_advisory_lock_shared(285714) IS NOT NULL; <waiting ...>
step s4_update: UPDATE pktab SET data = DEFAULT WHERE pg_advisory_lock_shared(571428) IS NOT NULL; <waiting ...>
step s1_chain: UPDATE pktab SET data = DEFAULT;
step s1_begin: BEGIN;
step s1_grablock: SELECT * FROM pktab FOR KEY SHARE;
id|data
--+----
 1|   2
(1 row)

step s1_advunlock1: SELECT pg_advisory_unlock(142857); <waiting ...>
step s2_update: <... completed>
step s1_advunlock1: <... completed>
pg_advisory_unlock
------------------
t                 
(1 row)

step s1_advunlock2: SELECT pg_advisory_unlock(285714); <waiting ...>
step s3_update: <... completed>
step s1_advunlock2: <... completed>
pg_advisory_unlock
------------------
t                 
(1 row)

step s1_advunlock3: SELECT pg_advisory_unlock(571428); <waiting ...>
step s4_update: <... completed>
step s1_advunlock3: <... completed>
pg_advisory_unlock
------------------
t                 
(1 row)

step s1_commit: COMMIT;