Parsed test spec with 2 sessions

starting permutation: w1_complete w2_fetch w2_commit
step w1_complete: 
    UPDATE ulak.queue
    SET status = 'completed', completed_at = NOW()
    WHERE ordering_key = 'order-X'
      AND status = 'processing';

step w2_fetch: 
    WITH blocked_keys AS (
        SELECT DISTINCT ordering_key FROM ulak.queue
        WHERE ordering_key IS NOT NULL AND status = 'processing'
    )
    SELECT payload::text, ordering_key
    FROM ulak.queue q
    WHERE q.status = 'pending'
      AND (q.ordering_key IS NULL
           OR (q.ordering_key NOT IN (SELECT ordering_key FROM blocked_keys)
               AND NOT EXISTS (
                   SELECT 1 FROM ulak.queue q2
                   WHERE q2.ordering_key = q.ordering_key
                     AND q2.status = 'pending'
                     AND q2.id < q.id)))
    ORDER BY q.id
    FOR UPDATE SKIP LOCKED;

payload        |ordering_key
---------------+------------
{"key": "X2"}  |order-X     
{"key": "Y1"}  |order-Y     
{"key": "free"}|            
(3 rows)

step w2_commit: COMMIT;