read-lock.py

import time
import psycopg2
import concurrent.futures


def first():
    conn = psycopg2.connect("dbname=buildhub_copy")
    with conn.cursor() as cur:
        cur.execute("""
        delete from records where parent_id='peter'
        """)
        timestamp = 1523875992003
        cur.execute("""
            insert into records (id, parent_id, collection_id, data, last_modified, deleted)
            values ('firefox_beta_58-0b15rc1_linux-i686_cs2', 'peter', 'record', ('{}')::JSONB,
                    from_epoch(%(timestamp)s),
                 FALSE)
        """, {'timestamp': timestamp})
    conn.commit()


def read_or_mess(i):
    # print('!', i)
    conn = psycopg2.connect("dbname=buildhub_copy")


    # If you COMMENT OUT this thing, the output will be:
    #
    #    1) RESULT: Deleted!
    #    0) COUNT: 1	#RECORDS: 0
    #
    # I.e. Wrong! The count doesn't match the number of records.
    #
    if i == 0:
        conn.set_session(isolation_level='REPEATABLE READ')


    with conn.cursor() as cur:
        if i == 0:
            cur.execute("""
                select count(*) from records
                WHERE parent_id='peter'
            """)
            count_total, = cur.fetchone()
            if i == 0:
                time.sleep(1)
            cur.execute("""
                select id from records
                WHERE parent_id='peter'
            """)
            records = cur.fetchmany()
            conn.commit()
            return (count_total, records)
        else:
            cur.execute("""
                delete from records where parent_id='peter'
            """)
            conn.commit()
            return "Deleted!"



first()
time.sleep(1)

with concurrent.futures.ThreadPoolExecutor() as executor:
    future_to_url = {executor.submit(read_or_mess, i): i for i in range(2)}
    for future in concurrent.futures.as_completed(future_to_url):
        i = future_to_url[future]
        result = future.result()
        if i == 0:
            print('{}) COUNT: {}\t#RECORDS: {}'.format(i, result[0], len(result[1])))
        else:
            print("{}) RESULT: {}".format(i, result))