[RelDB2] Concurrency-related stuff

[luisa-beerboom]

There seem to be some uncertainties in regards to locking and concurrency. Not necessarily blockers, but uncomfortable things nonetheless. Here's a list of topics:

• See this comment [relDB] Investigate safety of TreeSortMixin #3305 (comment) and the one above.
• test_create_sequential_number.py test_create_sequential_numbers_race_condition problem will need to be investigated (Impulse from @hjanott was to search for dead locks in postgres)
• db_connection_handling.py in create_os_conn_pool, try enabling prepared statements again, according to @hjanott they were disabled due to problems with concurrency.
• system/test_write.py: the two_write_requests_with_locked_fields test deals with locks and is currently skipped. It should be fixed or deleted. It is quite possible that the concept behind the test is obsolete.
• action/test_action_command_format.py: Implement locked fields or delete commented out code. Apparently 95% chance that the test is obsolete.

[luisa-beerboom]

• test_create_sequential_number.py test_create_sequential_numbers_race_condition problem will need to be investigated (Impulse from @hjanott was to search for dead locks in postgres)

how to do it:

alias psql-docker='docker exec -ti -u postgres dev-postgres-1 
bash -c '\''PAGER="less -S" psql -U openslides'\'''

CREATE VIEW lock_monitor AS(
SELECT
  COALESCE(blockingl.relation::regclass::text,blockingl.locktype) as 
locked_item,
  now() - blockeda.query_start AS waiting_duration, blockeda.pid AS 
blocked_pid,
  blockeda.query as blocked_query, blockedl.mode as blocked_mode,
  blockinga.pid AS blocking_pid, blockinga.query as blocking_query,
  blockingl.mode as blocking_mode
FROM pg_catalog.pg_locks blockedl
JOIN pg_stat_activity blockeda ON blockedl.pid = blockeda.pid
JOIN pg_catalog.pg_locks blockingl ON(
  ( (blockingl.transactionid=blockedl.transactionid) OR
  (blockingl.relation=blockedl.relation AND 
blockingl.locktype=blockedl.locktype)
  ) AND blockedl.pid != blockingl.pid)
JOIN pg_stat_activity blockinga ON blockingl.pid = blockinga.pid
  AND blockinga.datid = blockeda.datid
WHERE NOT blockedl.granted
AND blockinga.datname = current_database()
);
SELECT * from lock_monitor;