Multi table validations exhausted PostgreSQL connections

[nj1973]

When running a single validate command for many tables it was noticed that many PostgreSQL connections are opened, the command failed with the error below:

File "/usr/lib/python3.10/site-packages/sqlalchemy/engine/default.py", line 598, in connect
    return self.dbapi.connect(*cargs, **cparams)
  File "/usr/lib/python3.10/site-packages/psycopg2/__init__.py", line 122, in connect
    conn = _connect(dsn, connection_factory=connection_factory, **kwasync)
psycopg2.OperationalError: connection to server at "127.0.0.1", port 5432 failed: FATAL:  remaining connection slots are reserved for non-replication superuser connections

I've tested this on PostgreSQL and Oracle and can see the connection count climbing until the command eventually completes and all connections are released.

[nj1973]

Helper information (based on my DVT username starting with a specific string).

To monitor PostgreSQL connections I used:

SELECT pid,wait_event,state,query FROM pg_stat_activity where usename like 'dvt%';
\watch 1

To monitor Oracle connections I used:

column event format a30
column module format a10 trunc
column action format a10 trunc
set lines 200 tab off pages 100
SELECT sid,event,module,action,state FROM v$session where username like 'DVT%';

[nj1973]

Since merging this change we are occasionally seeing this warning from DVT:

07/25/2024 09:26:57 AM-WARNING: Exception closing connections: 'Backend' object has no attribute 'con'

We need to make the original fix more robust.