Fix connections.py: allow to connect to Redis using a Unix socket URL…

[drygdryg]

Allow to connect to Redis using a Unix socket URL without specifying the database number using querystring parameter. Currently, when trying to connect via a Unix socket, specifying a URL without a db number, we get a parsing error. This fix allows to skip specifying the db number and set db=0 by default . This code demonstrates buggy behavior of the URL parser:

from arq.connections import RedisSettings

settings = RedisSettings.from_dsn('unix:///run/redis/redis.sock')

Causes error:

Traceback (most recent call last):
  File "test.py", line 3, in <module>
    settings = RedisSettings.from_dsn('unix:///run/redis/redis.sock')
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv/lib/python3.11/site-packages/arq/connections.py", line 59, in from_dsn
    database = int(conf.path.lstrip('/')) if conf.path else 0
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: invalid literal for int() with base 10: 'run/redis/redis.sock'

While this works correcrly:

from arq.connections import RedisSettings

settings = RedisSettings.from_dsn('unix:///run/redis/redis.sock?db=0')