Add support for password functions (useful for RDS IAM auth)

[HLFrye]

Resolves this issue: #553

[elprans]

Please also add support for coroutines (via inspect.iscoroutinefunction()) to support async callbacks.

[HLFrye]

Good point, updated the branch with support for async password callbacks and an additional test for that case as well.

[elprans]

Looks like flake8 is unhappy about the extra newline here

[elprans]

... and here

[elprans]

LGTM. Please add a note about this to the connect() docstring. Thanks!

[Rigdon]

@HLFrye Thanks so much for this change. I have a question though regarding RDS IAM before I open a new issue. I haven't been able to get this to connect to our RDS instance despite our callable generating valid tokens. I actually can't figure out how to get asyncpg to connect using a generated auth token at all, either by passing our callable or by generating the token and passing it directly into the password kwarg for asyncpg.connect(). I'm consistently getting an auth error:

InvalidAuthorizationSpecificationError: pg_hba.conf rejects connection for host ...

I've confirmed that I can use our same connection code to connect to a different postgres database using a standard password, and also that all of the values used and the token generated are valid and can be used to connect via psycopg2 and psql.

The code is about as simple as possible:

    db_kwargs = dict(
        min_size=5,
        max_size=25,
        host=config["hostname"],
        port=config["port"],
        user=config["username"],
        database=config["dbname"],
        password=config["password"],
        timeout=config["connect_timeout"],
    )
    database = databases.Database(
        url="postgres://",
        **db_kwargs
    )
    await database.connect()

where config["password"] is a callable that call an aiobotocore session's generate_db_auth_token

Any help would be greatly appreciated!

[HLFrye]

@Rigdon I've seen this error message occur when trying to connect without SSL enabled. The answers in this issue (#238) shows the process to set that up.

[Rigdon]

@HLFrye That solved it! I went with the route described here. Thank you so much!