[batch] Use _batch_client() to query supported_regions() and default_region()

[jmarshall]

Change Description

As default_region() is used in __init__(), it needs to work in all scenarios, including those in which credentials are provided via __init__'s token parameter. Thus it will need self._token so can't be a staticmethod, so it might as well reuse _batch_client() and its session.

Do the same with supported_regions() for good measure.

Lift self.__batch_client=None to the top so it occurs before anything might use _batch_client(), e.g., our self.default_region() call. This also ensures the __batch_client attribute exists within __del__() even if __init__() raises an exception, preventing a common red herring warning.

Security Assessment

• This change potentially impacts the Hail Batch instance as deployed by Broad Institute in GCP

Impact Rating

• This change has a low security impact

Impact Description

Some requests that currently fail due to being entirely unauthenticated will be tried using the authentication token the caller intends to use instead, and will have an opportunity to succeed.

Appsec Review

• Required: The impact has been assessed and approved by appsec

[cjllanwarne]

My comments are mostly "nit" level so feel free to push back. But it looks like you ran into trouble with the static analysis (unused BatchClient import) and doctest (service_backend is not defined) so I figured I might as well comment and let you decide what to do about these ones too.

[jmarshall]

trouble with the static analysis (unused BatchClient import)

Oops. I suppose now that that's unused and so not imported we could stop renaming the other BatchClient in this source file:

from hailtop.batch_client.aioclient import BatchClient as AioBatchClient
#                                                      ^^^^^^^^^^^^^^^^^

but doing that might be more confusing…