Bug in Double application of Google Creds utility

[ydamit]

The setup_google_application_credentials() utility is applied twice in any use of the GoogleBigQuery connector that uses copy() or any other method that also instantiates the GoogleCloudStorage class.

Both of these classes make use of the setup_google_application_credentials() utility, which works fine the first time it's applied. The result under most conditions is that the GOOGLE_APPLICATION_CREDENTIALS env var is populated with the path to the temporary .json file holding the credentials.

The problem is that the second time this is applied, a temporary file is created whose contents are merely the path to the first temporary file, and the various clients involved don't know what to do with that. They try to parse that nested file path as a JSON string, and of course it throws an error.

I believe this might be most easily fixed by adding another condition to the if block to handle this edge case:

elif credentials.endswith('.json'):
    return True

[ydamit]

CC: @elyse-weiss and @zerlinac

[alxmrs]

I'm happy to take a look at this.

[SorenSpicknall]

Appears to be superseded by updated approach to passing auth information to BQ operations, but feel free to reopen if needed.