chore(spanner): source default opts from gapic client

[olavloite]

Sources the default client options from the generated client instead of the hand-written client. This makes sure that new default options that are introduced in the future will automatically be picked up by the Spanner client.

Fixes #3532

[olavloite]

@codyoss @skuruppu While testing something else today, I ran into a slight difference in behavior that was introduced by this change, and I want to check with you whether you consider that a problem:

1. Previously, the hand-written Spanner client would manage the default scopes itself, and set these when a new client was created. Now we source that from the default options of the generated client. The values were equal in both the hand-written and generated client. BUT: The hand-written client would set these by calling WithScopes, while the generated client calls WithDefaultScopes.
2. That means that when a connection is set up the client will now use a self-signed JWT token if the credentials that are used is a service account. Previous to this change, the client would use an OAuth2 flow. This works fine in most cases.
3. However if I use a different endpoint than the default endpoint, and my service account was generated for the default endpoint, I will get an Unauthenticated error. I can circumvent this by also calling WithScopes(...).

So this means that the following code snippet would work prior to this change, but will not work any more when using a service account that has been created for the default endpoint, but that is also authorized for the custom endpoint:

client, err := spanner.NewClient(
    context.Background(),
    "projects/my-project/instances/my-instance/databases/my-database",
    option.WithEndpoint("custom-endpoint.googleapis.com:443"),
)

The following is now needed to make it work:

client, err := spanner.NewClient(
    context.Background(),
    "projects/my-project/instances/my-instance/databases/my-database",
    option.WithEndpoint("custom-endpoint.googleapis.com:443"),
    option.WithScopes("https://www.googleapis.com/auth/spanner.data"),
)

Is this something that we need to do something about?

[codyoss]

@olavloite This sound like it might be a bug. I will investigate and get back to you. Thanks for bringing this to my attention.

[codyoss]

@olavloite This should no longer be an issue on HEAD. Thanks for the report!

[olavloite]

Great, thanks for the update.