feat: Secure Parameters support for the Java Client

[aseering]

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Fixes #<issue_number_goes_here> ☕️

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[gemini-code-assist]

Summary of Changes

Hello @aseering, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the Java Spanner client library by adding robust support for ClientContext. This feature enables users to embed secure, context-specific metadata with their database requests, facilitating advanced use cases like secure views that require dynamic, user-specific parameters. The implementation provides flexible configuration options and ensures seamless integration and correct behavior across the client's various operational layers.

Highlights

• ClientContext Support: Introduced a new ClientContext option within RequestOptions to allow passing side-channel or configuration information to the backend, such as user IDs for parameterized secure views.
• Flexible Configuration: The ClientContext can now be configured at multiple levels: as a default in SpannerOptions, on a Connection object, or as an Options parameter for individual queries and updates, with merging logic to handle precedence.
• API Integration: Integrated ClientContext propagation throughout the client library, affecting request building in AbstractReadContext, SessionImpl, TransactionRunnerImpl, and various UnitOfWork implementations in the connection layer.
• Public API Extension: Added setClientContext and getClientContext methods to the Connection interface, providing a public API for managing the client context on a per-connection basis.
• Comprehensive Testing: New and updated unit tests, including a dedicated ClientContextMockServerTest, ensure correct propagation, merging, and persistence of ClientContext across different operations and configuration scenarios.

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[gemini-code-assist]

Code Review

This pull request introduces support for ClientContext in the Spanner Java client, enabling the use of secure parameters. The changes add a clientContext option at both the global SpannerOptions level and the per-request Options level, with logic to merge them. My review identifies a potential issue where the default ClientContext is not being applied to CommitRequests, which is inconsistent with other request types. I've also included a couple of suggestions to refactor duplicated and verbose code for better maintainability.

I am having trouble creating individual review comments. Click here to see my feedback.

google-cloud-spanner/src/main/java/com/google/cloud/spanner/TransactionRunnerImpl.java (467-470)

The RequestOptions for the CommitRequest are built from per-transaction options but do not appear to merge with the default ClientContext that can be configured on SpannerOptions. This is inconsistent with how BeginTransactionRequest and ExecuteSqlRequest are handled in this PR. This could lead to the ClientContext being unexpectedly missing from commit requests when a default is expected to be applied.

Consider applying the same merge logic used in SessionImpl.beginTransactionAsync here to ensure consistent behavior.

          RequestOptions requestOptions = options.toRequestOptionsProto(true);
          RequestOptions.ClientContext defaultClientContext =
              session.getSpanner().getOptions().getClientContext();
          if (defaultClientContext != null) {
            RequestOptions.ClientContext.Builder clientContextBuilder =
                defaultClientContext.toBuilder();
            if (requestOptions.hasClientContext()) {
              clientContextBuilder.mergeFrom(requestOptions.getClientContext());
            }
            requestOptions =
                requestOptions.toBuilder().setClientContext(clientContextBuilder.build()).build();
          }
          if (!requestOptions.equals(RequestOptions.getDefaultInstance())) {
            requestBuilder.setRequestOptions(requestOptions);
          }

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SessionImpl.java (488-497)

The logic for merging the default ClientContext from SpannerOptions with the request-specific ClientContext is also present in AbstractReadContext.buildRequestOptions. To improve maintainability and reduce code duplication, consider extracting this logic into a shared helper method.

google-cloud-spanner/src/main/java/com/google/cloud/spanner/connection/ConnectionImpl.java (2091-2098)

The logic for appending the clientContext as an UpdateOption is a bit verbose. For consistency with mergeQueryRequestOptions, which uses an appendQueryOption helper, consider creating and using a similar helper method like appendUpdateOption.

    if (clientContext != null) {
      options = appendUpdateOption(options, Options.clientContext(clientContext));
    }