feat(auth): add OAuth2/OIDC authentication package

[cpunion]

Add AuthConfig, AuthCredential, AuthScheme types
Add AuthHandler for OAuth2 flow orchestration
Add CredentialManager and CredentialService for token management
Add OAuth2 token exchange helpers
Add unit tests for auth package

[gemini-code-assist]

Summary of Changes

Hello @cpunion, 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 introduces a robust and comprehensive authentication package designed to handle OAuth2 and OpenID Connect flows. It establishes core data structures for authentication configuration and credentials, provides a dedicated handler for orchestrating OAuth flows, and includes a manager for the entire credential lifecycle, from initial acquisition to refreshing expired tokens. This foundational work aims to streamline and standardize how authentication is handled across various tools within the system, ensuring secure and efficient credential management.

Highlights

• New Authentication Package: Introduced a new auth package to centralize and standardize authentication mechanisms, specifically for OAuth2 and OpenID Connect.
• Core Authentication Types: Added fundamental types such as AuthConfig (combines scheme and credentials), AuthCredential (holds various credential data like API keys, HTTP, OAuth2, Service Account), and AuthScheme (interface for different security schemes like APIKey, HTTP, OAuth2, OpenID Connect).
• OAuth2 Flow Orchestration: Implemented AuthHandler to manage the OAuth2/OIDC authorization flow, including generating authorization URIs and processing authentication responses.
• Credential Lifecycle Management: Developed CredentialManager to orchestrate the full lifecycle of credentials, from initial loading and validation to token exchange (e.g., auth code to access token), refreshing expired tokens, and saving credentials. This manager integrates with CredentialService for persistent storage.
• Credential Services and Registries: Provided CredentialService interfaces and in-memory implementations (InMemoryCredentialService, SessionStateCredentialService) for storing and retrieving credentials. Also introduced ExchangerRegistry and RefresherRegistry to manage different credential exchange and refresh strategies.
• Helper Functions and Utilities: Included helper functions for easily creating common authentication configurations (e.g., TokenToSchemeCredential, BearerTokenCredential, OAuth2ClientCredentials, ServiceAccountCredentials, DefaultCredentials).
• Updated Dependencies: Updated the golang.org/x/oauth2 dependency to v0.34.0 and added gopkg.in/yaml.v3 to go.mod and go.sum.

🧠 New Feature in Public Preview: You can now enable Memory to help Gemini Code Assist learn from your team's feedback. This makes future code reviews more consistent and personalized to your project's style. Click here to enable Memory in your admin console.

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description
Code Review	/gemini review	Performs a code review for the current pull request in its current state.
Pull Request Summary	/gemini summary	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in pull request comments and review comments.
Help	/gemini help	Displays a list of available commands.

Customization

To customize Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counter productive. You can react with 👍 and 👎 on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for Github and other Google products, sign up here.

You can also get AI-powered code generation, chat, as well as code reviews directly in the IDE at no cost with the Gemini Code Assist IDE Extension.

Footnotes

1. Review the Privacy Notices, Generative AI Prohibited Use Policy, Terms of Service, and learn how to configure Gemini Code Assist in GitHub here. Gemini can make mistakes, so double check it and use code with caution. ↩

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive authentication package for handling OAuth2/OIDC flows. The code is well-structured, with clear separation of concerns between configuration, credentials, schemes, handlers, and services. The implementation correctly uses the golang.org/x/oauth2 library for standard flows and includes thorough unit tests. I have a few suggestions to improve robustness and maintainability. The most critical issue is related to non-deterministic credential key generation, which could prevent reliable caching. I've also noted an opportunity to make the error handling in the random state generator more library-friendly and to improve the maintainability of the deep copy logic for credentials.