handle http server close gracefully in idp

[havetisyan]

Description

In the idp http flow, sometimes, the http server doesn't stay alive long enough to serve the /close page. The solution uses an intermediate authCode channel inside registerHandlers:

/oauth2/callback stores the raw query in the internal authCode channel, then redirects to /close -- the auth code is no longer visible in the browser URL bar
/close serves the HTML page, then drains authCode and forwards it to the outer code channel -- this triggers the shutdown goroutine
Since server.Shutdown() is graceful, it waits for the in-flight /close handler to finish writing its response before actually stopping the server

Contribution Checklist:

• The pull request does not introduce any breaking changes
• I have read the contribution guidelines.
• Create an issue and link to the pull request.

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[gemini-code-assist]

Summary of Changes

Hello, 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 addresses a critical issue in the IDP HTTP flow where the server could terminate prematurely, preventing the /close page from being successfully served. The solution introduces an intermediate channel to manage the authentication code transfer, ensuring that the /close handler can complete its operation and trigger a graceful server shutdown. This change enhances the robustness and reliability of the IDP authentication process by preventing race conditions related to server closure.

Highlights

• Graceful HTTP Server Shutdown: Implemented a mechanism to ensure the HTTP server gracefully shuts down, specifically allowing the /close page handler to complete its response before the server stops, resolving a potential race condition.
• Intermediate authCode Channel: Introduced a new buffered channel, authCode, within registerHandlers to temporarily store the OAuth2 callback query parameters.
• OAuth2 Callback Handler Update: Modified the /oauth2/callback handler to store the raw query in the authCode channel and then issue an HTTP redirect to the /close endpoint, preventing the auth code from being visible in the browser URL bar.
• Close Page Handler Enhancement: Updated the /close handler to retrieve the authentication code from the authCode channel, forward it to the main code channel, and then close the main code channel, triggering the server shutdown process.
• Test Suite Expansion: Added a new test, TestRegisterHandlersCallbackFollowRedirect, to thoroughly validate the end-to-end flow, including the redirect from /oauth2/callback to /close and the subsequent processing of the authentication code.

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[gemini-code-assist]

Code Review

This pull request refactors the IdP authentication flow to more gracefully handle HTTP server shutdown, resolving a race condition. The approach of using an intermediate channel to delay the shutdown signal until after the final page is served is sound. The changes are accompanied by good test coverage. I have a couple of suggestions to improve the robustness of the implementation and its tests.