Fix slow local startup caused by secret manager initialization

[nick-nlb]

Issue

b/516817315

Description

Following an update to grpcio from 1.76.0 to 1.80.0, we started to see a major lag in Flask server startup when running locally on MacOS.

For reference, the grpcio update was in Website: 6172. It was effected because of a major vulnerability in a library that used grpcio as a dependency.

This change likely altered behavior in terms of default timeouts, retry policies, etc, in such a way that the observed delay began to manifest.

The Fix

There are two parts to this fix. One is to refactor the secret manager client to a singleton. The pattern used here is similar to that found in server/lib/vertex_ai.py (without the threading locking as this only occurs on server start up).

This does solve the problem in itself, but produced warnings because of how Flask works in debug mode. It forks a child worker for actually serving the requests (with the master monitoring the file system for changes, among other things). The gRPC connections are long-lived and are forked along with the child worker, causing a warning in the underlying library that this is not safe.

The second part to the fix is to switch the transport to rest. This fixes the problem even without the refactoring (as well as the above issue with refactoring, because the rest requests are stateless and do not maintain a persistent connection, and so have no fork safety issues.

[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 significant performance regression in local Flask server startup observed after updating the grpcio library. By transitioning the Secret Manager client to a singleton and switching its transport layer to REST, the changes eliminate persistent connection overhead and resolve fork-related issues, ensuring a smoother development experience.

Highlights

• Secret Manager Singleton: Refactored the Secret Manager client into a singleton pattern to prevent redundant initialization and improve startup performance.
• Transport Protocol Switch: Switched the Secret Manager transport from gRPC to REST to resolve fork safety warnings in Flask debug mode and eliminate startup latency.

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[gemini-code-assist]

Code Review

This pull request introduces a global _secret_client variable in server/__init__.py to cache and reuse the GCP Secret Manager client across function calls, initializing it with the REST transport when first needed. There are no review comments to address, and I have no additional feedback to provide.

[juliawu]

Thank you for the much needed fix!