fix(pubsub): impersonate sa credentials CompletionQueue lifetime

[scotthart]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @scotthart, 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 the lifetime management of the CompletionQueue within the Pub/Sub SubscriberConnectionImpl, particularly in scenarios involving impersonated service account credentials. By refactoring the constructor to accept an externally managed BackgroundThreads object, the PR ensures proper resource handling. It also introduces new integration tests to validate the Pub/Sub client's behavior with impersonated credentials and updates the continuous integration environment to support these tests.

Highlights

• Refactored SubscriberConnectionImpl Constructor: The constructor for SubscriberConnectionImpl now accepts an externally managed std::shared_ptr<BackgroundThreads> object, allowing for explicit control over the CompletionQueue's lifetime.
• Enhanced Testability: A MockBackgroundThreads class was introduced, and numerous SubscriberConnectionTest cases were updated to inject this mock, significantly improving the testability of SubscriberConnectionImpl.
• New Integration Test for Impersonated Credentials: A new integration test, PublishPullAckWithImpersonatedCredentials, has been added to verify Pub/Sub functionality when using impersonated service account credentials.
• CI Configuration Updates: New environment variables (GOOGLE_CLOUD_CPP_PUBSUB_TEST_IMPERSONATED_SERVICE_ACCOUNT) were added to the CI configuration scripts (.ps1 and .sh) to support the new impersonated credentials integration tests.

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[gemini-code-assist]

Code Review

This pull request addresses a potential lifetime issue with the CompletionQueue used for impersonated service account credentials in Pub/Sub subscribers. The core change is to pass an existing BackgroundThreads object (which holds the CompletionQueue) into the SubscriberConnectionImpl, rather than having it create its own. This ensures that components like credentials and the subscriber connection can share the same background threads and completion queue, which is crucial for long-lived credential helpers.

The changes look correct and are supported by a new integration test. My main feedback is to reduce code duplication in the unit tests for SubscriberConnectionImpl. A significant amount of boilerplate for mocking background threads is repeated across multiple tests, and abstracting this into a helper function would improve the test suite's readability and maintainability.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.95%. Comparing base (a8aaa31) to head (f652998).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15833   +/-   ##
=======================================
  Coverage   92.94%   92.95%           
=======================================
  Files        2458     2458           
  Lines      227588   227621   +33     
=======================================
+ Hits       211539   211575   +36     
+ Misses      16049    16046    -3     

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