fix(metrics): resolve thread-safety race condition in MetricsInterceptor

[sinhasubham]

Title:
fix(metrics): resolve thread-safety race condition in MetricsInterceptor

Description: This PR fixes a thread-safety regression introduced in v3.60.0 that caused AlreadyExists: 409 errors during parallel batch operations.

The Bug: The
MetricsInterceptor was accessing the global SpannerMetricsTracerFactory.current_metrics_tracer after the gRPC call completed. In concurrent scenarios, this global variable could be overwritten by another thread starting a new request, leading to the interceptor attempting to record completion on the wrong (or uninitialized) tracer. This resulted in an AttributeError crash, which the client library would catch and retry, causing a duplicate INSERT attempt and the 409 error.

The Fix:

Thread Safety: The intercept method now captures the current_metrics_tracer into a local variable before making the gRPC call. This guarantees that record_attempt_completion uses the same tracer instance that started the attempt, regardless of what other threads do.

Testing: Added test_intercept_thread_safety which uses threading and mocks to deterministically reproduce the race condition and verify the fix.

[gemini-code-assist]

Summary of Changes

Hello @sinhasubham, 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 thread-safety regression in the MetricsInterceptor component, which was causing data integrity issues and retry-related errors in concurrent operations. By ensuring that the correct metrics tracer is consistently used for each gRPC call, the changes prevent erroneous metric reporting and eliminate the root cause of 409 errors during parallel batch processing.

Highlights

• Thread-Safety Fix: Resolved a thread-safety race condition in MetricsInterceptor where the global current_metrics_tracer could be overwritten by concurrent requests, leading to AttributeError crashes and subsequent 409 errors during retries.
• Tracer Capture: The intercept method now captures the current_metrics_tracer into a local variable before the gRPC call, ensuring the correct tracer instance is used throughout the call's lifecycle, regardless of other concurrent threads.
• Regression Testing: A new unit test, test_intercept_thread_safety, has been added to deterministically reproduce the race condition and verify the effectiveness of the thread-safety fix.

Changelog

• google/cloud/spanner_v1/metrics/metrics_interceptor.py
  • Introduced a local variable tracer to capture SpannerMetricsTracerFactory.current_metrics_tracer at the beginning of the intercept method.
  • Updated all subsequent references within the intercept method to use the local tracer variable instead of the global SpannerMetricsTracerFactory.current_metrics_tracer.
• tests/unit/test_metrics_interceptor.py
  • Added threading and time imports to support concurrency testing.
  • Implemented test_intercept_thread_safety to specifically test and confirm the fix for the MetricsInterceptor race condition.

Activity

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[gemini-code-assist]

Code Review

This pull request effectively resolves the thread-safety race condition in MetricsInterceptor by capturing the current_metrics_tracer in a local variable within the intercept method. This is a solid fix for the described bug. The addition of the test_intercept_thread_safety is excellent, as it deterministically reproduces the race condition and verifies the fix, preventing future regressions. The existing comment regarding a potential copy-paste error in the test code has been retained as it does not conflict with any provided rules.