fix: address race condition

[aseering]

Implements the double-check lock pattern for inline-begin-transaction.

[gemini-code-assist]

Summary of Changes

Hello @aseering, 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 potential race condition in the Google Cloud Spanner client library where multiple threads could concurrently attempt to initialize a transaction ID. By introducing a double-check locking pattern, the changes ensure that the _transaction_id is safely and correctly initialized only once, thereby improving the robustness and thread-safety of transaction management within the library.

Highlights

• Race Condition Fix: Implemented a double-check locking mechanism to prevent race conditions during the initialization of _transaction_id in key methods, ensuring thread safety.

Changelog

• google/cloud/spanner_v1/snapshot.py
  • Added a double-check locking pattern within the _get_streamed_result_set method to safely handle _transaction_id initialization.
• google/cloud/spanner_v1/transaction.py
  • Applied a double-check locking pattern to the execute_update method for thread-safe _transaction_id initialization.
  • Incorporated a double-check locking pattern into the batch_update method to prevent race conditions during _transaction_id setup.

Activity

• No human activity has been recorded on this pull request yet.

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[gemini-code-assist]

Code Review

This pull request correctly addresses a race condition in transaction creation by implementing a double-checked locking pattern across several methods. The logic is sound. I have one suggestion for google/cloud/spanner_v1/snapshot.py to remove a redundant line of code, which will improve clarity and align the implementation with the other changes in this PR.