fix(spanner): handle errors during stream restart in snapshot

[sinhasubham]

Handle errors during stream restart in snapshot

Root Cause
When _restart_on_unavailable caught a ServiceUnavailable or resumable InternalServerError, it attempted to re-initialize the iterator immediately within the except block. If this re-initialization failed (e.g. due to a persistent transient error), the exception would propagate unhandled, breaking the retry loop.

Fix
This change modifies the logic to reset the iterator to None and continue the loop, forcing the re-initialization to occur inside the try block. This ensures that subsequent errors during restart are properly caught and retried.

Testing
Added unit tests to cover this specific behavior

[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 reliability issue in the Spanner client library's snapshot iteration, where transient errors during stream restarts could lead to unhandled exceptions and premature termination of data retrieval. By refining the retry mechanism to properly re-initialize iterators after such errors, the change significantly improves the robustness and resilience of data streaming operations, preventing data loss or incomplete results due to intermittent service disruptions.

Highlights

• Robust Error Handling: The pull request enhances the error handling mechanism within the _restart_on_unavailable function for Spanner snapshots, specifically addressing issues where re-initialization failures during stream restarts could break the retry loop.
• Refactored Iterator Re-initialization: The logic for re-initializing iterators after catching ServiceUnavailable or resumable InternalServerError has been refactored. Instead of immediate re-initialization within the except block, the iterator is now explicitly set to None, forcing re-initialization to occur safely within the main try block on the next loop iteration.
• Improved Test Coverage: New unit tests have been added to specifically cover scenarios where ServiceUnavailable or resumable InternalServerError exceptions occur during the stream restart process, ensuring the fix behaves as expected and the retry mechanism is robust.

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[gemini-code-assist]

Code Review

This pull request effectively addresses a bug where errors during a stream restart in a snapshot were not handled correctly, which could break the retry loop. The fix of resetting the iterator to None and letting the main try block handle the re-initialization is a solid approach. The added unit tests properly cover the fixed scenario.

I've included a couple of suggestions to improve code maintainability by reducing duplication in both the implementation and the new tests. These changes should make the code cleaner and easier to manage in the future.

[sakthivelmanii]

why are you removing the trace?

[sinhasubham]

We aren't removing the observability; we're consolidating it. By setting iterator = None and calling continue, the loop restarts and re-enters the trace_call block at Line 111. This ensures every retry is still traced while removing the need for duplicate tracing logic inside the except blocks. By doing this we handle the problem of error inside except block which is crashing the application without going through retry mechanism.