fix(storage): gRPC misuse causing crashes due to concurrent writes from Flush() and Write()

[v-pratap]

A gRPC misuse issue can lead to crashes when using the async writer. The issue occurs due to a race condition between user code and the SDK's internal handling of Flush() operations.

Here's a breakdown of the problem:

When a Flush() is issued, the buffer is cleaned up asynchronously via OnQuery() after the flush future is marked complete. OnQuery() can then call WriteLoop(), which issues a write. However, user code can also call Write() after the Flush() future is complete, which can also call WriteLoop(). This can result in two concurrent writes to the same gRPC stream, which is against the gRPC API contract and can cause a crash.

[gemini-code-assist]

Summary of Changes

Hello @v-pratap, 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 resolves a critical race condition within the gRPC async writer that could lead to application crashes. The issue arose from Flush() and Write() operations potentially initiating concurrent writes to the same gRPC stream, violating the gRPC API contract. The solution involves introducing and carefully managing a writing_ flag to serialize write operations, ensuring only one write is active at a time. This prevents the problematic concurrent access and stabilizes the async writer. A new test case has been added to thoroughly validate this fix.

Highlights

• Race Condition Fix: Addressed a critical gRPC misuse issue that caused crashes due to a race condition between Flush() and Write() operations, leading to concurrent writes on the same gRPC stream.
• Concurrent Write Prevention: Implemented a writing_ flag to ensure that only one write operation (either from Flush() or Write()) is active at any given time, thereby preventing concurrent gRPC writes.
• Logic Reordering in OnQuery: Reordered the execution flow within the OnQuery callback to ensure that SetFlushed is called after the OnQuery processing, preventing potential issues with object lifetime management.
• New Test Case: Introduced a new test case, NoConcurrentWritesWhenFlushAndWriteRace, to specifically validate the fix and ensure that concurrent Flush() and Write() calls no longer result in concurrent gRPC writes.

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[gemini-code-assist]

Code Review

This pull request effectively resolves a critical race condition that could cause crashes due to concurrent writes on a gRPC stream. The solution, which involves introducing a writing_ flag for serialization and adjusting the order of asynchronous operations, is sound and well-implemented. The new test case, NoConcurrentWritesWhenFlushAndWriteRace, is particularly valuable as it specifically targets the race condition and confirms the fix. The changes significantly enhance the thread safety and reliability of the async writer. I have one minor style suggestion.

[gemini-code-assist]

This line contains trailing whitespace. The repository's style guide indicates it follows the Google C++ Style Guide, which advises against trailing whitespace for code cleanliness.¹

      was_finalizing = finalizing_;

Style Guide References

Footnotes

1. ↩

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.93%. Comparing base (c03c99b) to head (6b5569a).

Files with missing lines	Patch %	Lines
...e/internal/async/writer_connection_resumed_test.cc	87.30%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15802      +/-   ##
==========================================
- Coverage   92.94%   92.93%   -0.01%     
==========================================
  Files        2457     2457              
  Lines      227128   227190      +62     
==========================================
+ Hits       211099   211148      +49     
- Misses      16029    16042      +13     

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