test(lru): demonstrate TOCTOU race in lru implementation

[kislaykishore]

Description

When downloading sparse files, cached objects grow incrementally from 0 bytes as chunks arrive. Callers invoke cache.UpdateSize(key, delta) to dynamically update LRU size accounting (currentSize). (Note: Normal sequential file-cache is immune to this issue because its full file size is allocated upfront at insertion time).

• The Race: If a sparse object is concurrently evicted or erased via Erase(key) after its in-memory size changes but before UpdateSize finishes, Erase subtracts the new size instead of the size known to the cache.
• The Impact: This causes currentSize underflows (wrapping around unsigned uint64), leading to invariant check panics or aggressive, endless cache eviction. Additionally, if the same key is rapidly re-inserted, stale deltas from the old object can corrupt the new entry's accounting.

Link to the issue in case of a bug fix.

N/A

Testing details

1. Manual - Analyzed TOCTOU race condition and verified sparse-file vs normal file-cache accounting differences.
2. Unit tests - Executed go test -v ./internal/cache/lru/... on TestMapCache_UpdateSize_TOCTOU_Underflow with maxSize=2000 which successfully reproduced and demonstrated the underflow (CurrentSize 18446744073709551586 over maxSize 2000).
3. Integration tests - N/A

Any backward incompatible change? If so, please explain.

N/A

[gemini-code-assist]

Code Review

This pull request adds a new test, TestMapCache_UpdateSize_TOCTOU_Underflow, to demonstrate a TOCTOU race and size accounting underflow in the LRU cache. The reviewer pointed out that the cache size of 100 in the test is too small and can lead to false-positive test failures when cumulative updates exceed the cache limit. They suggested increasing the cache size to 2000 to ensure the test only fails on actual underflow issues.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.72%. Comparing base (4401c60) to head (0eda53e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4865      +/-   ##
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+ Coverage   83.68%   83.72%   +0.04%     
==========================================
  Files         170      170              
  Lines       20948    20948              
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+ Hits        17530    17539       +9     
+ Misses       2762     2757       -5     
+ Partials      656      652       -4     

Flag	Coverage Δ
unittests	83.72% <ø> (+0.04%)	⬆️

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