chore(spanner): core implementation for a dynamic channel pool

[olavloite]

Introduce modular channel pooling supporting static fixed-size pools and dynamic load-based scaling with Power of Two Least Busy (P2C) selection, parallel channel priming, and transaction affinity routing.

[gemini-code-assist]

Code Review

This pull request introduces a comprehensive dynamic channel pooling implementation for the Spanner client. It includes transaction channel affinity management, static and dynamic pool configurations, channel entry lifecycle tracking with RAII guards, a core channel pool engine utilizing Power of Two Least Busy selection, and background scaling workers for autonomous scale-up, scale-down, and priming. I have no feedback to provide as there are no review comments to assess.

[codecov]

Codecov Report

❌ Patch coverage is 97.58287% with 70 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.54%. Comparing base (76a72fc) to head (f092480).

Files with missing lines	Patch %	Lines
src/spanner/src/channel_pool/scaler.rs	97.75%	38 Missing ⚠️
src/spanner/src/channel_pool/pool.rs	95.15%	24 Missing ⚠️
src/spanner/src/channel_pool/entry.rs	97.66%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6522      +/-   ##
==========================================
+ Coverage   96.51%   96.54%   +0.03%     
==========================================
  Files         304      308       +4     
  Lines       87943    90839    +2896     
==========================================
+ Hits        84878    87702    +2824     
- Misses       3065     3137      +72     

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[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a dynamic channel pooling implementation for the Spanner client, adding modules for affinity routing, pool configuration, channel entry lifecycles, and background scaling/priming. The review feedback suggests several robustness and performance improvements: using sort_unstable_by_key to avoid temporary allocations during sorting, replacing direct as u64 casts with u64::try_from to prevent truncation of large durations, and using is_finite() instead of is_nan() to properly validate float configuration parameters.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a dynamic channel pooling mechanism for Spanner, implementing capacity management, load-balanced channel selection (Power of Two Least Busy), error penalization, transaction affinity pinning, and background scaling. Feedback on the implementation highlights two critical issues in the background scaling loops: a potential CPU thrashing risk in the scale-up worker loop during its cooldown period, and a background task leak in the scale-down monitor loop that can persist for up to three minutes after the client is dropped. Suggestions are provided to sleep during the remaining cooldown and to use a shutdown notification signal to terminate the monitor loop immediately.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a dynamic channel pooling engine for the Spanner client, implementing capacity management, Power of Two Least Busy (P2C) channel selection, transaction affinity routing, and background scaling/priming workers. The review feedback highlights three critical concurrency and logic issues: a bug in the scale-up worker where cooldown sleep discards the triggering notification, a dangerous unstable sort on active entries using concurrent atomic loads that violates sorting invariants, and a check-then-act race condition when updating error penalty state across separate atomic variables.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a dynamic channel pooling system for the Spanner client, featuring configurable static and dynamic pool strategies, atomic channel entry management, and background scaling workers. The reviewer provided a valuable suggestion to use Weak references in the scale-up worker loop to avoid holding strong Arc references across await points, which ensures cleaner resource cleanup during shutdown.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a dynamic channel pooling implementation for the Spanner client, including transaction affinity routing, Power of Two Least Busy selection, and background scaling/priming. The feedback suggests wrapping physical channel creation in a timeout to prevent indefinite blocking during network hangs, and updating the scale-up cooldown timestamp when returning early to avoid busy-wakeup storms under sustained load.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a dynamic channel pooling mechanism for the Spanner client, including support for static and dynamic pool configurations, Power of Two Least Busy (P2C) load balancing, and transaction affinity management. The implementation includes background scaling workers for dynamic adjustments and RAII guards for resource management. Feedback suggests refining the cooldown throttling logic to distinguish between capacity-based and wakeup-based delays, and considering an active eviction mechanism for permanently unhealthy channels.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request introduces a new channel_pool module for the Spanner client, implementing support for both static and dynamic channel pooling. The changes include logic for transaction affinity, channel entry management, P2C (Power of Two Least Busy) load balancing, and background scaling workers to handle dynamic capacity management. I have no feedback to provide on these changes.