chore(spanner): add location-aware routing hint generation and key recipe caching

[olavloite]

• Add LocationRouter::resolve_route for unified, single-pass resolution of target ServerConnection and RoutingHint, eliminating sampling drift and TOCTOU races between transport selection and hint stamping.
• Add KeyRecipeCache to ingest and store RecipeList definitions by schema generation, supporting zero-allocation table/index lookups, CLOCK (Second-Chance) eviction for query recipes, and atomic cache invalidation on schema version bumps.
• Update CacheUpdater::process_cache_update to ingest owned CacheUpdate payloads by value, enabling zero-copy recipe transfer.
• Wire single-pass streaming route resolution into DatabaseClient, populating RoutingHint on streaming read/query requests when location routing is active.

[gemini-code-assist]

Code Review

This pull request implements location-aware routing for the Spanner client, enabling the resolution of optimal server connections and routing hints in a single atomic pass. It introduces schema generation tracking to invalidate stale cache entries and updates the CacheUpdater and KeyRecipeCache to handle these changes efficiently. The review identified a type mismatch in extract_transaction_id and a move-borrow conflict in update_from_recipe_list, both of which require fixes to ensure compilation.

[codecov]

Codecov Report

❌ Patch coverage is 99.80658% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.57%. Comparing base (807e7b6) to head (360072b).

Files with missing lines	Patch %	Lines
src/spanner/src/routing/cache_updater.rs	98.31%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6563      +/-   ##
==========================================
+ Coverage   96.53%   96.57%   +0.03%     
==========================================
  Files         304      304              
  Lines       90380    91285     +905     
==========================================
+ Hits        87249    88157     +908     
+ Misses       3131     3128       -3     

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[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request integrates location-aware routing hints into Spanner streaming RPCs by resolving the optimal server connection and routing hint in a single atomic pass. It introduces schema generation tracking and invalidation within the key recipe cache, refactors cache updates to be processed by value, and implements unified route resolution in the location router. A review comment correctly identifies an issue in the tablet fallback path where querying eligible tablets with prefer_leader = true after a leader failure prevents routing to healthy follower replicas, suggesting a fallback to false instead.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request implements location-aware routing hints for Spanner streaming RPCs by introducing a single-pass route resolution mechanism (resolve_route) that atomically determines the target server connection and constructs the corresponding RoutingHint. It also updates KeyRecipeCache to track schema generation versions, invalidating stale cache entries on schema bumps, and refactors CacheUpdater to process updates by value. Additionally, the routing fallback logic is improved to fail over to healthy follower replicas when a leader is on cooldown. Feedback on the changes points out a critical synchronization issue in cache_subscriber.rs where bypassing the update lock when processing cache updates can lead to data races and out-of-sync database IDs.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request implements location-aware routing enhancements for the Spanner database client, including atomic resolution of server connections and routing hints, improved concurrency in CacheUpdater via RwLock, and schema generation tracking in KeyRecipeCache to manage cache invalidations. The reviewer provided an optimization suggestion for KeyRecipeCache::update_from_recipe_list to reduce lock contention by deferring the deallocation of displaced recipes until after the write lock is released.

[olavloite]

/gemini review

[gemini-code-assist]

Code Review

This pull request integrates location-aware request routing and routing hint generation into the Spanner client. It refactors the routing resolution logic to perform a single-pass atomic resolution of both the target ServerConnection and the RoutingHint, ensuring consistency and preventing TOCTOU races. The cache update mechanism is optimized by moving the database ID and update synchronization locks (RwLock) directly into CacheUpdater, allowing concurrent incremental updates under a shared read lock while synchronizing database transitions under an exclusive write lock. Additionally, KeyRecipeCache now tracks schema generations to handle stale updates and invalidate outdated recipes. Comprehensive unit and integration tests have been added to verify these routing, caching, and fallback behaviors. I have no feedback to provide as there are no review comments to evaluate.