Add automatic pool size reduction (pruning) to ChannelDbConnectionPool

[apoorvdeshmukh]

Description

Summary

Implements automatic pool size reduction for ChannelDbConnectionPool (connection pool V2). When demand drops, excess idle connections are gradually closed based on observed usage patterns rather than reacting to instantaneous idle counts.

Design

• Sampling-based pruning: A one-shot timer fires every 10 seconds, recording the current idle connection count into a circular buffer.
• Median-based decision: Once the buffer fills, samples are sorted and the median is used as the prune target. This smooths out brief traffic lulls and avoids over-pruning.
• Sample window: LoadBalanceTimeout / PruningInterval samples (defaults to 300s / 10s = 30 samples when Connection Lifetime is 0), clamped to MaxSampleSize (300).
• MinPoolSize floor: Pruning never reduces the pool below MinPoolSize.
• Fixed-size pools skip pruning: When MinPoolSize >= MaxPoolSize, no PoolPruner is created.

Architecture

Pruning logic is encapsulated in a dedicated PoolPruner class (IDisposable):

• PoolPruner owns the timer, sample buffer, and sampling/pruning logic.
• ChannelDbConnectionPool holds a nullable Pruner auto-property — null for fixed-size pools.
• The pool calls Pruner?.UpdateTimer() after connections are opened or closed.
• The pool exposes PruneConnections(int count) for the pruner to call back into.
• ChannelDbConnectionPool implements IDisposable (delegates to Shutdown()).

Changes

• PoolPruner.cs (new): Encapsulates all pruning state and logic — timer, sample buffer, median calculation, enable/disable, and disposal.
• ChannelDbConnectionPool.cs: Added PoolPruner? Pruner auto-property, PruneConnections() callback method, IDisposable implementation, and pruner creation in constructor.
• ChannelDbConnectionPoolPruningTest.cs (new): Unit tests for pruning behavior.

Thread Safety

• Timer callback acquires lock(_timer) to read/write shared sample state.
• Dispose() acquires the same lock with a _disposed guard to prevent double-disposal.
• UpdateTimer() checks _pool.IsRunning both before and inside the lock to avoid post-shutdown races.
• Prune loop runs outside the lock and checks IsRunning and MinPoolSize floor on each iteration.
• Timer uses ADP.UnsafeCreateTimer to avoid capturing ExecutionContext onto the long-lived timer.

Issues

AB#44848

Testing

Tests

• ChannelDbConnectionPoolPruningTest.cs: 19 test methods (28 runtime cases including Theory data) covering:
  • Timer creation and configuration (fixed-size pool skips, sample size calculation, clamp)
  • UpdateTimer enable/disable logic
  • Pruning callback: sampling, buffer reset, MinPoolSize floor, in-use connection safety
  • Sample values verified after each callback
  • DivideRoundingUp correctness (7 cases)
  • Shutdown/Dispose behavior
  • Integration: full prune to zero, median prevents aggressive pruning during brief lulls
• All tests use using var pool for deterministic cleanup via IDisposable
• GetPruner() helper eliminates null-forgiving operator noise

Guidelines

Please review the contribution guidelines before submitting a pull request:

• Contributing
• Code of Conduct
• Best Practices
• Coding Style
• Review Process

[Copilot]

Pull request overview

Adds an automatic “pruning” mechanism to ChannelDbConnectionPool (pool V2) to gradually reduce excess idle connections during low demand by sampling idle counts over time and pruning based on the median sample.

Changes:

• Introduces pruning state, sampling logic, and a one-shot timer callback to periodically evaluate and close excess idle connections.
• Wires pruning timer enable/disable behavior into connection add/remove paths and disposes the timer during pool shutdown.
• Adds a dedicated unit test suite validating pruning configuration, sampling/median behavior, floors (MinPoolSize), and shutdown behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs	Implements the pruning timer, sampling window/median computation, and integrates timer lifecycle with pool growth/shrink/shutdown.
src/Microsoft.Data.SqlClient/tests/UnitTests/ConnectionPool/ChannelDbConnectionPoolPruningTest.cs	Adds unit tests covering pruning timer creation/configuration, sampling/median pruning behavior, and shutdown semantics.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

[codecov]

Codecov Report

❌ Patch coverage is 98.27586% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.64%. Comparing base (bfbdd30) to head (3e43ad2).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...rosoft/Data/SqlClient/ConnectionPool/PoolPruner.cs	97.26%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4304      +/-   ##
==========================================
- Coverage   66.69%   64.64%   -2.05%     
==========================================
  Files         284      280       -4     
  Lines       43238    66162   +22924     
==========================================
+ Hits        28836    42773   +13937     
- Misses      14402    23389    +8987     

Flag	Coverage Δ
CI-SqlClient	?
PR-SqlClient-Project	64.64% <98.27%> (?)

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[mdaigle]

Would like to see some improvements to the tests, otherwise looks good!

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

[paulmedynski]

Looks good - just a few minor tweaks.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.

[mdaigle]

👍