Complete connection pool pruning (Story 2/3/4)

[apoorvdeshmukh]

Summary

Wraps up the V2 ChannelDbConnectionPool idle-pruning feature started in Story 1: derive the pruning window from Connection Idle Timeout (Story 2) plus resilience/robustness tests (Stories 3 & 4). Part of parent #37338.

What changed

Story 2 — window derived from Connection Idle Timeout (was LoadBalanceTimeout, which defaulted to 0 -> hard-coded 300s):

• Both the sampling interval and sample count are derived so the window spans the full idle timeout while the sample count stays bounded:
  • interval = clamp(ceil(IdleTimeout / 300), 10s, 1 day)
  • sampleSize = min(300, ceil(IdleTimeout / interval))
• Short/typical timeouts keep the 10s cadence; large timeouts pin sampleSize at 300 and stretch the interval (EventSource trace on stretch). The 1-day cap guards Timer.Change overflow.
• Pruner constructed only when MinPoolSize < MaxPoolSize and IdleTimeout != 0.
• No new keyword / AppContext switch. Default 300s -> 10s, 30 samples = Story 1 defaults (no behavior change).

Story 3 (tests): median ignores a transient demand spike; prune -> regrow -> re-arm cycle. No prune-margin change (deferred).

Story 4 (test): pruning callback racing with Shutdown() is an inert no-op.

Regression: multi-threaded checkout/return asserts Count > MinPoolSize <=> timer enabled.

Notes for reviewer (@Malcolm)

• Adaptive interval per our discussion: bound both interval and sample count (max 300); stretch the interval instead of shrinking the window for large idle timeouts.
• Chose to derive the window from Connection Idle Timeout rather than add a V2-specific Connection Pruning Interval keyword (ambiguous to users re: V1 vs V2 pool).

Testing

ChannelDbConnectionPoolPruningTest — 38/38 on net8.0 and net9.0; net462 driver builds.

Checklist

• Tests added/updated
• No public API changes
• No breaking changes (defaults unchanged)
• Verified against customer repro (N/A — internal pool tuning)

Suggested release note

Connection pooling (V2 ChannelDbConnectionPool): the background idle-connection pruner now sizes its sampling window from Connection Idle Timeout and is disabled when idle timeout is 0. Default behavior is unchanged.

Fixes #45165

[Copilot]

Pull request overview

Completes the V2 ChannelDbConnectionPool idle-pruning feature by deriving the pruning sampling window from the existing Connection Idle Timeout setting (instead of LoadBalanceTimeout + a fallback constant), and adds resilience/robustness unit tests for pruning behavior and shutdown races.

Changes:

• Size the pruning window from PoolGroupOptions.IdleTimeout (sampleSize = ceil(idleTimeout / 10s), clamped to MaxSampleSize with a pooler trace when clamped).
• Only construct the pruner when MinPoolSize < MaxPoolSize and IdleTimeout != 0.
• Expand unit tests to cover spike resilience, prune→disable→regrow cycles, shutdown-race no-op behavior, and concurrent checkout/return invariants.

Reviewed changes

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

File	Description
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/ChannelDbConnectionPool.cs	Gates pruner construction on IdleTimeout != 0 and documents the new rationale.
src/Microsoft.Data.SqlClient/src/Microsoft/Data/SqlClient/ConnectionPool/PoolPruner.cs	Computes sample window from idle-timeout seconds, clamps with EventSource trace, and uses Timeout.InfiniteTimeSpan consistently.
src/Microsoft.Data.SqlClient/tests/UnitTests/ConnectionPool/ChannelDbConnectionPoolPruningTest.cs	Updates existing tests for idle-timeout-based sizing and adds resilience/robustness/concurrency regression coverage.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.37%. Comparing base (fdebcd2) to head (dde8d05).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4463      +/-   ##
==========================================
- Coverage   65.83%   62.37%   -3.47%     
==========================================
  Files         287      283       -4     
  Lines       43763    66823   +23060     
==========================================
+ Hits        28812    41681   +12869     
- Misses      14951    25142   +10191     

Flag	Coverage Δ
CI-SqlClient	?
PR-SqlClient-Project	62.37% <100.00%> (?)

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

src/Microsoft.Data.SqlClient/tests/UnitTests/ConnectionPool/ChannelDbConnectionPoolPruningTest.cs:10

• using System.Threading.Tasks; is unused in this test file, which will trigger CS8019 and fail the build (TreatWarningsAsErrors=true).

using System.Threading;
using System.Threading.Tasks;
using System.Transactions;

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.