[6.0] Fix connection pool concurrency issue (#3632)

[cheenamalhotra]

Ports #3632 to release/6.0

[Copilot]

Pull Request Overview

This PR fixes a concurrency issue in the connection pool by introducing a new SemaphoreHolder class that properly manages semaphore acquisition and release using the disposable pattern. The fix ensures that semaphores are automatically released even when exceptions occur, preventing potential deadlocks in connection pool operations.

Key Changes

• Introduces a new SemaphoreHolder class that implements IDisposable for automatic semaphore cleanup
• Refactors semaphore usage in TryGetConnection and PoolCreateRequest methods to use the new pattern
• Adds comprehensive connection pool stress tests to validate concurrent behavior

Reviewed Changes

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

File	Description
ConnectionPoolStressTest.cs	New test file with comprehensive stress testing for connection pool concurrency scenarios
Microsoft.Data.SqlClient.ManualTesting.Tests.csproj	Includes the new stress test file in the test project
DbConnectionPool.cs	Implements the SemaphoreHolder class and refactors semaphore management to prevent concurrency issues

[codecov]

Codecov Report

❌ Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.82%. Comparing base (90bc83c) to head (93d7455).
⚠️ Report is 6 commits behind head on release/6.0.

Files with missing lines	Patch %	Lines
...rc/Microsoft/Data/ProviderBase/DbConnectionPool.cs	76.92%	3 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release/6.0    #3654      +/-   ##
===============================================
+ Coverage        72.73%   75.82%   +3.08%     
===============================================
  Files              285      244      -41     
  Lines            59162    40211   -18951     
===============================================
- Hits             43034    30489   -12545     
+ Misses           16128     9722    -6406     

Flag	Coverage Δ
addons	92.58% <ø> (ø)
netcore	75.69% <76.92%> (+0.16%)	⬆️
netfx	?

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[benrr101]

The test code is mostly unused, please either remove it or operationalize it.
Take a look at the placement of the PreapreConstrainedRegions call in theDbConnectionPool class.

[benrr101]

These tests need a lot of work before I'd approve of them normally. But I'm in a bind because this work has already been accepted on main, and the fix is important enough. I'd prefer it if you didn't submit this test file as part of this PR and back out the file in main. But considering you want it released asap since it's not really harming anything to add more incomplete code to the mountain of incomplete code in our test folders.